os: make RemoveAll("") fail silently on unix

CL 146020 changed the behavior of RemoveAll("") on unix systems using
the *at functions to return syscall.EINVAL instead of nil. Adjust the
*at implementation to retain this behavior as is the case on the *noat
systems.

Additionally, also make sure RemoveAll("") on systems not using the "at
functions (e.g. nacl and js/wasm) follow the same behavior (which wasn't
the case previously).

Fixes #28830

Change-Id: I8383c1423fefe871d18ff49134a1d23077ec6867
Reviewed-on: https://go-review.googlesource.com/c/150158
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>

src/os/removeall_at.go

@@ -13,8 +13,14 @@ import (
 )
 
 func RemoveAll(path string) error {
+	if path == "" {
+		// fail silently to retain compatibility with previous behavior
+		// of RemoveAll. See issue 28830.
+		return nil
+	}
+
 	// Not allowed in unix
-	if path == "" || endsWithDot(path) {
+	if endsWithDot(path) {
 		return syscall.EINVAL
 	}
 

src/os/removeall_noat.go

@@ -16,6 +16,12 @@ import (
 // it encounters. If the path does not exist, RemoveAll
 // returns nil (no error).
 func RemoveAll(path string) error {
+	if path == "" {
+		// fail silently to retain compatibility with previous behavior
+		// of RemoveAll. See issue 28830.
+		return nil
+	}
+
 	// Simple case: if Remove works, we're done.
 	err := Remove(path)
 	if err == nil || IsNotExist(err) {

src/os/removeall_test.go

@@ -21,6 +21,10 @@ func TestRemoveAll(t *testing.T) {
 	}
 	defer RemoveAll(tmpDir)
 
+	if err := RemoveAll(""); err != nil {
+		t.Errorf("RemoveAll(\"\"): %v; want nil", err)
+	}
+
 	file := filepath.Join(tmpDir, "file")
 	path := filepath.Join(tmpDir, "_TestRemoveAll_")
 	fpath := filepath.Join(path, "file")