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.

9 files changed, 145 insertions, 64 deletions

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 {