path/filepath: make Abs handle paths like c:a.txt properly

Fixes #8145.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/126440043

src/pkg/path/filepath/path.go

@@ -231,6 +231,10 @@ func EvalSymlinks(path string) (string, error) {
 // working directory to turn it into an absolute path.  The absolute
 // path name for a given file is not guaranteed to be unique.
 func Abs(path string) (string, error) {
+	return abs(path)
+}
+
+func unixAbs(path string) (string, error) {
 	if IsAbs(path) {
 		return Clean(path), nil
 	}

src/pkg/path/filepath/path_plan9.go

@@ -28,3 +28,7 @@ func splitList(path string) []string {
 	}
 	return strings.Split(path, string(ListSeparator))
 }
+
+func abs(path string) (string, error) {
+	return unixAbs(path)
+}

src/pkg/path/filepath/path_test.go

@@ -628,6 +628,8 @@ var winisabstests = []IsAbsTest{
 	{`\`, false},
 	{`\Windows`, false},
 	{`c:a\b`, false},
+	{`c:\a\b`, true},
+	{`c:/a/b`, true},
 	{`\\host\share\foo`, true},
 	{`//host/share/foo/bar`, true},
 }
@@ -807,6 +809,19 @@ func TestAbs(t *testing.T) {
 		}
 	}
 
+	if runtime.GOOS == "windows" {
+		vol := filepath.VolumeName(root)
+		var extra []string
+		for _, path := range absTests {
+			if strings.Index(path, "$") != -1 {
+				continue
+			}
+			path = vol + path
+			extra = append(extra, path)
+		}
+		absTests = append(absTests, extra...)
+	}
+
 	err = os.Chdir(absTestDirs[0])
 	if err != nil {
 		t.Fatal("chdir failed: ", err)

src/pkg/path/filepath/path_unix.go

@@ -30,3 +30,7 @@ func splitList(path string) []string {
 	}
 	return strings.Split(path, string(ListSeparator))
 }
+
+func abs(path string) (string, error) {
+	return unixAbs(path)
+}

src/pkg/path/filepath/path_windows.go

@@ -6,6 +6,7 @@ package filepath
 
 import (
 	"strings"
+	"syscall"
 )
 
 func isSlash(c uint8) bool {
@@ -103,3 +104,7 @@ func splitList(path string) []string {
 
 	return list
 }
+
+func abs(path string) (string, error) {
+	return syscall.FullPath(path)
+}