Log a warning if the homeserver is unreachable at startup

sync2/client.go

@@ -20,6 +20,9 @@ var ProxyVersion = ""
 var HTTP401 error = fmt.Errorf("HTTP 401")
 
 type Client interface {
+	// Versions fetches and parses the list of Matrix versions that the homeserver
+	// advertises itself as supporting.
+	Versions(ctx context.Context) (version []string, err error)
 	// WhoAmI asks the homeserver to lookup the access token using the CSAPI /whoami
 	// endpoint. The response must contain a device ID (meaning that we assume the
 	// homeserver supports Matrix >= 1.1.)
@@ -49,6 +52,34 @@ func NewHTTPClient(shortTimeout, longTimeout time.Duration, destHomeServer strin
 	}
 }
 
+func (v *HTTPClient) Versions(ctx context.Context) ([]string, error) {
+	req, err := http.NewRequestWithContext(ctx, "GET", v.DestinationServer+"/_matrix/client/versions", nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("User-Agent", "sync-v3-proxy-"+ProxyVersion)
+	res, err := v.Client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	if res.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("/versions returned HTTP %d", res.StatusCode)
+	}
+	defer res.Body.Close()
+	body, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		return nil, err
+	}
+	var parsedRes struct {
+		Result []string `json:"versions"`
+	}
+	err = json.Unmarshal(body, &parsedRes)
+	if err != nil {
+		return nil, fmt.Errorf("could not parse /versions response: %w", err)
+	}
+	return parsedRes.Result, nil
+}
+
 // Return sync2.HTTP401 if this request returns 401
 func (v *HTTPClient) WhoAmI(ctx context.Context, accessToken string) (string, string, error) {
 	req, err := http.NewRequestWithContext(ctx, "GET", v.DestinationServer+"/_matrix/client/r0/account/whoami", nil)

sync2/poller_test.go

@@ -1188,6 +1188,9 @@ type mockClient struct {
 	fn func(authHeader, since string) (*SyncResponse, int, error)
 }
 
+func (c *mockClient) Versions(ctx context.Context) ([]string, error) {
+	return []string{"v1.1"}, nil
+}
 func (c *mockClient) DoSyncV2(ctx context.Context, authHeader, since string, isFirst, toDeviceOnly bool) (*SyncResponse, int, error) {
 	return c.fn(authHeader, since)
 }

v3.go

@@ -1,6 +1,7 @@
 package slidingsync
 
 import (
+	"context"
 	"embed"
 	"encoding/json"
 	"fmt"
@@ -86,6 +87,13 @@ func allowCORS(next http.Handler) http.HandlerFunc {
 func Setup(destHomeserver, postgresURI, secret string, opts Opts) (*handler2.Handler, http.Handler) {
 	// Setup shared DB and HTTP client
 	v2Client := sync2.NewHTTPClient(opts.HTTPTimeout, opts.HTTPLongTimeout, destHomeserver)
+
+	// Sanity check that we can contact the upstream homeserver.
+	_, err := v2Client.Versions(context.Background())
+	if err != nil {
+		logger.Warn().Err(err).Str("dest", destHomeserver).Msg("Could not contact upstream homeserver. Is SYNCV3_SERVER set correctly?")
+	}
+
 	db, err := sqlx.Open("postgres", postgresURI)
 	if err != nil {
 		sentry.CaptureException(err)