Set a proper HTTP user-agent header (#9482)

Previously, mattermost-server would always request with the default
user-agent of Go's net/http package that is `Go-http-client/1.1` or
something similar.
This has several disadvantages, one is that the default user-agent
made it pretty hard to distinguish mattermost requests from other
service requests in a network log for example.

Now a user-agent of the form `mattermost-<current-version>` is set in
the client.

- [x] Added or updated unit tests (required for all new features)

3 files changed, 39 insertions, 5 deletions

diff --git a/services/httpservice/client.go b/services/httpservice/client.go
index 268f63b24..1e7b7b5f9 100644
--- a/services/httpservice/client.go
+++ b/services/httpservice/client.go
@@ -10,6 +10,8 @@ import (
 	"net"
 	"net/http"
 	"time"
+
+	"github.com/mattermost/mattermost-server/model"
 )
 
 const (
@@ -28,6 +30,8 @@ func IsReservedIP(ip net.IP) bool {
 	return false
 }
 
+var defaultUserAgent string
+
 func init() {
 	for _, cidr := range []string{
 		// See https://tools.ietf.org/html/rfc6890
@@ -48,6 +52,7 @@ func init() {
 		}
 		reservedIPRanges = append(reservedIPRanges, parsed)
 	}
+	defaultUserAgent = "mattermost-" + model.CurrentVersion
 }
 
 type DialContextFunction func(ctx context.Context, network, addr string) (net.Conn, error)
@@ -97,6 +102,15 @@ func dialContextFilter(dial DialContextFunction, allowHost func(host string) boo
 	}
 }
 
+type Client struct {
+	*http.Client
+}
+
+func (c *Client) Do(req *http.Request) (*http.Response, error) {
+	req.Header.Set("User-Agent", defaultUserAgent)
+	return c.Client.Do(req)
+}
+
 // NewHTTPClient returns a variation the default implementation of Client.
 // It uses a Transport with the same settings as the default Transport
 // but with the following modifications:
@@ -104,7 +118,7 @@ func dialContextFilter(dial DialContextFunction, allowHost func(host string) boo
 //   "connectTimeout")
 // - timeout for the end-to-end request (defined as constant
 //   "requestTimeout")
-func NewHTTPClient(enableInsecureConnections bool, allowHost func(host string) bool, allowIP func(ip net.IP) bool) *http.Client {
+func NewHTTPClient(enableInsecureConnections bool, allowHost func(host string) bool, allowIP func(ip net.IP) bool) *Client {
 	dialContext := (&net.Dialer{
 		Timeout:   connectTimeout,
 		KeepAlive: 30 * time.Second,
@@ -129,5 +143,5 @@ func NewHTTPClient(enableInsecureConnections bool, allowHost func(host string) b
 		Timeout: requestTimeout,
 	}
 
-	return client
+	return &Client{Client: client}
 }
diff --git a/services/httpservice/client_test.go b/services/httpservice/client_test.go
index ceb133140..174ddf2de 100644
--- a/services/httpservice/client_test.go
+++ b/services/httpservice/client_test.go
@@ -118,3 +118,24 @@ func TestDialContextFilter(t *testing.T) {
 		}
 	}
 }
+
+func TestUserAgentIsSet(t *testing.T) {
+	testUserAgent := "test-user-agent"
+	defaultUserAgent = testUserAgent
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		ua := req.UserAgent()
+		if ua == "" {
+			t.Error("expected user-agent to be non-empty")
+		}
+		if ua != testUserAgent {
+			t.Errorf("expected user-agent to be %q but was %q", testUserAgent, ua)
+		}
+	}))
+	defer ts.Close()
+	client := NewHTTPClient(true, nil, nil)
+	req, err := http.NewRequest("GET", ts.URL, nil)
+	if err != nil {
+		t.Fatal("NewRequest failed", err)
+	}
+	client.Do(req)
+}
diff --git a/services/httpservice/httpservice.go b/services/httpservice/httpservice.go
index 5ed42a12d..6e776890f 100644
--- a/services/httpservice/httpservice.go
+++ b/services/httpservice/httpservice.go
@@ -5,7 +5,6 @@ package httpservice
 
 import (
 	"net"
-	"net/http"
 	"strings"
 
 	"github.com/mattermost/mattermost-server/services/configservice"
@@ -13,7 +12,7 @@ import (
 
 // Wraps the functionality for creating a new http.Client to encapsulate that and allow it to be mocked when testing
 type HTTPService interface {
-	MakeClient(trustURLs bool) *http.Client
+	MakeClient(trustURLs bool) *Client
 	Close()
 }
 
@@ -25,7 +24,7 @@ func MakeHTTPService(configService configservice.ConfigService) HTTPService {
 	return &HTTPServiceImpl{configService}
 }
 
-func (h *HTTPServiceImpl) MakeClient(trustURLs bool) *http.Client {
+func (h *HTTPServiceImpl) MakeClient(trustURLs bool) *Client {
 	insecure := h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections != nil && *h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections
 
 	if trustURLs {