[FIXED] Call ReconnectErrHandler for initial connection failures with RetryOnFailedConnect (#1915)

When RetryOnFailedConnect is enabled and initial connection fails,
ReconnectErrHandler is now called to report the error, allowing users
to debug initial connection issues.

Resolves issue mentioned in https://github.com/nats-io/nats.go/issues/1870#issuecomment-2853851326

Signed-off-by: Piotr Piotrowski <piotr@synadia.com>

nats.go

@@ -1660,14 +1660,17 @@ func (o Options) Connect() (*Conn, error) {
 	// Create reader/writer
 	nc.newReaderWriter()
 
+	// Spin up the async cb dispatcher before connect so it's ready
+	// to handle callbacks, especially when RetryOnFailedConnect is used
+	// and initial connection fails.
+	go nc.ach.asyncCBDispatcher()
+
 	connectionEstablished, err := nc.connect()
 	if err != nil {
+		nc.ach.close()
 		return nil, err
 	}
 
-	// Spin up the async cb dispatcher on success
-	go nc.ach.asyncCBDispatcher()
-
 	if connectionEstablished && nc.Opts.ConnectedCB != nil {
 		nc.ach.push(func() { nc.Opts.ConnectedCB(nc) })
 	}
@@ -2554,7 +2557,7 @@ func (nc *Conn) connect() (bool, error) {
 		nc.setup()
 		nc.changeConnStatus(RECONNECTING)
 		nc.bw.switchToPending()
-		go nc.doReconnect(ErrNoServers, false)
+		go nc.doReconnect(err, false)
 		err = nil
 	} else {
 		nc.current = nil
@@ -2877,6 +2880,7 @@ func (nc *Conn) doReconnect(err error, forceReconnect bool) {
 
 	// Clear any errors.
 	nc.err = nil
+
 	// Perform appropriate callback if needed for a disconnect.
 	// DisconnectedErrCB has priority over deprecated DisconnectedCB
 	if !nc.initc {
@@ -2885,6 +2889,12 @@ func (nc *Conn) doReconnect(err error, forceReconnect bool) {
 		} else if nc.Opts.DisconnectedCB != nil {
 			nc.ach.push(func() { nc.Opts.DisconnectedCB(nc) })
 		}
+	} else if nc.Opts.RetryOnFailedConnect && nc.initc && err != nil {
+		// For initial connection failure with RetryOnFailedConnect,
+		// report the error via ReconnectErrCB if available
+		if nc.Opts.ReconnectErrCB != nil {
+			nc.ach.push(func() { nc.Opts.ReconnectErrCB(nc, err) })
+		}
 	}
 
 	// This is used to wait on go routines exit if we start them in the loop

test/conn_test.go

@@ -2920,6 +2920,77 @@ func TestRetryOnFailedConnect(t *testing.T) {
 	}
 }
 
+func TestRetryOnFailedConnectReconnectErrCB(t *testing.T) {
+	errChan := make(chan error, 10)
+
+	nc, err := nats.Connect(nats.DefaultURL,
+		nats.RetryOnFailedConnect(true),
+		nats.MaxReconnects(0), // Limited retries for faster test
+		nats.ReconnectWait(10*time.Millisecond),
+		nats.ReconnectErrHandler(func(_ *nats.Conn, err error) {
+			errChan <- err
+		}),
+	)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer nc.Close()
+
+	// Verify the first error is the initial connection error
+	select {
+	case err := <-errChan:
+		if !errors.Is(err, nats.ErrNoServers) {
+			t.Fatalf("Expected ErrNoServers for initial connection failure, got: %v", err)
+		}
+	case <-time.After(200 * time.Millisecond):
+		t.Fatal("Should have received initial connection error in ReconnectErrCB")
+	}
+}
+
+func TestRetryOnFailedConnectWithAuthError(t *testing.T) {
+	o := test.DefaultTestOptions
+	o.Username = "user"
+	o.Password = "password"
+	s := RunServerWithOptions(&o)
+	defer s.Shutdown()
+
+	errChan := make(chan error, 10)
+	closedCh := make(chan bool, 1)
+
+	// Try to connect without credentials
+	nc, err := nats.Connect(nats.DefaultURL,
+		nats.RetryOnFailedConnect(true),
+		nats.MaxReconnects(2),
+		nats.ReconnectWait(10*time.Millisecond),
+		nats.ReconnectErrHandler(func(_ *nats.Conn, err error) {
+			errChan <- err
+		}),
+		nats.ClosedHandler(func(_ *nats.Conn) {
+			closedCh <- true
+		}),
+	)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer nc.Close()
+
+	// Wait for closed due to auth failure
+	select {
+	case <-closedCh:
+	case <-time.After(500 * time.Millisecond):
+		t.Fatal("Connection should have closed due to auth failure")
+	}
+
+	select {
+	case err := <-errChan:
+		if !errors.Is(err, nats.ErrAuthorization) {
+			t.Fatalf("Expected ErrAuthorization for auth failure, got: %v", err)
+		}
+	case <-time.After(500 * time.Millisecond):
+		t.Fatal("Should have received authorization error in ReconnectErrCB")
+	}
+}
+
 func TestRetryOnFailedConnectWithTLSError(t *testing.T) {
 	opts := test.DefaultTestOptions
 	opts.Port = 4222