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Fix two data races in shell tool #2127

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Merged
dgageot merged 1 commit into from
Mar 16, 2026
+34 −28
Merged

Fix two data races in shell tool #2127

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62 changes: 34 additions & 28 deletions pkg/tools/builtin/shell.go
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Original file line number Diff line number Diff line change
Expand Up @@ -74,9 +74,11 @@ type backgroundJob struct {
err error
}

// limitedWriter wraps a buffer and stops writing after maxSize bytes
// limitedWriter wraps a buffer and stops writing after maxSize bytes.
// It uses an external mutex (mu) so that readers of the underlying buffer
// can share the same lock.
type limitedWriter struct {
mu sync.Mutex
mu *sync.RWMutex
buf *bytes.Buffer
written int64
maxSize int64
Expand All @@ -86,20 +88,12 @@ func (lw *limitedWriter) Write(p []byte) (n int, err error) {
lw.mu.Lock()
defer lw.mu.Unlock()

if lw.written >= lw.maxSize {
return len(p), nil // Discard but report success
if remaining := lw.maxSize - lw.written; remaining > 0 {
toWrite := min(int64(len(p)), remaining)
lw.buf.Write(p[:toWrite]) // bytes.Buffer.Write never errors
lw.written += toWrite
}

remaining := lw.maxSize - lw.written
toWrite := min(int64(len(p)), remaining)

n, err = lw.buf.Write(p[:toWrite])
lw.written += int64(n)

if err == nil && int64(n) < int64(len(p)) {
return len(p), nil // Report full write even if truncated
}
return n, err
return len(p), nil // always report full write
}

type RunShellArgs struct {
Expand Down Expand Up @@ -184,6 +178,15 @@ func (h *shellHandler) runNativeCommand(timeoutCtx, ctx context.Context, command
select {
case <-timeoutCtx.Done():
_ = kill(cmd.Process, pg)
// Wait for cmd.Wait() to complete so that the internal pipe-copy
// goroutines finish writing to outBuf before we read it.
// Use a grace period: if SIGTERM is ignored, escalate to SIGKILL.
select {
case <-done:
case <-time.After(3 * time.Second):
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⚠️ Minor: Theoretical goroutine leak scenario

After sending SIGTERM and waiting 3 seconds, the code escalates to cmd.Process.Kill() (SIGKILL) and then unconditionally waits on <-done.

In the extremely unlikely scenario where SIGKILL fails or is ignored (essentially impossible on Unix/Linux as SIGKILL cannot be caught or ignored), the goroutine would block indefinitely waiting for cmd.Wait() to complete.

Impact: Theoretical only - SIGKILL is uncatchable by design on Unix-like systems, making this scenario virtually impossible in practice.

Recommendation: Consider adding a final timeout after Kill() for defense-in-depth, though this is not a practical concern.

_ = cmd.Process.Kill()
<-done
}
case cmdErr = <-done:
}

Expand All @@ -200,10 +203,20 @@ func (h *shellHandler) RunShellBackground(_ context.Context, params RunShellBack
cmd.Dir = h.resolveWorkDir(params.Cwd)
cmd.SysProcAttr = platformSpecificSysProcAttr()

outputBuf := &bytes.Buffer{}
limitedWriter := &limitedWriter{buf: outputBuf, maxSize: 10 * 1024 * 1024}
cmd.Stdout = limitedWriter
cmd.Stderr = limitedWriter
job := &backgroundJob{
id: jobID,
cmd: params.Cmd,
cwd: params.Cwd,
output: &bytes.Buffer{},
startTime: time.Now(),
}

// The limitedWriter shares the job's outputMu so that readers
// (ViewBackgroundJob, ListBackgroundJobs) and the pipe-copy
// goroutines spawned by exec.Cmd use the same lock.
lw := &limitedWriter{mu: &job.outputMu, buf: job.output, maxSize: 10 * 1024 * 1024}
cmd.Stdout = lw
cmd.Stderr = lw

if err := cmd.Start(); err != nil {
return tools.ResultError(fmt.Sprintf("Error starting background command: %s", err)), nil
Expand All @@ -215,15 +228,8 @@ func (h *shellHandler) RunShellBackground(_ context.Context, params RunShellBack
return tools.ResultError(fmt.Sprintf("Error creating process group: %s", err)), nil
}

job := &backgroundJob{
id: jobID,
cmd: params.Cmd,
cwd: params.Cwd,
process: cmd.Process,
processGroup: pg,
output: outputBuf,
startTime: time.Now(),
}
job.process = cmd.Process
job.processGroup = pg
job.status.Store(statusRunning)
h.jobs.Store(jobID, job)

Expand Down
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