path/filepath: make Join handle UNC paths on Windows

Unless the first element is a Universal Naming Convention (UNC)[0]
path, Join shouldn't create a UNC path on Windows.

For example, Join inadvertently creates a UNC path on Windows when
told to join at least three non-empty path elements, where the first
element is `\` or `/`.

This CL prevents creation of a UNC path prefix when the first path
element isn't a UNC path.

Since this introduces some amount of Windows-specific logic, Join is
moved to a per GOOS implementation.

Fixes #9167.

[0]: http://msdn.microsoft.com/en-us/library/gg465305.aspx

Change-Id: Ib6eda597106cb025137673b33c4828df1367f75b
Reviewed-on: https://go-review.googlesource.com/2211
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

1 files changed, 20 insertions, 8 deletions

diff --git a/src/path/filepath/path_test.go b/src/path/filepath/path_test.go
index 399284b97d..c4f74b97ff 100644
--- a/src/path/filepath/path_test.go
+++ b/src/path/filepath/path_test.go
@@ -242,6 +242,7 @@ var jointests = []JoinTest{
 
 	// one parameter
 	{[]string{""}, ""},
+	{[]string{"/"}, "/"},
 	{[]string{"a"}, "a"},
 
 	// two parameters
@@ -249,10 +250,16 @@ var jointests = []JoinTest{
 	{[]string{"a", ""}, "a"},
 	{[]string{"", "b"}, "b"},
 	{[]string{"/", "a"}, "/a"},
+	{[]string{"/", "a/b"}, "/a/b"},
 	{[]string{"/", ""}, "/"},
+	{[]string{"//", "a"}, "/a"},
+	{[]string{"/a", "b"}, "/a/b"},
 	{[]string{"a/", "b"}, "a/b"},
 	{[]string{"a/", ""}, "a"},
 	{[]string{"", ""}, ""},
+
+	// three parameters
+	{[]string{"/", "a", "b"}, "/a/b"},
 }
 
 var winjointests = []JoinTest{
@@ -262,13 +269,17 @@ var winjointests = []JoinTest{
 	{[]string{`C:\`, `Windows`}, `C:\Windows`},
 	{[]string{`C:`, `Windows`}, `C:\Windows`},
 	{[]string{`\\host\share`, `foo`}, `\\host\share\foo`},
+	{[]string{`\\host\share\foo`}, `\\host\share\foo`},
 	{[]string{`//host/share`, `foo/bar`}, `\\host\share\foo\bar`},
-}
-
-// join takes a []string and passes it to Join.
-func join(elem []string, args ...string) string {
-	args = elem
-	return filepath.Join(args...)
+	{[]string{`\`}, `\`},
+	{[]string{`\`, ``}, `\`},
+	{[]string{`\`, `a`}, `\a`},
+	{[]string{`\\`, `a`}, `\a`},
+	{[]string{`\`, `a`, `b`}, `\a\b`},
+	{[]string{`\\`, `a`, `b`}, `\a\b`},
+	{[]string{`\`, `\\a\b`, `c`}, `\a\b\c`},
+	{[]string{`\\a`, `b`, `c`}, `\a\b\c`},
+	{[]string{`\\a\`, `b`, `c`}, `\a\b\c`},
 }
 
 func TestJoin(t *testing.T) {
@@ -276,8 +287,9 @@ func TestJoin(t *testing.T) {
 		jointests = append(jointests, winjointests...)
 	}
 	for _, test := range jointests {
-		if p := join(test.elem); p != filepath.FromSlash(test.path) {
-			t.Errorf("join(%q) = %q, want %q", test.elem, p, test.path)
+		expected := filepath.FromSlash(test.path)
+		if p := filepath.Join(test.elem...); p != expected {
+			t.Errorf("join(%q) = %q, want %q", test.elem, p, expected)
 		}
 	}
 }