Relax 4k post message limit (#8478)

* MM-9661: rename POST_MESSAGE_MAX_RUNES to \0_v1

* MM-9661: s/4000/POST_MESSAGE_MAX_RUNES_V1/ in tests

* MM-9661: introduce POST_MESSAGE_MAX_RUNES_V2

* MM-9661: migrate Postgres Posts.Message column to TEXT from VARCHAR(4000)

This is safe to do in a production instance since the underyling type is
not changing. We explicitly don't do this automatically for MySQL, but
also don't need to since the ORM would have already created a TEXT column
for MySQL in that case.

* MM-9661: emit MaxPostSize in client config

This value remains unconfigurable at this time, but exposes the current
limit to the client. The limit remains at 4k in this commit.

* MM-9661: introduce and use SqlPostStore.GetMaxPostSize

Enforce a byte limitation in the database, and use 1/4 of that value as
the rune count limitation (assuming a worst case UTF-8 representation).

* move maxPostSizeCached, lastPostsCache and lastPostTimeCache out of the global context and onto the SqlPostStore

* address feedback from code review:

* ensure sqlstore unit tests are actually being run
* move global caches into SqlPostStore
* leverage sync.Once to address a race condition

* modify upgrade semantics to match new db semantics

gorp's behaviour on creating columns with a maximum length on Postgres
differs from MySQL:
* Postgres
  * gorp uses TEXT for string columns without a maximum length
  * gorp uses VARCHAR(N) for string columns with a maximum length of N
* MySQL
  * gorp uses TEXT for string columns with a maximum length >= 256
  * gorp uses VARCHAR(N) for string columns with a maximum length of N
  * gorp defaults to a maximum length of 255, implying VARCHAR(255)

So the Message column has been TEXT on MySQL but VARCHAR(4000) on
Postgres. With the new, longer limits of 65535, and without changes to
gorp, the expected behaviour is TEXT on MySQL and VARCHAR(65535) on
Postgres. This commit makes the upgrade semantics match the new database
semantics.

Ideally, we'd revisit the gorp behaviour at a later time.

* allow TestMaxPostSize test cases to actually run in parallel

* default maxPostSizeCached to POST_MESSAGE_MAX_RUNES_V1 in case the once initializer panics

* fix casting error

* MM-9661: skip the schema migration for Postgres

It turns out resizing VARCHAR requires a rewrite in some versions of
Postgres, but migrating VARCHAR to TEXT does not. Given the increasing
complexity, let's defer the migration to the enduser instead.

17 files changed, 349 insertions, 140 deletions

diff --git a/api4/system.go b/api4/system.go
index 4ae8ee7b9..b34f2af6b 100644
--- a/api4/system.go
+++ b/api4/system.go
@@ -248,7 +248,7 @@ func getClientConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Write([]byte(model.MapToJson(c.App.ClientConfigWithNoAccounts())))
+	w.Write([]byte(model.MapToJson(c.App.ClientConfigWithComputed())))
 }
 
 func getClientLicense(c *Context, w http.ResponseWriter, r *http.Request) {
diff --git a/app/app.go b/app/app.go
index 6329a80d3..cd9fdaa66 100644
--- a/app/app.go
+++ b/app/app.go
@@ -139,7 +139,7 @@ func New(options ...Option) (outApp *App, outErr error) {
 
 		message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_CONFIG_CHANGED, "", "", "", nil)
 
-		message.Add("config", app.ClientConfigWithNoAccounts())
+		message.Add("config", app.ClientConfigWithComputed())
 		app.Go(func() {
 			app.Publish(message)
 		})
diff --git a/app/config.go b/app/config.go
index ccd7236a0..761fe3ec9 100644
--- a/app/config.go
+++ b/app/config.go
@@ -273,15 +273,17 @@ func (a *App) GetSiteURL() string {
 	return a.siteURL
 }
 
-// ClientConfigWithNoAccounts gets the configuration in a format suitable for sending to the client.
-func (a *App) ClientConfigWithNoAccounts() map[string]string {
+// ClientConfigWithComputed gets the configuration in a format suitable for sending to the client.
+func (a *App) ClientConfigWithComputed() map[string]string {
 	respCfg := map[string]string{}
 	for k, v := range a.ClientConfig() {
 		respCfg[k] = v
 	}
 
-	// NoAccounts is not actually part of the configuration, but is expected by the client.
+	// These properties are not configurable, but nevertheless represent configuration expected
+	// by the client.
 	respCfg["NoAccounts"] = strconv.FormatBool(a.IsFirstUserAccount())
+	respCfg["MaxPostSize"] = strconv.Itoa(a.MaxPostSize())
 
 	return respCfg
 }
diff --git a/app/config_test.go b/app/config_test.go
index 051fa8fd8..4fc7df5e2 100644
--- a/app/config_test.go
+++ b/app/config_test.go
@@ -64,12 +64,15 @@ func TestAsymmetricSigningKey(t *testing.T) {
 	assert.NotEmpty(t, th.App.ClientConfig()["AsymmetricSigningPublicKey"])
 }
 
-func TestClientConfigWithNoAccounts(t *testing.T) {
+func TestClientConfigWithComputed(t *testing.T) {
 	th := Setup().InitBasic()
 	defer th.TearDown()
 
-	config := th.App.ClientConfigWithNoAccounts()
+	config := th.App.ClientConfigWithComputed()
 	if _, ok := config["NoAccounts"]; !ok {
 		t.Fatal("expected NoAccounts in returned config")
 	}
+	if _, ok := config["MaxPostSize"]; !ok {
+		t.Fatal("expected MaxPostSize in returned config")
+	}
 }
diff --git a/app/import.go b/app/import.go
index e2e3aa1b7..23a315be7 100644
--- a/app/import.go
+++ b/app/import.go
@@ -1086,7 +1086,7 @@ func (a *App) ImportReaction(data *ReactionImportData, post *model.Post, dryRun
 }
 
 func (a *App) ImportReply(data *ReplyImportData, post *model.Post, dryRun bool) *model.AppError {
-	if err := validateReplyImportData(data, post.CreateAt); err != nil {
+	if err := validateReplyImportData(data, post.CreateAt, a.MaxPostSize()); err != nil {
 		return err
 	}
 
@@ -1136,7 +1136,7 @@ func (a *App) ImportReply(data *ReplyImportData, post *model.Post, dryRun bool)
 }
 
 func (a *App) ImportPost(data *PostImportData, dryRun bool) *model.AppError {
-	if err := validatePostImportData(data); err != nil {
+	if err := validatePostImportData(data, a.MaxPostSize()); err != nil {
 		return err
 	}
 
@@ -1271,14 +1271,14 @@ func validateReactionImportData(data *ReactionImportData, parentCreateAt int64)
 	return nil
 }
 
