os/exec: add Cmd.String

The initial implementation is intentionally simple.
Let's see how far it gets us.

Fixes #30638

Change-Id: I240afae2b401744ab2ff2a69952c4eb0fd3feb56
Reviewed-on: https://go-review.googlesource.com/c/go/+/168518
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec.go

@@ -190,6 +190,25 @@ func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
 	return cmd
 }
 
+// String returns a human-readable description of c.
+// It is intended only for debugging.
+// In particular, it is not suitable for use as input to a shell.
+// The output of String may vary across Go releases.
+func (c *Cmd) String() string {
+	if c.lookPathErr != nil {
+		// failed to resolve path; report the original requested path (plus args)
+		return strings.Join(c.Args, " ")
+	}
+	// report the exact executable path (plus args)
+	b := new(strings.Builder)
+	b.WriteString(c.Path)
+	for _, a := range c.Args[1:] {
+		b.WriteByte(' ')
+		b.WriteString(a)
+	}
+	return b.String()
+}
+
 // interfaceEqual protects against panics from doing equality tests on
 // two interfaces with non-comparable underlying types.
 func interfaceEqual(a, b interface{}) bool {

src/os/exec/exec_test.go

@@ -1150,3 +1150,37 @@ func TestDedupEnvEcho(t *testing.T) {
 		t.Errorf("output = %q; want %q", got, want)
 	}
 }
+
+func TestString(t *testing.T) {
+	echoPath, err := exec.LookPath("echo")
+	if err != nil {
+		t.Skip(err)
+	}
+	tests := [...]struct {
+		path string
+		args []string
+		want string
+	}{
+		{"echo", nil, echoPath},
+		{"echo", []string{"a"}, echoPath + " a"},
+		{"echo", []string{"a", "b"}, echoPath + " a b"},
+	}
+	for _, test := range tests {
+		cmd := exec.Command(test.path, test.args...)
+		if got := cmd.String(); got != test.want {
+			t.Errorf("String(%q, %q) = %q, want %q", test.path, test.args, got, test.want)
+		}
+	}
+}
+
+func TestStringPathNotResolved(t *testing.T) {
+	_, err := exec.LookPath("makemeasandwich")
+	if err == nil {
+		t.Skip("wow, thanks")
+	}
+	cmd := exec.Command("makemeasandwich", "-lettuce")
+	want := "makemeasandwich -lettuce"
+	if got := cmd.String(); got != want {
+		t.Errorf("String(%q, %q) = %q, want %q", "makemeasandwich", "-lettuce", got, want)
+	}
+}