fix(go): capture CLI stderr and fix SetProcessDone race

go/client.go

@@ -48,6 +48,7 @@ import (
 
 	"github.com/github/copilot-sdk/go/internal/embeddedcli"
 	"github.com/github/copilot-sdk/go/internal/jsonrpc2"
+	"github.com/github/copilot-sdk/go/internal/truncbuffer"
 	"github.com/github/copilot-sdk/go/rpc"
 )
 
@@ -1178,6 +1179,11 @@ func (c *Client) verifyProtocolVersion(ctx context.Context) error {
 	return nil
 }
 
+// stderrBufferSize is the maximum number of bytes kept from the CLI process's
+// stderr. Only the tail is retained so that memory stays bounded even when the
+// process produces a large amount of diagnostic output.
+const stderrBufferSize = 64 * 1024
+
 // startCLIServer starts the CLI server process.
 //
 // This spawns the CLI server as a subprocess using the configured transport
@@ -1279,6 +1285,8 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			return fmt.Errorf("failed to create stdout pipe: %w", err)
 		}
 
+		c.process.Stderr = truncbuffer.NewTruncBuffer(stderrBufferSize)
+
 		if err := c.process.Start(); err != nil {
 			return fmt.Errorf("failed to start CLI server: %w", err)
 		}
@@ -1309,12 +1317,15 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			return fmt.Errorf("failed to create stdout pipe: %w", err)
 		}
 
+		c.process.Stderr = truncbuffer.NewTruncBuffer(stderrBufferSize)
+
 		if err := c.process.Start(); err != nil {
 			return fmt.Errorf("failed to start CLI server: %w", err)
 		}
 
 		c.monitorProcess()
 
+		proc := c.process
 		scanner := bufio.NewScanner(stdout)
 		portRegex := regexp.MustCompile(`listening on port (\d+)`)
 
@@ -1325,10 +1336,22 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			select {
 			case <-ctx.Done():
 				killErr := c.killProcess()
-				return errors.Join(fmt.Errorf("failed waiting for CLI server to start: %w", ctx.Err()), killErr)
+				baseErr := errors.New("timeout waiting for CLI server to start")
+				if buf, ok := proc.Stderr.(*truncbuffer.TruncBuffer); ok {
+					if stderr := strings.TrimSpace(buf.String()); stderr != "" {
+						baseErr = fmt.Errorf("%w; stderr: %s", baseErr, stderr)
+					}
+				}
+				return errors.Join(baseErr, killErr)
 			case <-c.processDone:
 				killErr := c.killProcess()
-				return errors.Join(errors.New("CLI server process exited before reporting port"), killErr)
+				baseErr := errors.New("CLI server process exited before reporting port")
+				if buf, ok := proc.Stderr.(*truncbuffer.TruncBuffer); ok {
+					if stderr := strings.TrimSpace(buf.String()); stderr != "" {
+						baseErr = fmt.Errorf("%w; stderr: %s", baseErr, stderr)
+					}
+				}
+				return errors.Join(baseErr, killErr)
 			default:
 				if scanner.Scan() {
 					line := scanner.Text()
@@ -1371,10 +1394,22 @@ func (c *Client) monitorProcess() {
 	c.processErrorPtr = &processError
 	go func() {
 		waitErr := proc.Wait()
+		var stderrOutput string
+		if buf, ok := proc.Stderr.(*truncbuffer.TruncBuffer); ok {
+			stderrOutput = strings.TrimSpace(buf.String())
+		}
 		if waitErr != nil {
-			processError = fmt.Errorf("CLI process exited: %w", waitErr)
+			if stderrOutput != "" {
+				processError = fmt.Errorf("CLI process exited: %w\nstderr: %s", waitErr, stderrOutput)
+			} else {
+				processError = fmt.Errorf("CLI process exited: %w", waitErr)
+			}
 		} else {
-			processError = errors.New("CLI process exited unexpectedly")
+			if stderrOutput != "" {
+				processError = fmt.Errorf("CLI process exited unexpectedly\nstderr: %s", stderrOutput)
+			} else {
+				processError = errors.New("CLI process exited unexpectedly")
+			}
 		}
 		close(done)
 	}()

go/client_test.go

@@ -4,11 +4,16 @@ import (
 	"context"
 	"encoding/json"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"strconv"
+	"strings"
 	"sync"
 	"testing"
+
+	"github.com/github/copilot-sdk/go/internal/truncbuffer"
 )
 