-func validateReplyImportData(data *ReplyImportData, parentCreateAt int64) *model.AppError {
+func validateReplyImportData(data *ReplyImportData, parentCreateAt int64, maxPostSize int) *model.AppError {
 	if data.User == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_reply_import_data.user_missing.error", nil, "", http.StatusBadRequest)
 	}
 
 	if data.Message == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_reply_import_data.message_missing.error", nil, "", http.StatusBadRequest)
-	} else if utf8.RuneCountInString(*data.Message) > model.POST_MESSAGE_MAX_RUNES {
+	} else if utf8.RuneCountInString(*data.Message) > maxPostSize {
 		return model.NewAppError("BulkImport", "app.import.validate_reply_import_data.message_length.error", nil, "", http.StatusBadRequest)
 	}
 
@@ -1293,7 +1293,7 @@ func validateReplyImportData(data *ReplyImportData, parentCreateAt int64) *model
 	return nil
 }
 
-func validatePostImportData(data *PostImportData) *model.AppError {
+func validatePostImportData(data *PostImportData, maxPostSize int) *model.AppError {
 	if data.Team == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_post_import_data.team_missing.error", nil, "", http.StatusBadRequest)
 	}
@@ -1308,7 +1308,7 @@ func validatePostImportData(data *PostImportData) *model.AppError {
 
 	if data.Message == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_post_import_data.message_missing.error", nil, "", http.StatusBadRequest)
-	} else if utf8.RuneCountInString(*data.Message) > model.POST_MESSAGE_MAX_RUNES {
+	} else if utf8.RuneCountInString(*data.Message) > maxPostSize {
 		return model.NewAppError("BulkImport", "app.import.validate_post_import_data.message_length.error", nil, "", http.StatusBadRequest)
 	}
 
@@ -1326,7 +1326,7 @@ func validatePostImportData(data *PostImportData) *model.AppError {
 
 	if data.Replies != nil {
 		for _, reply := range *data.Replies {
-			validateReplyImportData(&reply, *data.CreateAt)
+			validateReplyImportData(&reply, *data.CreateAt, maxPostSize)
 		}
 	}
 
@@ -1446,7 +1446,7 @@ func validateDirectChannelImportData(data *DirectChannelImportData) *model.AppEr
 }
 
 func (a *App) ImportDirectPost(data *DirectPostImportData, dryRun bool) *model.AppError {
-	if err := validateDirectPostImportData(data); err != nil {
+	if err := validateDirectPostImportData(data, a.MaxPostSize()); err != nil {
 		return err
 	}
 
@@ -1572,7 +1572,7 @@ func (a *App) ImportDirectPost(data *DirectPostImportData, dryRun bool) *model.A
 	return nil
 }
 
-func validateDirectPostImportData(data *DirectPostImportData) *model.AppError {
+func validateDirectPostImportData(data *DirectPostImportData, maxPostSize int) *model.AppError {
 	if data.ChannelMembers == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_direct_post_import_data.channel_members_required.error", nil, "", http.StatusBadRequest)
 	}
@@ -1591,7 +1591,7 @@ func validateDirectPostImportData(data *DirectPostImportData) *model.AppError {
 
 	if data.Message == nil {
 		return model.NewAppError("BulkImport", "app.import.validate_direct_post_import_data.message_missing.error", nil, "", http.StatusBadRequest)
-	} else if utf8.RuneCountInString(*data.Message) > model.POST_MESSAGE_MAX_RUNES {
+	} else if utf8.RuneCountInString(*data.Message) > maxPostSize {
 		return model.NewAppError("BulkImport", "app.import.validate_direct_post_import_data.message_length.error", nil, "", http.StatusBadRequest)
 	}
 
@@ -1624,7 +1624,7 @@ func validateDirectPostImportData(data *DirectPostImportData) *model.AppError {
 
 	if data.Replies != nil {
 		for _, reply := range *data.Replies {
-			validateReplyImportData(&reply, *data.CreateAt)
+			validateReplyImportData(&reply, *data.CreateAt, maxPostSize)
 		}
 	}
 
@@ -1640,12 +1640,13 @@ func validateDirectPostImportData(data *DirectPostImportData) *model.AppError {
 func (a *App) OldImportPost(post *model.Post) {
 	// Workaround for empty messages, which may be the case if they are webhook posts.
 	firstIteration := true
+	maxPostSize := a.MaxPostSize()
 	for messageRuneCount := utf8.RuneCountInString(post.Message); messageRuneCount > 0 || firstIteration; messageRuneCount = utf8.RuneCountInString(post.Message) {
 		firstIteration = false
 		var remainder string
-		if messageRuneCount > model.POST_MESSAGE_MAX_RUNES {
-			remainder = string(([]rune(post.Message))[model.POST_MESSAGE_MAX_RUNES:])
-			post.Message = truncateRunes(post.Message, model.POST_MESSAGE_MAX_RUNES)
+		if messageRuneCount > maxPostSize {
+			remainder = string(([]rune(post.Message))[maxPostSize:])
+			post.Message = truncateRunes(post.Message, maxPostSize)
 		} else {
 			remainder = ""
 		}
diff --git a/app/import_test.go b/app/import_test.go
index f294c8731..23213d81b 100644
--- a/app/import_test.go
+++ b/app/import_test.go
@@ -644,12 +644,13 @@ func TestImportValidateReactionImportData(t *testing.T) {
 func TestImportValidateReplyImportData(t *testing.T) {
 	// Test with minimum required valid properties.
 	parentCreateAt := model.GetMillis() - 100
+	maxPostSize := 10000
 	data := ReplyImportData{
 		User:     ptrStr("username"),
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err != nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err != nil {
 		t.Fatal("Validation failed but should have been valid.")
 	}
 
@@ -658,7 +659,7 @@ func TestImportValidateReplyImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -666,7 +667,7 @@ func TestImportValidateReplyImportData(t *testing.T) {
 		User:     ptrStr("username"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -674,17 +675,17 @@ func TestImportValidateReplyImportData(t *testing.T) {
 		User:    ptrStr("username"),
 		Message: ptrStr("message"),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
 	// Test with invalid message.
 	data = ReplyImportData{
 		User:     ptrStr("username"),
-		Message:  ptrStr(strings.Repeat("1234567890", 500)),
+		Message:  ptrStr(strings.Repeat("0", maxPostSize+1)),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to too long message.")
 	}
 
@@ -694,7 +695,7 @@ func TestImportValidateReplyImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(0),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to 0 create-at value.")
 	}
 
@@ -703,12 +704,13 @@ func TestImportValidateReplyImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(parentCreateAt - 100),
 	}
-	if err := validateReplyImportData(&data, parentCreateAt); err == nil {
+	if err := validateReplyImportData(&data, parentCreateAt, maxPostSize); err == nil {
 		t.Fatal("Should have failed due parent with newer create-at value.")
 	}
 }
 
 func TestImportValidatePostImportData(t *testing.T) {
+	maxPostSize := 10000
 
 	// Test with minimum required valid properties.
 	data := PostImportData{
@@ -718,7 +720,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err != nil {
+	if err := validatePostImportData(&data, maxPostSize); err != nil {
 		t.Fatal("Validation failed but should have been valid.")
 	}
 
