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>

src/path/filepath/path.go

@@ -196,13 +196,10 @@ func Split(path string) (dir, file string) {
 // Join joins any number of path elements into a single path, adding
 // a Separator if necessary. The result is Cleaned, in particular
 // all empty strings are ignored.
+// On Windows, the result is a UNC path if and only if the first path
+// element is a UNC path.
 func Join(elem ...string) string {
-	for i, e := range elem {
-		if e != "" {
-			return Clean(strings.Join(elem[i:], string(Separator)))
-		}
-	}
-	return ""
+	return join(elem)
 }
 
 // Ext returns the file name extension used by path.

src/path/filepath/path_plan9.go

@@ -32,3 +32,13 @@ func splitList(path string) []string {
 func abs(path string) (string, error) {
 	return unixAbs(path)
 }
+
+func join(elem []string) string {
+	// If there's a bug here, fix the logic in ./path_unix.go too.
+	for i, e := range elem {
+		if e != "" {
+			return Clean(strings.Join(elem[i:], string(Separator)))
+		}
+	}
+	return ""
+}

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)
 		}
 	}
 }

src/path/filepath/path_unix.go

@@ -34,3 +34,13 @@ func splitList(path string) []string {
 func abs(path string) (string, error) {
 	return unixAbs(path)
 }
+
+func join(elem []string) string {
+	// If there's a bug here, fix the logic in ./path_plan9.go too.
+	for i, e := range elem {
+		if e != "" {
+			return Clean(strings.Join(elem[i:], string(Separator)))
+		}
+	}
+	return ""
+}

src/path/filepath/path_windows.go

@@ -108,3 +108,40 @@ func splitList(path string) []string {
 func abs(path string) (string, error) {
 	return syscall.FullPath(path)
 }
+
+func join(elem []string) string {
+	for i, e := range elem {
+		if e != "" {
+			return joinNonEmpty(elem[i:])
+		}
+	}
+	return ""
+}
+
+// joinNonEmpty is like join, but it assumes that the first element is non-empty.
+func joinNonEmpty(elem []string) string {
+	// The following logic prevents Join from inadvertently creating a
+	// UNC path on Windows. Unless the first element is a UNC path, Join
+	// shouldn't create a UNC path. See golang.org/issue/9167.
+	p := Clean(strings.Join(elem, string(Separator)))
+	if !isUNC(p) {
+		return p
+	}
+	// p == UNC only allowed when the first element is a UNC path.
+	head := Clean(elem[0])
+	if isUNC(head) {
+		return p
+	}
+	// head + tail == UNC, but joining two non-UNC paths should not result
+	// in a UNC path. Undo creation of UNC path.
+	tail := Clean(strings.Join(elem[1:], string(Separator)))
+	if head[len(head)-1] == Separator {
+		return head + tail
+	}
+	return head + string(Separator) + tail
+}
+
+// isUNC returns true if path is a UNC path.
+func isUNC(path string) bool {
+	return volumeNameLen(path) > 2
+}