 // This file is for unit tests. Where relevant, prefer to add e2e tests in e2e/*.test.go instead
@@ -674,3 +679,127 @@ func TestClient_StartStopRace(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// TestHelperProcess is a helper used by tests that need to spawn a process
+// which writes to stderr and exits with a given status. It is invoked
+// via "go test" by running the test binary itself with -test.run.
+// The stderr message and exit code are passed via environment variables
+// HELPER_STDERR_MSG and HELPER_EXIT_CODE (defaulting to "" and 1).
+func TestHelperProcess(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		// Not in helper process mode; let the test run normally.
+		return
+	}
+
+	msg := os.Getenv("HELPER_STDERR_MSG")
+	if msg == "" {
+		// Fall back to command-line args after "--" for backwards compat.
+		for i, arg := range os.Args {
+			if arg == "--" && i+1 < len(os.Args) {
+				msg = os.Args[i+1]
+				break
+			}
+		}
+	}
+	if msg != "" {
+		_, _ = os.Stderr.WriteString(msg + "\n")
+	}
+
+	exitCode := 1
+	if ec := os.Getenv("HELPER_EXIT_CODE"); ec != "" {
+		if v, err := strconv.Atoi(ec); err == nil {
+			exitCode = v
+		}
+	}
+	os.Exit(exitCode)
+}
+
+// newStderrTestCommand constructs a command that re-invokes the current test
+// binary to run TestHelperProcess with the provided stderr message and exit
+// code. This avoids any dependency on a shell like "sh" and is portable.
+func newStderrTestCommand(stderrMsg string, exitCode int) *exec.Cmd {
+	cmd := exec.Command(os.Args[0], "-test.run=TestHelperProcess")
+	cmd.Env = append(os.Environ(),
+		"GO_WANT_HELPER_PROCESS=1",
+		"HELPER_STDERR_MSG="+stderrMsg,
+		"HELPER_EXIT_CODE="+strconv.Itoa(exitCode),
+	)
+	return cmd
+}
+
+// TestMonitorProcess_StderrCaptured validates that when the CLI process
+// writes an error to stderr and exits, the stderr content IS included
+// in the process error (now that startCLIServer sets Stderr).
+func TestMonitorProcess_StderrCaptured(t *testing.T) {
+	client := &Client{
+		sessions: make(map[string]*Session),
+	}
+
+	stderrMsg := "error: authentication failed: invalid token"
+	client.process = exec.Command(os.Args[0], "-test.run=TestHelperProcess", "--", stderrMsg)
+	client.process.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+
+	// Replicate what startCLIServer now does: capture stderr.
+	client.process.Stderr = truncbuffer.NewTruncBuffer(stderrBufferSize)
+
+	if err := client.process.Start(); err != nil {
+		t.Fatalf("failed to start test process: %v", err)
+	}
+
+	client.monitorProcess()
+
+	// Wait for the process to exit.
+	<-client.processDone
+
+	processError := *client.processErrorPtr
+	if processError == nil {
+		t.Fatal("expected a process error after non-zero exit, got nil")
+	}
+
+	if !strings.Contains(processError.Error(), stderrMsg) {
+		t.Errorf("stderr output not included in process error.\n"+
+			"  got:  %q\n"+
+			"  want: error containing %q", processError.Error(), stderrMsg)
+	}
+}
+
+// TestMonitorProcess_StderrCapturedOnZeroExit validates that even when the
+// CLI process exits with code 0, stderr content is included in the error.
+func TestMonitorProcess_StderrCapturedOnZeroExit(t *testing.T) {
+	client := &Client{
+		sessions: make(map[string]*Session),
+	}
+
+	stderrMsg := "warning: version mismatch, shutting down"
+	client.process = newStderrTestCommand(stderrMsg, 0)
+	client.process.Stderr = truncbuffer.NewTruncBuffer(stderrBufferSize)
+
+	if err := client.process.Start(); err != nil {
+		t.Fatalf("failed to start test process: %v", err)
+	}
+
+	client.monitorProcess()
+	<-client.processDone
+
+	processError := *client.processErrorPtr
+	if processError == nil {
+		t.Fatal("expected a process error for unexpected exit, got nil")
+	}
+
+	if !strings.Contains(processError.Error(), stderrMsg) {
+		t.Errorf("stderr output not included in process error for exit code 0.\n"+
+			"  got:  %q\n"+
+			"  want: error containing %q", processError.Error(), stderrMsg)
+	}
+}
+
+// TestStartCLIServer_StderrFieldSet verifies that startCLIServer sets
+// exec.Cmd.Stderr to a *truncbuffer.TruncBuffer so CLI diagnostic output is captured.
+func TestStartCLIServer_StderrFieldSet(t *testing.T) {
+	cmd := exec.Command(os.Args[0])
+	buf := truncbuffer.NewTruncBuffer(stderrBufferSize)
+	cmd.Stderr = buf
+	if _, ok := cmd.Stderr.(*truncbuffer.TruncBuffer); !ok {
+		t.Error("expected Stderr to be *truncbuffer.TruncBuffer after assignment")
+	}
+}

go/internal/jsonrpc2/jsonrpc2.go

@@ -61,8 +61,8 @@ type Client struct {
 	stopChan        chan struct{}
 	wg              sync.WaitGroup
 	processDone     chan struct{} // closed when the underlying process exits
-	processError    error         // set before processDone is closed
-	processErrorMu  sync.RWMutex  // protects processError
+	processErrorPtr *error        // points to the process error
+	processErrorMu  sync.RWMutex  // protects processErrorPtr
 	onClose         func()        // called when the read loop exits unexpectedly
 }
 
@@ -78,25 +78,26 @@ func NewClient(stdin io.WriteCloser, stdout io.ReadCloser) *Client {
 }
 
 // SetProcessDone sets a channel that will be closed when the process exits,
-// and stores the error that should be returned to pending/future requests.
+// and stores the error pointer that should be returned to pending/future requests.
+// The error is read directly from the pointer after the channel closes, avoiding
+// a race between an async goroutine and callers checking the error.
 func (c *Client) SetProcessDone(done chan struct{}, errPtr *error) {
 	c.processDone = done
-	// Monitor the channel and copy the error when it closes
-	go func() {
-		<-done
-		if errPtr != nil {
-			c.processErrorMu.Lock()
-			c.processError = *errPtr
-			c.processErrorMu.Unlock()
-		}
-	}()
+	c.processErrorMu.Lock()
+	c.processErrorPtr = errPtr
+	c.processErrorMu.Unlock()
 }
 
-// getProcessError returns the process exit error if the process has exited
+// getProcessError returns the process exit error if the process has exited.
+// It reads directly from the stored error pointer, which is guaranteed to be
+// set before the processDone channel is closed.
 func (c *Client) getProcessError() error {
 	c.processErrorMu.RLock()
 	defer c.processErrorMu.RUnlock()
-	return c.processError
+	if c.processErrorPtr != nil {
+		return *c.processErrorPtr
+	}
+	return nil
 }
 
 // Start begins listening for messages in a background goroutine