@@ -729,7 +731,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -739,7 +741,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -749,7 +751,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -759,7 +761,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		User:     ptrStr("username"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -769,7 +771,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		User:    ptrStr("username"),
 		Message: ptrStr("message"),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -778,10 +780,10 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Team:     ptrStr("teamname"),
 		Channel:  ptrStr("channelname"),
 		User:     ptrStr("username"),
-		Message:  ptrStr(strings.Repeat("1234567890", 500)),
+		Message:  ptrStr(strings.Repeat("0", maxPostSize+1)),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to too long message.")
 	}
 
@@ -793,7 +795,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(0),
 	}
-	if err := validatePostImportData(&data); err == nil {
+	if err := validatePostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to 0 create-at value.")
 	}
 
@@ -817,7 +819,7 @@ func TestImportValidatePostImportData(t *testing.T) {
 		Reactions: &reactions,
 		Replies:   &replies,
 	}
-	if err := validatePostImportData(&data); err != nil {
+	if err := validatePostImportData(&data, maxPostSize); err != nil {
 		t.Fatal("Should have succeeded.")
 	}
 }
@@ -933,6 +935,7 @@ func TestImportValidateDirectChannelImportData(t *testing.T) {
 }
 
 func TestImportValidateDirectPostImportData(t *testing.T) {
+	maxPostSize := 10000
 
 	// Test with minimum required valid properties.
 	data := DirectPostImportData{
@@ -944,7 +947,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err != nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err != nil {
 		t.Fatal("Validation failed but should have been valid.")
 	}
 
@@ -954,7 +957,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -966,7 +969,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -978,7 +981,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		User:     ptrStr("username"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -990,7 +993,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		User:    ptrStr("username"),
 		Message: ptrStr("message"),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to missing required property.")
 	}
 
@@ -1001,7 +1004,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:        ptrStr("message"),
 		CreateAt:       ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to unsuitable number of members.")
 	}
 
@@ -1013,7 +1016,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to unsuitable number of members.")
 	}
 
@@ -1034,7 +1037,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to unsuitable number of members.")
 	}
 
@@ -1049,7 +1052,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err != nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err != nil {
 		t.Fatal("Validation failed but should have been valid.")
 	}
 
@@ -1060,10 +1063,10 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 			model.NewId(),
 		},
 		User:     ptrStr("username"),
-		Message:  ptrStr(strings.Repeat("1234567890", 500)),
+		Message:  ptrStr(strings.Repeat("0", maxPostSize+1)),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to too long message.")
 	}
 
@@ -1077,7 +1080,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(0),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Should have failed due to 0 create-at value.")
 	}
 
@@ -1097,7 +1100,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err == nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err == nil {
 		t.Fatal("Validation should have failed due to non-member flagged.")
 	}
 
@@ -1115,7 +1118,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Message:  ptrStr("message"),
 		CreateAt: ptrInt64(model.GetMillis()),
 	}
-	if err := validateDirectPostImportData(&data); err != nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err != nil {
 		t.Fatal(err)
 	}
 
@@ -1146,7 +1149,7 @@ func TestImportValidateDirectPostImportData(t *testing.T) {
 		Replies:   &replies,
 	}
 
-	if err := validateDirectPostImportData(&data); err != nil {
+	if err := validateDirectPostImportData(&data, maxPostSize); err != nil {
 		t.Fatal(err)
 	}
 }
diff --git a/app/post.go b/app/post.go
index 5067777ab..d9445155b 100644
--- a/app/post.go
+++ b/app/post.go
@@ -965,3 +965,14 @@ func (a *App) ImageProxyRemover() (f func(string) string) {
 		return url
 	}
 }
+
+func (a *App) MaxPostSize() int {
+	maxPostSize := model.POST_MESSAGE_MAX_RUNES_V1
+	if result := <-a.Srv.Store.Post().GetMaxPostSize(); result.Err != nil {
+		l4g.Error(result.Err)
+	} else {
+		maxPostSize = result.Data.(int)
+	}
+
+	return maxPostSize
+}
diff --git a/app/post_test.go b/app/post_test.go
index 2472e40c6..8455656d7 100644
--- a/app/post_test.go
+++ b/app/post_test.go
@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -17,6 +18,8 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/store"
+	"github.com/mattermost/mattermost-server/store/storetest"
 )
 
 func TestUpdatePostEditAt(t *testing.T) {
@@ -346,3 +349,59 @@ func TestMakeOpenGraphURLsAbsolute(t *testing.T) {
 		})
 	}
 }
+
+func TestMaxPostSize(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		Description         string
+		StoreMaxPostSize    int
+		ExpectedMaxPostSize int
+		ExpectedError       *model.AppError
+	}{
+		{
+			"error fetching max post size",
+			0,
+			model.POST_MESSAGE_MAX_RUNES_V1,
+			model.NewAppError("TestMaxPostSize", "this is an error", nil, "", http.StatusBadRequest),
+		},
+		{
+			"4000 rune limit",
+			4000,
+			4000,
+			nil,
+		},
+		{
+			"16383 rune limit",
+			16383,
+			16383,
+			nil,
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+		t.Run(testCase.Description, func(t *testing.T) {
+			t.Parallel()
+
+			mockStore := &storetest.Store{}
+			defer mockStore.AssertExpectations(t)
+
+			mockStore.PostStore.On("GetMaxPostSize").Return(
+				storetest.NewStoreChannel(store.StoreResult{
+					Data: testCase.StoreMaxPostSize,
+					Err:  testCase.ExpectedError,
+				}),
+			)
+
+			app := App{
+				Srv: &Server{
+					Store: mockStore,
+				},
+				config: atomic.Value{},
+			}
+
+			assert.Equal(t, testCase.ExpectedMaxPostSize, app.MaxPostSize())
+		})
+	}
+}
diff --git a/app/webhook.go b/app/webhook.go
index abfc388b5..5c3e963ce 100644
--- a/app/webhook.go
+++ b/app/webhook.go
@@ -143,7 +143,7 @@ func (a *App) TriggerWebhook(payload *model.OutgoingWebhookPayload, hook *model.
 	}
 }
 
-func SplitWebhookPost(post *model.Post) ([]*model.Post, *model.AppError) {
+func SplitWebhookPost(post *model.Post, maxPostSize int) ([]*model.Post, *model.AppError) {
 	splits := make([]*model.Post, 0)
 	remainingText := post.Message
 
@@ -159,12 +159,12 @@ func SplitWebhookPost(post *model.Post) ([]*model.Post, *model.AppError) {
 		return nil, model.NewAppError("SplitWebhookPost", "web.incoming_webhook.split_props_length.app_error", map[string]interface{}{"Max": model.POST_PROPS_MAX_USER_RUNES}, "", http.StatusBadRequest)
 	}
 
