os: depend on Readlink only when necessary

Currently Readlink gets linked into the binary even when Executable is
not needed.

This reduces a simple "os.Stdout.Write([]byte("hello"))" by ~10KiB.

Previously the executable path was read during init time, because
deleting the executable would make "Readlink" return "(deleted)" suffix.
There's probably a slight chance that the init time reading would return
it anyways.

Updates #6853

Change-Id: Ic76190c5b64d9320ceb489cd6a553108614653d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/311790
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

src/os/executable_test.go

@@ -86,3 +86,68 @@ func init() {
 		os.Exit(0)
 	}
 }
+
+func TestExecutableDeleted(t *testing.T) {
+	testenv.MustHaveExec(t)
+	switch runtime.GOOS {
+	case "windows":
+		t.Skip("windows does not support deleting running binary")
+	case "openbsd", "freebsd":
+		t.Skipf("%v does not support reading deleted binary name", runtime.GOOS)
+	}
+
+	dir := t.TempDir()
+
+	src := filepath.Join(dir, "testdel.go")
+	exe := filepath.Join(dir, "testdel.exe")
+
+	err := os.WriteFile(src, []byte(testExecutableDeletion), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err := osexec.Command(testenv.GoToolPath(t), "build", "-o", exe, src).CombinedOutput()
+	t.Logf("build output:\n%s", out)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err = osexec.Command(exe).CombinedOutput()
+	t.Logf("exec output:\n%s", out)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+const testExecutableDeletion = `package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	before, err := os.Executable()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to read executable name before deletion: %v\n", err)
+		os.Exit(1)
+	}
+
+	err = os.Remove(before)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to remove executable: %v\n", err)
+		os.Exit(1)
+	}
+
+	after, err := os.Executable()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to read executable name after deletion: %v\n", err)
+		os.Exit(1)
+	}
+
+	if before != after {
+		fmt.Fprintf(os.Stderr, "before and after do not match: %v != %v\n", before, after)
+		os.Exit(1)
+	}
+}
+`