fix: ensure websocket close messages are truncated to 123 bytes (#779)

It's possible for websocket close messages to be too long, which cause
them to silently fail without a proper close message. See error below:

```
2022-03-31 17:08:34.862 [INFO]	(stdlib)	<close_notjs.go:72>	"2022/03/31 17:08:34 websocket: failed to marshal close frame: reason string max is 123 but got \"insert provisioner daemon:Cannot encode []database.ProvisionerType into oid 19098 - []database.ProvisionerType must implement Encoder or be converted to a string\" with length 161"
```

coderd/httpapi/httpapi.go

@@ -115,3 +115,21 @@ func Read(rw http.ResponseWriter, r *http.Request, value interface{}) bool {
 	}
 	return true
 }
+
+const websocketCloseMaxLen = 123
+
+// WebsocketCloseSprintf formats a websocket close message and ensures it is
+// truncated to the maximum allowed length.
+func WebsocketCloseSprintf(format string, vars ...any) string {
+	msg := fmt.Sprintf(format, vars...)
+
+	// Cap msg length at 123 bytes. nhooyr/websocket only allows close messages
+	// of this length.
+	if len(msg) > websocketCloseMaxLen {
+		// Trim the string to 123 bytes. If we accidentally cut in the middle of
+		// a UTF-8 character, remove it from the string.
+		return strings.ToValidUTF8(string(msg[123]), "")
+	}
+
+	return msg
+}

coderd/httpapi/httpapi_test.go

@@ -5,8 +5,10 @@ import (
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/coder/coder/coderd/httpapi"
@@ -142,3 +144,23 @@ func TestReadUsername(t *testing.T) {
 		})
 	}
 }
+
+func WebsocketCloseMsg(t *testing.T) {
+	t.Parallel()
+
+	t.Run("TruncateSingleByteCharacters", func(t *testing.T) {
+		t.Parallel()
+
+		msg := strings.Repeat("d", 255)
+		trunc := httpapi.WebsocketCloseSprintf(msg)
+		assert.LessOrEqual(t, len(trunc), 123)
+	})
+
+	t.Run("TruncateMultiByteCharacters", func(t *testing.T) {
+		t.Parallel()
+
+		msg := strings.Repeat("こんにちは", 10)
+		trunc := httpapi.WebsocketCloseSprintf(msg)
+		assert.LessOrEqual(t, len(trunc), 123)
+	})
+}

coderd/provisionerdaemons.go

@@ -56,7 +56,7 @@ func (api *api) provisionerDaemonsListen(rw http.ResponseWriter, r *http.Request
 		Provisioners: []database.ProvisionerType{database.ProvisionerTypeEcho, database.ProvisionerTypeTerraform},
 	})
 	if err != nil {
-		_ = conn.Close(websocket.StatusInternalError, fmt.Sprintf("insert provisioner daemon:% s", err))
+		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("insert provisioner daemon: %s", err))
 		return
 	}
 
@@ -67,7 +67,7 @@ func (api *api) provisionerDaemonsListen(rw http.ResponseWriter, r *http.Request
 	config.LogOutput = io.Discard
 	session, err := yamux.Server(websocket.NetConn(r.Context(), conn, websocket.MessageBinary), config)
 	if err != nil {
-		_ = conn.Close(websocket.StatusInternalError, fmt.Sprintf("multiplex server: %s", err))
+		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("multiplex server: %s", err))
 		return
 	}
 	mux := drpcmux.New()
@@ -80,13 +80,13 @@ func (api *api) provisionerDaemonsListen(rw http.ResponseWriter, r *http.Request
 		Logger:       api.Logger.Named(fmt.Sprintf("provisionerd-%s", daemon.Name)),
 	})
 	if err != nil {
-		_ = conn.Close(websocket.StatusInternalError, fmt.Sprintf("drpc register provisioner daemon: %s", err))
+		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("drpc register provisioner daemon: %s", err))
 		return
 	}
 	server := drpcserver.New(mux)
 	err = server.Serve(r.Context(), session)
 	if err != nil {
-		_ = conn.Close(websocket.StatusInternalError, fmt.Sprintf("serve: %s", err))
+		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("serve: %s", err))
 		return
 	}
 	_ = conn.Close(websocket.StatusGoingAway, "")

coderd/workspaceresources.go

@@ -108,7 +108,7 @@ func (api *api) workspaceResourceDial(rw http.ResponseWriter, r *http.Request) {
 		Pubsub:    api.Pubsub,
 	})
 	if err != nil {
-		_ = conn.Close(websocket.StatusInternalError, fmt.Sprintf("serve: %s", err))
+		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("serve: %s", err))
 		return
 	}
 }