-	for utf8.RuneCountInString(remainingText) > model.POST_MESSAGE_MAX_RUNES {
+	for utf8.RuneCountInString(remainingText) > maxPostSize {
 		split := base
 		x := 0
 		for index := range remainingText {
 			x++
-			if x > model.POST_MESSAGE_MAX_RUNES {
+			if x > maxPostSize {
 				split.Message = remainingText[:index]
 				remainingText = remainingText[index:]
 				break
@@ -266,7 +266,7 @@ func (a *App) CreateWebhookPost(userId string, channel *model.Channel, text, ove
 		}
 	}
 
-	splits, err := SplitWebhookPost(post)
+	splits, err := SplitWebhookPost(post, a.MaxPostSize())
 	if err != nil {
 		return nil, err
 	}
diff --git a/app/webhook_test.go b/app/webhook_test.go
index 4d2bc58fa..8931100ac 100644
--- a/app/webhook_test.go
+++ b/app/webhook_test.go
@@ -383,23 +383,25 @@ func TestSplitWebhookPost(t *testing.T) {
 		Expected []*model.Post
 	}
 
+	maxPostSize := 10000
+
 	for name, tc := range map[string]TestCase{
 		"LongPost": {
 			Post: &model.Post{
-				Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES*3/2),
+				Message: strings.Repeat("本", maxPostSize*3/2),
 			},
 			Expected: []*model.Post{
 				{
-					Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES),
+					Message: strings.Repeat("本", maxPostSize),
 				},
 				{
-					Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES/2),
+					Message: strings.Repeat("本", maxPostSize/2),
 				},
 			},
 		},
 		"LongPostAndMultipleAttachments": {
 			Post: &model.Post{
-				Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES*3/2),
+				Message: strings.Repeat("本", maxPostSize*3/2),
 				Props: map[string]interface{}{
 					"attachments": []*model.SlackAttachment{
 						&model.SlackAttachment{
@@ -416,10 +418,10 @@ func TestSplitWebhookPost(t *testing.T) {
 			},
 			Expected: []*model.Post{
 				{
-					Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES),
+					Message: strings.Repeat("本", maxPostSize),
 				},
 				{
-					Message: strings.Repeat("本", model.POST_MESSAGE_MAX_RUNES/2),
+					Message: strings.Repeat("本", maxPostSize/2),
 					Props: map[string]interface{}{
 						"attachments": []*model.SlackAttachment{
 							&model.SlackAttachment{
@@ -452,7 +454,7 @@ func TestSplitWebhookPost(t *testing.T) {
 		},
 	} {
 		t.Run(name, func(t *testing.T) {
-			splits, err := SplitWebhookPost(tc.Post)
+			splits, err := SplitWebhookPost(tc.Post, maxPostSize)
 			if tc.Expected == nil {
 				require.NotNil(t, err)
 			} else {
diff --git a/i18n/en.json b/i18n/en.json
index d623ce864..0ce0a47e6 100644
--- a/i18n/en.json
+++ b/i18n/en.json
@@ -6447,6 +6447,18 @@
     "translation": "We couldn't select the posts to delete for the user (too many), please re-run"
   },
   {
+    "id": "store.sql_post.query_max_post_size.error",
+    "translation": "We couldn't determine the maximum supported post size"
+  },
+  {
+    "id": "store.sql_post.query_max_post_size.unrecognized_driver",
+    "translation": "No implementation found to determine the maximum supported post size"
+  },
+  {
+    "id": "store.sql_post.query_max_post_size.max_post_size_bytes",
+    "translation": "Post.Message supports at most %d bytes"
+  },
+  {
     "id": "store.sql_post.save.app_error",
     "translation": "We couldn't save the Post"
   },
diff --git a/model/post.go b/model/post.go
index 4a774b5d4..ae0627a03 100644
--- a/model/post.go
+++ b/model/post.go
@@ -40,7 +40,9 @@ const (
 	POST_FILEIDS_MAX_RUNES      = 150
 	POST_FILENAMES_MAX_RUNES    = 4000
 	POST_HASHTAGS_MAX_RUNES     = 1000
-	POST_MESSAGE_MAX_RUNES      = 4000
+	POST_MESSAGE_MAX_RUNES_V1   = 4000
+	POST_MESSAGE_MAX_BYTES_V2   = 65535                         // Maximum size of a TEXT column in MySQL
+	POST_MESSAGE_MAX_RUNES_V2   = POST_MESSAGE_MAX_BYTES_V2 / 4 // Assume a worst-case representation
 	POST_PROPS_MAX_RUNES        = 8000
 	POST_PROPS_MAX_USER_RUNES   = POST_PROPS_MAX_RUNES - 400 // Leave some room for system / pre-save modifications
 	POST_CUSTOM_TYPE_PREFIX     = "custom_"
@@ -141,7 +143,7 @@ func (o *Post) Etag() string {
 	return Etag(o.Id, o.UpdateAt)
 }
 
-func (o *Post) IsValid() *AppError {
+func (o *Post) IsValid(maxPostSize int) *AppError {
 
 	if len(o.Id) != 26 {
 		return NewAppError("Post.IsValid", "model.post.is_valid.id.app_error", nil, "", http.StatusBadRequest)
@@ -179,7 +181,7 @@ func (o *Post) IsValid() *AppError {
 		return NewAppError("Post.IsValid", "model.post.is_valid.original_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if utf8.RuneCountInString(o.Message) > POST_MESSAGE_MAX_RUNES {
+	if utf8.RuneCountInString(o.Message) > maxPostSize {
 		return NewAppError("Post.IsValid", "model.post.is_valid.msg.app_error", nil, "id="+o.Id, http.StatusBadRequest)
 	}
 
diff --git a/model/post_test.go b/model/post_test.go
index 5d5e7c9ec..af350d76e 100644
--- a/model/post_test.go
+++ b/model/post_test.go
@@ -23,72 +23,73 @@ func TestPostJson(t *testing.T) {
 
 func TestPostIsValid(t *testing.T) {
 	o := Post{}
+	maxPostSize := 10000
 
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.Id = NewId()
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.CreateAt = GetMillis()
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.UpdateAt = GetMillis()
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.UserId = NewId()
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.ChannelId = NewId()
 	o.RootId = "123"
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.RootId = ""
 	o.ParentId = "123"
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.ParentId = NewId()
 	o.RootId = ""
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.ParentId = ""
-	o.Message = strings.Repeat("0", 4001)
-	if err := o.IsValid(); err == nil {
+	o.Message = strings.Repeat("0", maxPostSize+1)
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
-	o.Message = strings.Repeat("0", 4000)
-	if err := o.IsValid(); err != nil {
+	o.Message = strings.Repeat("0", maxPostSize)
+	if err := o.IsValid(maxPostSize); err != nil {
 		t.Fatal(err)
 	}
 
 	o.Message = "test"
-	if err := o.IsValid(); err != nil {
+	if err := o.IsValid(maxPostSize); err != nil {
 		t.Fatal(err)
 	}
 
 	o.Type = "junk"
-	if err := o.IsValid(); err == nil {
+	if err := o.IsValid(maxPostSize); err == nil {
 		t.Fatal("should be invalid")
 	}
 
