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, 3 insertions, 6 deletions

diff --git a/src/path/filepath/path.go b/src/path/filepath/path.go
index 3bde14b91e..ebdd9f5968 100644
--- a/src/path/filepath/path.go
+++ b/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.