internal/testenv: probe for symlink on wasip1

Certain WASI runtimes do not support generic symlinks, and
instead return permission errors when they are attempted.
Perform a runtime probe of symlink support in hasSymlink
on wasip1 to determine whether the runtime supports
generic symlinks.

Also perform the same probe on android.

For #59583

Change-Id: Iae5b704e670650d38ee350a5a98f99dcce8b5b28
Reviewed-on: https://go-review.googlesource.com/c/go/+/490115
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
TryBot-Bypass: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/internal/testenv/testenv.go

@@ -387,7 +387,7 @@ func HasSymlink() bool {
 func MustHaveSymlink(t testing.TB) {
 	ok, reason := hasSymlink()
 	if !ok {
-		t.Skipf("skipping test: cannot make symlinks on %s/%s%s", runtime.GOOS, runtime.GOARCH, reason)
+		t.Skipf("skipping test: cannot make symlinks on %s/%s: %s", runtime.GOOS, runtime.GOARCH, reason)
 	}
 }
 

src/internal/testenv/testenv_notunix.go

@@ -8,6 +8,7 @@ package testenv
 
 import (
 	"errors"
+	"io/fs"
 	"os"
 )
 
@@ -16,5 +17,5 @@ import (
 var Sigquit = os.Kill
 
 func syscallIsNotSupported(err error) bool {
-	return errors.Is(err, errors.ErrUnsupported)
+	return errors.Is(err, fs.ErrPermission) || errors.Is(err, errors.ErrUnsupported)
 }

src/internal/testenv/testenv_notwin.go

@@ -7,13 +7,39 @@
 package testenv
 
 import (
+	"fmt"
+	"os"
+	"path/filepath"
 	"runtime"
 )
 
 func hasSymlink() (ok bool, reason string) {
 	switch runtime.GOOS {
-	case "android", "plan9":
+	case "plan9":
 		return false, ""
+	case "android", "wasip1":
+		// For wasip1, some runtimes forbid absolute symlinks,
+		// or symlinks that escape the current working directory.
+		// Perform a simple test to see whether the runtime
+		// supports symlinks or not. If we get a permission
+		// error, the runtime does not support symlinks.
+		dir, err := os.MkdirTemp("", "")
+		if err != nil {
+			return false, ""
+		}
+		defer func() {
+			_ = os.RemoveAll(dir)
+		}()
+		fpath := filepath.Join(dir, "testfile.txt")
+		if err := os.WriteFile(fpath, nil, 0644); err != nil {
+			return false, ""
+		}
+		if err := os.Symlink(fpath, filepath.Join(dir, "testlink")); err != nil {
+			if SyscallIsNotSupported(err) {
+				return false, fmt.Sprintf("symlinks unsupported: %s", err.Error())
+			}
+			return false, ""
+		}
 	}
 
 	return true, ""