 	o.Type = POST_CUSTOM_TYPE_PREFIX + "type"
-	if err := o.IsValid(); err != nil {
+	if err := o.IsValid(maxPostSize); err != nil {
 		t.Fatal(err)
 	}
 }
diff --git a/store/sqlstore/post_store.go b/store/sqlstore/post_store.go
index 3ff9a3e1b..182cf4891 100644
--- a/store/sqlstore/post_store.go
+++ b/store/sqlstore/post_store.go
@@ -4,13 +4,13 @@
 package sqlstore
 
 import (
+	"bytes"
 	"fmt"
 	"net/http"
 	"regexp"
 	"strconv"
 	"strings"
-
-	"bytes"
+	"sync"
 
 	l4g "github.com/alecthomas/log4go"
 	"github.com/mattermost/mattermost-server/einterfaces"
@@ -21,7 +21,11 @@ import (
 
 type SqlPostStore struct {
 	SqlStore
-	metrics einterfaces.MetricsInterface
+	metrics           einterfaces.MetricsInterface
+	lastPostTimeCache *utils.Cache
+	lastPostsCache    *utils.Cache
+	maxPostSizeOnce   sync.Once
+	maxPostSizeCached int
 }
 
 const (
@@ -32,12 +36,9 @@ const (
 	LAST_POSTS_CACHE_SEC  = 900 // 15 minutes
 )
 
-var lastPostTimeCache = utils.NewLru(LAST_POST_TIME_CACHE_SIZE)
-var lastPostsCache = utils.NewLru(LAST_POSTS_CACHE_SIZE)
-
-func (s SqlPostStore) ClearCaches() {
-	lastPostTimeCache.Purge()
-	lastPostsCache.Purge()
+func (s *SqlPostStore) ClearCaches() {
+	s.lastPostTimeCache.Purge()
+	s.lastPostsCache.Purge()
 
 	if s.metrics != nil {
 		s.metrics.IncrementMemCacheInvalidationCounter("Last Post Time - Purge")
@@ -47,8 +48,11 @@ func (s SqlPostStore) ClearCaches() {
 
 func NewSqlPostStore(sqlStore SqlStore, metrics einterfaces.MetricsInterface) store.PostStore {
 	s := &SqlPostStore{
-		SqlStore: sqlStore,
-		metrics:  metrics,
+		SqlStore:          sqlStore,
+		metrics:           metrics,
+		lastPostTimeCache: utils.NewLru(LAST_POST_TIME_CACHE_SIZE),
+		lastPostsCache:    utils.NewLru(LAST_POSTS_CACHE_SIZE),
+		maxPostSizeCached: model.POST_MESSAGE_MAX_RUNES_V1,
 	}
 
 	for _, db := range sqlStore.GetAllConns() {
@@ -59,18 +63,18 @@ func NewSqlPostStore(sqlStore SqlStore, metrics einterfaces.MetricsInterface) st
 		table.ColMap("RootId").SetMaxSize(26)
 		table.ColMap("ParentId").SetMaxSize(26)
 		table.ColMap("OriginalId").SetMaxSize(26)
-		table.ColMap("Message").SetMaxSize(4000)
+		table.ColMap("Message").SetMaxSize(model.POST_MESSAGE_MAX_BYTES_V2)
 		table.ColMap("Type").SetMaxSize(26)
 		table.ColMap("Hashtags").SetMaxSize(1000)
 		table.ColMap("Props").SetMaxSize(8000)
-		table.ColMap("Filenames").SetMaxSize(4000)
+		table.ColMap("Filenames").SetMaxSize(model.POST_FILENAMES_MAX_RUNES)
 		table.ColMap("FileIds").SetMaxSize(150)
 	}
 
 	return s
 }
 
-func (s SqlPostStore) CreateIndexesIfNotExists() {
+func (s *SqlPostStore) CreateIndexesIfNotExists() {
 	s.CreateIndexIfNotExists("idx_posts_update_at", "Posts", "UpdateAt")
 	s.CreateIndexIfNotExists("idx_posts_create_at", "Posts", "CreateAt")
 	s.CreateIndexIfNotExists("idx_posts_delete_at", "Posts", "DeleteAt")
@@ -86,15 +90,23 @@ func (s SqlPostStore) CreateIndexesIfNotExists() {
 	s.CreateFullTextIndexIfNotExists("idx_posts_hashtags_txt", "Posts", "Hashtags")
 }
 
-func (s SqlPostStore) Save(post *model.Post) store.StoreChannel {
+func (s *SqlPostStore) Save(post *model.Post) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		if len(post.Id) > 0 {
 			result.Err = model.NewAppError("SqlPostStore.Save", "store.sql_post.save.existing.app_error", nil, "id="+post.Id, http.StatusBadRequest)
 			return
 		}
 
+		var maxPostSize int
+		if result := <-s.GetMaxPostSize(); result.Err != nil {
+			result.Err = model.NewAppError("SqlPostStore.Save", "store.sql_post.save.app_error", nil, "id="+post.Id+", "+result.Err.Error(), http.StatusInternalServerError)
+			return
+		} else {
+			maxPostSize = result.Data.(int)
+		}
+
 		post.PreSave()
-		if result.Err = post.IsValid(); result.Err != nil {
+		if result.Err = post.IsValid(maxPostSize); result.Err != nil {
 			return
 		}
 
@@ -122,7 +134,7 @@ func (s SqlPostStore) Save(post *model.Post) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) Update(newPost *model.Post, oldPost *model.Post) store.StoreChannel {
+func (s *SqlPostStore) Update(newPost *model.Post, oldPost *model.Post) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		newPost.UpdateAt = model.GetMillis()
 		newPost.PreCommit()
@@ -133,7 +145,15 @@ func (s SqlPostStore) Update(newPost *model.Post, oldPost *model.Post) store.Sto
 		oldPost.Id = model.NewId()
 		oldPost.PreCommit()
 
-		if result.Err = newPost.IsValid(); result.Err != nil {
+		var maxPostSize int
+		if result := <-s.GetMaxPostSize(); result.Err != nil {
+			result.Err = model.NewAppError("SqlPostStore.Save", "store.sql_post.update.app_error", nil, "id="+newPost.Id+", "+result.Err.Error(), http.StatusInternalServerError)
+			return
+		} else {
+			maxPostSize = result.Data.(int)
+		}
+
+		if result.Err = newPost.IsValid(maxPostSize); result.Err != nil {
 			return
 		}
 
@@ -155,11 +175,19 @@ func (s SqlPostStore) Update(newPost *model.Post, oldPost *model.Post) store.Sto
 	})
 }
 
-func (s SqlPostStore) Overwrite(post *model.Post) store.StoreChannel {
+func (s *SqlPostStore) Overwrite(post *model.Post) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		post.UpdateAt = model.GetMillis()
 
