os/exec: only use cachedLookExtensions if Cmd.Path is unmodified

Caching the invocation of lookExtensions on an absolute path in Command
and reusing the cached result in Start is only viable if Cmd.Path isn't
set to a different value after Command returns.

For #66586.
Fixes #68314.

Change-Id: I57007850aca2011b11344180c00faded737617b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/596875
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 21 insertions, 1 deletions

diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index dbe59fea11..a0bb89e203 100644
--- a/src/os/exec/exec_test.go
+++ b/src/os/exec/exec_test.go
@@ -1838,7 +1838,7 @@ func TestPathRace(t *testing.T) {
 
 func TestAbsPathExec(t *testing.T) {
 	testenv.MustHaveExec(t)
-	testenv.MustHaveGoBuild(t) // must have GOROOT/bin/gofmt, but close enough
+	testenv.MustHaveGoBuild(t) // must have GOROOT/bin/{go,gofmt}
 
 	// A simple exec of a full path should work.
 	// Go 1.22 broke this on Windows, requiring ".exe"; see #66586.
@@ -1863,4 +1863,24 @@ func TestAbsPathExec(t *testing.T) {
 	if err == nil {
 		t.Errorf("using exec.Cmd{Path: %#q}: unexpected success", cmd.Path)
 	}
+
+	// A simple exec after modifying Cmd.Path should work.
+	// This broke on Windows. See go.dev/issue/68314.
+	t.Run("modified", func(t *testing.T) {
+		if exec.Command(filepath.Join(testenv.GOROOT(t), "bin/go")).Run() == nil {
+			// The implementation of the test case below relies on the go binary
+			// exiting with a non-zero exit code when run without any arguments.
+			// In the unlikely case that changes, we need to use another binary.
+			t.Fatal("test case needs updating to verify fix for go.dev/issue/68314")
+		}
+		exe1 := filepath.Join(testenv.GOROOT(t), "bin/go")
+		exe2 := filepath.Join(testenv.GOROOT(t), "bin/gofmt")
+		cmd := exec.Command(exe1)
+		cmd.Path = exe2
+		cmd.Args = []string{cmd.Path}
+		err := cmd.Run()
+		if err != nil {
+			t.Error("ran wrong binary")
+		}
+	})
 }