os: don't treat RemoveAll("/x") as RemoveAll("x")

Fixes #31468

Change-Id: I5c4e61631b8af35bfc14b0cb9bc77feec100e340
Reviewed-on: https://go-review.googlesource.com/c/go/+/172058
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/export_unix_test.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
+
+package os
+
+var SplitPath = splitPath

src/os/os_unix_test.go

@@ -288,3 +288,28 @@ func TestNewFileNonBlock(t *testing.T) {
 	t.Parallel()
 	newFileTest(t, false)
 }
+
+func TestSplitPath(t *testing.T) {
+	t.Parallel()
+	for _, tt := range []struct{ path, wantDir, wantBase string }{
+		{"a", ".", "a"},
+		{"a/", ".", "a"},
+		{"a//", ".", "a"},
+		{"a/b", "a", "b"},
+		{"a/b/", "a", "b"},
+		{"a/b/c", "a/b", "c"},
+		{"/a", "/", "a"},
+		{"/a/", "/", "a"},
+		{"/a/b", "/a", "b"},
+		{"/a/b/", "/a", "b"},
+		{"/a/b/c", "/a/b", "c"},
+		{"//a", "/", "a"},
+		{"//a/", "/", "a"},
+		{"///a", "/", "a"},
+		{"///a/", "/", "a"},
+	} {
+		if dir, base := SplitPath(tt.path); dir != tt.wantDir || base != tt.wantBase {
+			t.Errorf("splitPath(%q) = %q, %q, want %q, %q", tt.path, dir, base, tt.wantDir, tt.wantBase)
+		}
+	}
+}

src/os/path_unix.go

@@ -38,20 +38,30 @@ func basename(name string) string {
 func splitPath(path string) (string, string) {
 	// if no better parent is found, the path is relative from "here"
 	dirname := "."
-	// if no slashes in path, base is path
-	basename := path
+
+	// Remove all but one leading slash.
+	for len(path) > 1 && path[0] == '/' && path[1] == '/' {
+		path = path[1:]
+	}
 
 	i := len(path) - 1
 
-	// Remove trailing slashes
+	// Remove trailing slashes.
 	for ; i > 0 && path[i] == '/'; i-- {
 		path = path[:i]
 	}
 
+	// if no slashes in path, base is path
+	basename := path
+
 	// Remove leading directory path
 	for i--; i >= 0; i-- {
 		if path[i] == '/' {
+			if i == 0 {
+				dirname = path[:1]
+			} else {
 				dirname = path[:i]
+			}
 			basename = path[i+1:]
 			break
 		}