-		if result.Err = post.IsValid(); result.Err != nil {
+		var maxPostSize int
+		if result := <-s.GetMaxPostSize(); result.Err != nil {
+			result.Err = model.NewAppError("SqlPostStore.Save", "store.sql_post.overwrite.app_error", nil, "id="+post.Id+", "+result.Err.Error(), http.StatusInternalServerError)
+			return
+		} else {
+			maxPostSize = result.Data.(int)
+		}
+
+		if result.Err = post.IsValid(maxPostSize); result.Err != nil {
 			return
 		}
 
@@ -171,7 +199,7 @@ func (s SqlPostStore) Overwrite(post *model.Post) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) GetFlaggedPosts(userId string, offset int, limit int) store.StoreChannel {
+func (s *SqlPostStore) GetFlaggedPosts(userId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		pl := model.NewPostList()
 
@@ -189,7 +217,7 @@ func (s SqlPostStore) GetFlaggedPosts(userId string, offset int, limit int) stor
 	})
 }
 
-func (s SqlPostStore) GetFlaggedPostsForTeam(userId, teamId string, offset int, limit int) store.StoreChannel {
+func (s *SqlPostStore) GetFlaggedPostsForTeam(userId, teamId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		pl := model.NewPostList()
 
@@ -234,7 +262,7 @@ func (s SqlPostStore) GetFlaggedPostsForTeam(userId, teamId string, offset int,
 	})
 }
 
-func (s SqlPostStore) GetFlaggedPostsForChannel(userId, channelId string, offset int, limit int) store.StoreChannel {
+func (s *SqlPostStore) GetFlaggedPostsForChannel(userId, channelId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		pl := model.NewPostList()
 
@@ -263,7 +291,7 @@ func (s SqlPostStore) GetFlaggedPostsForChannel(userId, channelId string, offset
 	})
 }
 
-func (s SqlPostStore) Get(id string) store.StoreChannel {
+func (s *SqlPostStore) Get(id string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		pl := model.NewPostList()
 
@@ -308,7 +336,7 @@ func (s SqlPostStore) Get(id string) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) GetSingle(id string) store.StoreChannel {
+func (s *SqlPostStore) GetSingle(id string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var post model.Post
 		err := s.GetReplica().SelectOne(&post, "SELECT * FROM Posts WHERE Id = :Id AND DeleteAt = 0", map[string]interface{}{"Id": id})
@@ -325,12 +353,12 @@ type etagPosts struct {
 	UpdateAt int64
 }
 
-func (s SqlPostStore) InvalidateLastPostTimeCache(channelId string) {
-	lastPostTimeCache.Remove(channelId)
+func (s *SqlPostStore) InvalidateLastPostTimeCache(channelId string) {
+	s.lastPostTimeCache.Remove(channelId)
 
 	// Keys are "{channelid}{limit}" and caching only occurs on limits of 30 and 60
-	lastPostsCache.Remove(channelId + "30")
-	lastPostsCache.Remove(channelId + "60")
+	s.lastPostsCache.Remove(channelId + "30")
+	s.lastPostsCache.Remove(channelId + "60")
 
 	if s.metrics != nil {
 		s.metrics.IncrementMemCacheInvalidationCounter("Last Post Time - Remove by Channel Id")
@@ -338,10 +366,10 @@ func (s SqlPostStore) InvalidateLastPostTimeCache(channelId string) {
 	}
 }
 
-func (s SqlPostStore) GetEtag(channelId string, allowFromCache bool) store.StoreChannel {
+func (s *SqlPostStore) GetEtag(channelId string, allowFromCache bool) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		if allowFromCache {
-			if cacheItem, ok := lastPostTimeCache.Get(channelId); ok {
+			if cacheItem, ok := s.lastPostTimeCache.Get(channelId); ok {
 				if s.metrics != nil {
 					s.metrics.IncrementMemCacheHitCounter("Last Post Time")
 				}
@@ -366,11 +394,11 @@ func (s SqlPostStore) GetEtag(channelId string, allowFromCache bool) store.Store
 			result.Data = fmt.Sprintf("%v.%v", model.CurrentVersion, et.UpdateAt)
 		}
 
-		lastPostTimeCache.AddWithExpiresInSecs(channelId, et.UpdateAt, LAST_POST_TIME_CACHE_SEC)
+		s.lastPostTimeCache.AddWithExpiresInSecs(channelId, et.UpdateAt, LAST_POST_TIME_CACHE_SEC)
 	})
 }
 
-func (s SqlPostStore) Delete(postId string, time int64) store.StoreChannel {
+func (s *SqlPostStore) Delete(postId string, time int64) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		_, err := s.GetMaster().Exec("Update Posts SET DeleteAt = :DeleteAt, UpdateAt = :UpdateAt WHERE Id = :Id OR RootId = :RootId", map[string]interface{}{"DeleteAt": time, "UpdateAt": time, "Id": postId, "RootId": postId})
 		if err != nil {
@@ -379,7 +407,7 @@ func (s SqlPostStore) Delete(postId string, time int64) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) permanentDelete(postId string) store.StoreChannel {
+func (s *SqlPostStore) permanentDelete(postId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		_, err := s.GetMaster().Exec("DELETE FROM Posts WHERE Id = :Id OR RootId = :RootId", map[string]interface{}{"Id": postId, "RootId": postId})
 		if err != nil {
@@ -388,7 +416,7 @@ func (s SqlPostStore) permanentDelete(postId string) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) permanentDeleteAllCommentByUser(userId string) store.StoreChannel {
+func (s *SqlPostStore) permanentDeleteAllCommentByUser(userId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		_, err := s.GetMaster().Exec("DELETE FROM Posts WHERE UserId = :UserId AND RootId != ''", map[string]interface{}{"UserId": userId})
 		if err != nil {
@@ -397,7 +425,7 @@ func (s SqlPostStore) permanentDeleteAllCommentByUser(userId string) store.Store
 	})
 }
 
-func (s SqlPostStore) PermanentDeleteByUser(userId string) store.StoreChannel {
+func (s *SqlPostStore) PermanentDeleteByUser(userId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		// First attempt to delete all the comments for a user
 		if r := <-s.permanentDeleteAllCommentByUser(userId); r.Err != nil {
@@ -437,7 +465,7 @@ func (s SqlPostStore) PermanentDeleteByUser(userId string) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) PermanentDeleteByChannel(channelId string) store.StoreChannel {
+func (s *SqlPostStore) PermanentDeleteByChannel(channelId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		if _, err := s.GetMaster().Exec("DELETE FROM Posts WHERE ChannelId = :ChannelId", map[string]interface{}{"ChannelId": channelId}); err != nil {
 			result.Err = model.NewAppError("SqlPostStore.PermanentDeleteByChannel", "store.sql_post.permanent_delete_by_channel.app_error", nil, "channel_id="+channelId+", "+err.Error(), http.StatusInternalServerError)
@@ -445,7 +473,7 @@ func (s SqlPostStore) PermanentDeleteByChannel(channelId string) store.StoreChan
 	})
 }
 
-func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFromCache bool) store.StoreChannel {
+func (s *SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFromCache bool) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		if limit > 1000 {
 			result.Err = model.NewAppError("SqlPostStore.GetLinearPosts", "store.sql_post.get_posts.app_error", nil, "channelId="+channelId, http.StatusBadRequest)
@@ -454,7 +482,7 @@ func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFro
 
