2 files changed, 33 insertions, 1 deletions
diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index e84ebfc453..e0c509cbef 100644
--- a/src/os/exec/exec.go
+++ b/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
 		}
 	}()
 
diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index 1decebdc22..bf2f3da535 100644
--- a/src/os/exec/exec_test.go
+++ b/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
+	}
+}