fix(agent/reaper): stop reaper goroutine in tests to prevent ECHILD race (#22844)

Each ForkReap call started a reap.ReapChildren goroutine that never
stopped (done=nil). Goroutines accumulated across subtests, racing to 
call Wait4(-1, WNOHANG) and stealing the child's wait status before 
ForkReap's Wait4(pid) could collect it.

Add a WithDone option to pass the done channel through to ReapChildren,
and use it in tests via a withDone(t) helper.

agent/reaper/reaper.go

@@ -42,9 +42,20 @@ func WithLogger(logger slog.Logger) Option {
 	}
 }
 
+// WithDone sets a channel that, when closed, stops the reaper
+// goroutine. Callers that invoke ForkReap more than once in the
+// same process (e.g. tests) should use this to prevent goroutine
+// accumulation.
+func WithDone(ch chan struct{}) Option {
+	return func(o *options) {
+		o.Done = ch
+	}
+}
+
 type options struct {
 	ExecArgs     []string
 	PIDs         reap.PidCh
 	CatchSignals []os.Signal
 	Logger       slog.Logger
+	Done         chan struct{}
 }

agent/reaper/reaper_test.go

@@ -18,6 +18,15 @@ import (
 	"github.com/coder/coder/v2/testutil"
 )
 
+// withDone returns an option that stops the reaper goroutine when t
+// completes, preventing goroutine accumulation across subtests.
+func withDone(t *testing.T) reaper.Option {
+	t.Helper()
+	done := make(chan struct{})
+	t.Cleanup(func() { close(done) })
+	return reaper.WithDone(done)
+}
+
 // TestReap checks that's the reaper is successfully reaping
 // exited processes and passing the PIDs through the shared
 // channel.
@@ -36,6 +45,7 @@ func TestReap(t *testing.T) {
 		reaper.WithPIDCallback(pids),
 		// Provide some argument that immediately exits.
 		reaper.WithExecArgs("/bin/sh", "-c", "exit 0"),
+		withDone(t),
 	)
 	require.NoError(t, err)
 	require.Equal(t, 0, exitCode)
@@ -89,6 +99,7 @@ func TestForkReapExitCodes(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			exitCode, err := reaper.ForkReap(
 				reaper.WithExecArgs("/bin/sh", "-c", tt.command),
+				withDone(t),
 			)
 			require.NoError(t, err)
 			require.Equal(t, tt.expectedCode, exitCode, "exit code mismatch for %q", tt.command)
@@ -118,6 +129,7 @@ func TestReapInterrupt(t *testing.T) {
 		exitCode, err := reaper.ForkReap(
 			reaper.WithPIDCallback(pids),
 			reaper.WithCatchSignals(os.Interrupt),
+			withDone(t),
 			// Signal propagation does not extend to children of children, so
 			// we create a little bash script to ensure sleep is interrupted.
 			reaper.WithExecArgs("/bin/sh", "-c", fmt.Sprintf("pid=0; trap 'kill -USR2 %d; kill -TERM $pid' INT; sleep 10 &\npid=$!; kill -USR1 %d; wait", os.Getpid(), os.Getpid())),

agent/reaper/reaper_unix.go

@@ -64,7 +64,7 @@ func ForkReap(opt ...Option) (int, error) {
 		o(opts)
 	}
 
-	go reap.ReapChildren(opts.PIDs, nil, nil, nil)
+	go reap.ReapChildren(opts.PIDs, nil, opts.Done, nil)
 
 	pwd, err := os.Getwd()
 	if err != nil {