 		// Caching only occurs on limits of 30 and 60, the common limits requested by MM clients
 		if allowFromCache && offset == 0 && (limit == 60 || limit == 30) {
-			if cacheItem, ok := lastPostsCache.Get(fmt.Sprintf("%s%v", channelId, limit)); ok {
+			if cacheItem, ok := s.lastPostsCache.Get(fmt.Sprintf("%s%v", channelId, limit)); ok {
 				if s.metrics != nil {
 					s.metrics.IncrementMemCacheHitCounter("Last Posts Cache")
 				}
@@ -498,7 +526,7 @@ func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFro
 
 			// Caching only occurs on limits of 30 and 60, the common limits requested by MM clients
 			if offset == 0 && (limit == 60 || limit == 30) {
-				lastPostsCache.AddWithExpiresInSecs(fmt.Sprintf("%s%v", channelId, limit), list, LAST_POSTS_CACHE_SEC)
+				s.lastPostsCache.AddWithExpiresInSecs(fmt.Sprintf("%s%v", channelId, limit), list, LAST_POSTS_CACHE_SEC)
 			}
 
 			result.Data = list
@@ -506,12 +534,12 @@ func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFro
 	})
 }
 
-func (s SqlPostStore) GetPostsSince(channelId string, time int64, allowFromCache bool) store.StoreChannel {
+func (s *SqlPostStore) GetPostsSince(channelId string, time int64, allowFromCache bool) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		if allowFromCache {
 			// If the last post in the channel's time is less than or equal to the time we are getting posts since,
 			// we can safely return no posts.
-			if cacheItem, ok := lastPostTimeCache.Get(channelId); ok && cacheItem.(int64) <= time {
+			if cacheItem, ok := s.lastPostTimeCache.Get(channelId); ok && cacheItem.(int64) <= time {
 				if s.metrics != nil {
 					s.metrics.IncrementMemCacheHitCounter("Last Post Time")
 				}
@@ -576,22 +604,22 @@ func (s SqlPostStore) GetPostsSince(channelId string, time int64, allowFromCache
 				}
 			}
 
-			lastPostTimeCache.AddWithExpiresInSecs(channelId, latestUpdate, LAST_POST_TIME_CACHE_SEC)
+			s.lastPostTimeCache.AddWithExpiresInSecs(channelId, latestUpdate, LAST_POST_TIME_CACHE_SEC)
 
 			result.Data = list
 		}
 	})
 }
 
-func (s SqlPostStore) GetPostsBefore(channelId string, postId string, numPosts int, offset int) store.StoreChannel {
+func (s *SqlPostStore) GetPostsBefore(channelId string, postId string, numPosts int, offset int) store.StoreChannel {
 	return s.getPostsAround(channelId, postId, numPosts, offset, true)
 }
 
-func (s SqlPostStore) GetPostsAfter(channelId string, postId string, numPosts int, offset int) store.StoreChannel {
+func (s *SqlPostStore) GetPostsAfter(channelId string, postId string, numPosts int, offset int) store.StoreChannel {
 	return s.getPostsAround(channelId, postId, numPosts, offset, false)
 }
 
-func (s SqlPostStore) getPostsAround(channelId string, postId string, numPosts int, offset int, before bool) store.StoreChannel {
+func (s *SqlPostStore) getPostsAround(channelId string, postId string, numPosts int, offset int, before bool) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var direction string
 		var sort string
@@ -672,7 +700,7 @@ func (s SqlPostStore) getPostsAround(channelId string, postId string, numPosts i
 	})
 }
 
-func (s SqlPostStore) getRootPosts(channelId string, offset int, limit int) store.StoreChannel {
+func (s *SqlPostStore) getRootPosts(channelId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var posts []*model.Post
 		_, err := s.GetReplica().Select(&posts, "SELECT * FROM Posts WHERE ChannelId = :ChannelId AND DeleteAt = 0 ORDER BY CreateAt DESC LIMIT :Limit OFFSET :Offset", map[string]interface{}{"ChannelId": channelId, "Offset": offset, "Limit": limit})
@@ -684,7 +712,7 @@ func (s SqlPostStore) getRootPosts(channelId string, offset int, limit int) stor
 	})
 }
 
-func (s SqlPostStore) getParentsPosts(channelId string, offset int, limit int) store.StoreChannel {
+func (s *SqlPostStore) getParentsPosts(channelId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var posts []*model.Post
 		_, err := s.GetReplica().Select(&posts, `
@@ -771,7 +799,7 @@ var specialSearchChar = []string{
 	":",
 }
 
-func (s SqlPostStore) Search(teamId string, userId string, params *model.SearchParams) store.StoreChannel {
+func (s *SqlPostStore) Search(teamId string, userId string, params *model.SearchParams) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		queryParams := map[string]interface{}{
 			"TeamId": teamId,
@@ -945,7 +973,7 @@ func (s SqlPostStore) Search(teamId string, userId string, params *model.SearchP
 	})
 }
 
-func (s SqlPostStore) AnalyticsUserCountsWithPostsByDay(teamId string) store.StoreChannel {
+func (s *SqlPostStore) AnalyticsUserCountsWithPostsByDay(teamId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		query :=
 			`SELECT DISTINCT
@@ -998,7 +1026,7 @@ func (s SqlPostStore) AnalyticsUserCountsWithPostsByDay(teamId string) store.Sto
 	})
 }
 
-func (s SqlPostStore) AnalyticsPostCountsByDay(teamId string) store.StoreChannel {
+func (s *SqlPostStore) AnalyticsPostCountsByDay(teamId string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		query :=
 			`SELECT
@@ -1053,7 +1081,7 @@ func (s SqlPostStore) AnalyticsPostCountsByDay(teamId string) store.StoreChannel
 	})
 }
 
-func (s SqlPostStore) AnalyticsPostCount(teamId string, mustHaveFile bool, mustHaveHashtag bool) store.StoreChannel {
+func (s *SqlPostStore) AnalyticsPostCount(teamId string, mustHaveFile bool, mustHaveHashtag bool) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		query :=
 			`SELECT
@@ -1084,7 +1112,7 @@ func (s SqlPostStore) AnalyticsPostCount(teamId string, mustHaveFile bool, mustH
 	})
 }
 
-func (s SqlPostStore) GetPostsCreatedAt(channelId string, time int64) store.StoreChannel {
+func (s *SqlPostStore) GetPostsCreatedAt(channelId string, time int64) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		query := `SELECT * FROM Posts WHERE CreateAt = :CreateAt AND ChannelId = :ChannelId`
 
@@ -1099,7 +1127,7 @@ func (s SqlPostStore) GetPostsCreatedAt(channelId string, time int64) store.Stor
 	})
 }
 
