os/exec: second call to Cmd.Start is always an error

Previously it would return an error only if the first call
resulted in process creation, contra the intent of the
comment at exec.Cmd:

// A Cmd cannot be reused after calling its [Cmd.Start], [Cmd.Run],
// [Cmd.Output], or [Cmd.CombinedOutput] methods.

Also, clear the Cmd.goroutines slice in case of failure to
start a process, so that the closures can be GC'd and their
pipe fds finalized and closed.

Fixes #76746

Change-Id: Ic63a4dced0aa52c2d4be7d44f6dcfc84ee22282c
Reviewed-on: https://go-review.googlesource.com/c/go/+/728642
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/os/exec/exec.go

@@ -102,6 +102,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync/atomic"
 	"syscall"
 	"time"
 )
@@ -354,6 +355,9 @@ type Cmd struct {
 	// the work of resolving the extension, so Start doesn't need to do it again.
 	// This is only used on Windows.
 	cachedLookExtensions struct{ in, out string }
+
+	// startCalled records that Start was attempted, regardless of outcome.
+	startCalled atomic.Bool
 }
 
 // A ctxResult reports the result of watching the Context associated with a
@@ -635,7 +639,8 @@ func (c *Cmd) Run() error {
 func (c *Cmd) Start() error {
 	// Check for doubled Start calls before we defer failure cleanup. If the prior
 	// call to Start succeeded, we don't want to spuriously close its pipes.
-	if c.Process != nil {
+	// It is an error to call Start twice even if the first call did not create a process.
+	if c.startCalled.Swap(true) {
 		return errors.New("exec: already started")
 	}
 
@@ -647,6 +652,7 @@ func (c *Cmd) Start() error {
 		if !started {
 			closeDescriptors(c.parentIOPipes)
 			c.parentIOPipes = nil
+			c.goroutine = nil // aid GC, finalization of pipe fds
 		}
 	}()
 

src/os/exec/exec_test.go

@@ -1839,3 +1839,29 @@ func TestAbsPathExec(t *testing.T) {
 		}
 	})
 }
+
+// Calling Start twice is an error, regardless of outcome.
+func TestStart_twice(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	cmd := exec.Command("/bin/nonesuch")
+	for i, want := range []string{
+		cond(runtime.GOOS == "windows",
+			`exec: "/bin/nonesuch": executable file not found in %PATH%`,
+			"fork/exec /bin/nonesuch: no such file or directory"),
+		"exec: already started",
+	} {
+		err := cmd.Start()
+		if got := fmt.Sprint(err); got != want {
+			t.Errorf("Start call #%d return err %q, want %q", i+1, got, want)
+		}
+	}
+}
+
+func cond[T any](cond bool, t, f T) T {
+	if cond {
+		return t
+	} else {
+		return f
+	}
+}