-func (s SqlPostStore) GetPostsByIds(postIds []string) store.StoreChannel {
+func (s *SqlPostStore) GetPostsByIds(postIds []string) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		keys := bytes.Buffer{}
 		params := make(map[string]interface{})
@@ -1127,7 +1155,7 @@ func (s SqlPostStore) GetPostsByIds(postIds []string) store.StoreChannel {
 	})
 }
 
-func (s SqlPostStore) GetPostsBatchForIndexing(startTime int64, endTime int64, limit int) store.StoreChannel {
+func (s *SqlPostStore) GetPostsBatchForIndexing(startTime int64, endTime int64, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var posts []*model.PostForIndexing
 		_, err1 := s.GetSearchReplica().Select(&posts,
@@ -1167,7 +1195,7 @@ func (s SqlPostStore) GetPostsBatchForIndexing(startTime int64, endTime int64, l
 	})
 }
 
-func (s SqlPostStore) PermanentDeleteBatch(endTime int64, limit int64) store.StoreChannel {
+func (s *SqlPostStore) PermanentDeleteBatch(endTime int64, limit int64) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var query string
 		if s.DriverName() == "postgres" {
@@ -1191,7 +1219,7 @@ func (s SqlPostStore) PermanentDeleteBatch(endTime int64, limit int64) store.Sto
 	})
 }
 
-func (s SqlPostStore) GetOldest() store.StoreChannel {
+func (s *SqlPostStore) GetOldest() store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var post model.Post
 		err := s.GetReplica().SelectOne(&post, "SELECT * FROM Posts ORDER BY CreateAt LIMIT 1")
@@ -1202,3 +1230,66 @@ func (s SqlPostStore) GetOldest() store.StoreChannel {
 		result.Data = &post
 	})
 }
+
+func (s *SqlPostStore) determineMaxPostSize() int {
+	var maxPostSize int = model.POST_MESSAGE_MAX_RUNES_V1
+	var maxPostSizeBytes int32
+
+	if s.DriverName() == model.DATABASE_DRIVER_POSTGRES {
+		// The Post.Message column in Postgres has historically been VARCHAR(4000), but
+		// may be manually enlarged to support longer posts.
+		if err := s.GetReplica().SelectOne(&maxPostSizeBytes, `
+			SELECT
+				COALESCE(character_maximum_length, 0)
+			FROM
+				information_schema.columns
+			WHERE
+				table_name = 'posts'
+			AND	column_name = 'message'
+		`); err != nil {
+			l4g.Error(utils.T("store.sql_post.query_max_post_size.error") + err.Error())
+		}
+	} else if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
+		// The Post.Message column in MySQL has historically been TEXT, with a maximum
+		// limit of 65535.
+		if err := s.GetReplica().SelectOne(&maxPostSizeBytes, `
+			SELECT 
+				COALESCE(CHARACTER_MAXIMUM_LENGTH, 0)
+			FROM 
+				INFORMATION_SCHEMA.COLUMNS
+			WHERE 
+				table_schema = DATABASE()
+			AND	table_name = 'Posts'
+			AND	column_name = 'Message'
+			LIMIT 0, 1
+		`); err != nil {
+			l4g.Error(utils.T("store.sql_post.query_max_post_size.error") + err.Error())
+		}
+	} else {
+		l4g.Warn(utils.T("store.sql_post.query_max_post_size.unrecognized_driver"))
+	}
+
+	l4g.Trace(utils.T("store.sql_post.query_max_post_size.max_post_size_bytes"), maxPostSizeBytes)
+
+	// Assume a worst-case representation of four bytes per rune.
+	maxPostSize = int(maxPostSizeBytes) / 4
+
+	// To maintain backwards compatibility, don't yield a maximum post
+	// size smaller than the previous limit, even though it wasn't
+	// actually possible to store 4000 runes in all cases.
+	if maxPostSize < model.POST_MESSAGE_MAX_RUNES_V1 {
+		maxPostSize = model.POST_MESSAGE_MAX_RUNES_V1
+	}
+
+	return maxPostSize
+}
+
+// GetMaxPostSize returns the maximum number of runes that may be stored in a post.
+func (s *SqlPostStore) GetMaxPostSize() store.StoreChannel {
+	return store.Do(func(result *store.StoreResult) {
+		s.maxPostSizeOnce.Do(func() {
+			s.maxPostSizeCached = s.determineMaxPostSize()
+		})
+		result.Data = s.maxPostSizeCached
+	})
+}
diff --git a/store/store.go b/store/store.go
index f070a45db..773dfff02 100644
--- a/store/store.go
+++ b/store/store.go
@@ -198,6 +198,7 @@ type PostStore interface {
 	GetPostsBatchForIndexing(startTime int64, endTime int64, limit int) StoreChannel
 	PermanentDeleteBatch(endTime int64, limit int64) StoreChannel
 	GetOldest() StoreChannel
+	GetMaxPostSize() StoreChannel
 }
 
 type UserStore interface {
diff --git a/store/storetest/mocks/PostStore.go b/store/storetest/mocks/PostStore.go
index c405d5030..bdd0d1d16 100644
--- a/store/storetest/mocks/PostStore.go
+++ b/store/storetest/mocks/PostStore.go
@@ -422,3 +422,18 @@ func (_m *PostStore) Update(newPost *model.Post, oldPost *model.Post) store.Stor
 
 	return r0
 }
+
+func (_m *PostStore) GetMaxPostSize() store.StoreChannel {
+	ret := _m.Called()
+
+	var r0 store.StoreChannel
+	if rf, ok := ret.Get(0).(func() store.StoreChannel); ok {
+		r0 = rf()
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(store.StoreChannel)
+		}
+	}
+
+	return r0
+}
diff --git a/store/storetest/post_store.go b/store/storetest/post_store.go
index 91fc40213..44ce47d9d 100644
--- a/store/storetest/post_store.go
+++ b/store/storetest/post_store.go
@@ -43,6 +43,7 @@ func TestPostStore(t *testing.T, ss store.Store) {
 	t.Run("GetPostsBatchForIndexing", func(t *testing.T) { testPostStoreGetPostsBatchForIndexing(t, ss) })
 	t.Run("PermanentDeleteBatch", func(t *testing.T) { testPostStorePermanentDeleteBatch(t, ss) })
 	t.Run("GetOldest", func(t *testing.T) { testPostStoreGetOldest(t, ss) })
+	t.Run("TestGetMaxPostSize", func(t *testing.T) { testGetMaxPostSize(t, ss) })
 }
 
 func testPostStoreSave(t *testing.T, ss store.Store) {
@@ -1783,3 +1784,8 @@ func testPostStoreGetOldest(t *testing.T, ss store.Store) {
 
 	assert.EqualValues(t, o2.Id, r1.Id)
 }
+
+func testGetMaxPostSize(t *testing.T, ss store.Store) {
+	assert.Equal(t, model.POST_MESSAGE_MAX_RUNES_V2, (<-ss.Post().GetMaxPostSize()).Data.(int))
+	assert.Equal(t, model.POST_MESSAGE_MAX_RUNES_V2, (<-ss.Post().GetMaxPostSize()).Data.(int))
+}