path/filepath: add EvalSymlinks function

R=rsc, niemeyer, r2, rog, iant2, r
CC=golang-dev
https://golang.org/cl/4235060

src/pkg/path/filepath/path.go

@@ -8,11 +8,17 @@
 package filepath
 
 import (
+	"bytes"
 	"os"
 	"sort"
 	"strings"
 )
 
+const (
+	SeparatorString     = string(Separator)
+	ListSeparatorString = string(ListSeparator)
+)
+
 // Clean returns the shortest path name equivalent to path
 // by purely lexical processing.  It applies the following rules
 // iteratively until no further processing can be done:
@@ -113,7 +119,7 @@ func ToSlash(path string) string {
 	if Separator == '/' {
 		return path
 	}
-	return strings.Replace(path, string(Separator), "/", -1)
+	return strings.Replace(path, SeparatorString, "/", -1)
 }
 
 // FromSlash returns the result of replacing each slash ('/') character
@@ -122,7 +128,7 @@ func FromSlash(path string) string {
 	if Separator == '/' {
 		return path
 	}
-	return strings.Replace(path, "/", string(Separator), -1)
+	return strings.Replace(path, "/", SeparatorString, -1)
 }
 
 // SplitList splits a list of paths joined by the OS-specific ListSeparator.
@@ -130,7 +136,7 @@ func SplitList(path string) []string {
 	if path == "" {
 		return []string{}
 	}
-	return strings.Split(path, string(ListSeparator), -1)
+	return strings.Split(path, ListSeparatorString, -1)
 }
 
 // Split splits path immediately following the final Separator,
@@ -150,7 +156,7 @@ func Split(path string) (dir, file string) {
 func Join(elem ...string) string {
 	for i, e := range elem {
 		if e != "" {
-			return Clean(strings.Join(elem[i:], string(Separator)))
+			return Clean(strings.Join(elem[i:], SeparatorString))
 		}
 	}
 	return ""
@@ -169,6 +175,62 @@ func Ext(path string) string {
 	return ""
 }
 
+// EvalSymlinks returns the path name after the evaluation of any symbolic
+// links.
+// If path is relative it will be evaluated relative to the current directory.
+func EvalSymlinks(path string) (string, os.Error) {
+	const maxIter = 255
+	originalPath := path
+	// consume path by taking each frontmost path element,
+	// expanding it if it's a symlink, and appending it to b
+	var b bytes.Buffer
+	for n := 0; path != ""; n++ {
+		if n > maxIter {
+			return "", os.NewError("EvalSymlinks: too many links in " + originalPath)
+		}
+
+		// find next path component, p
+		i := strings.IndexRune(path, Separator)
+		var p string
+		if i == -1 {
+			p, path = path, ""
+		} else {
+			p, path = path[:i], path[i+1:]
+		}
+
+		if p == "" {
+			if b.Len() == 0 {
+				// must be absolute path
+				b.WriteRune(Separator)
+			}
+			continue
+		}
+
+		fi, err := os.Lstat(b.String() + p)
+		if err != nil {
+			return "", err
+		}
+		if !fi.IsSymlink() {
+			b.WriteString(p)
+			if path != "" {
+				b.WriteRune(Separator)
+			}
+			continue
+		}
+
+		// it's a symlink, put it at the front of path
+		dest, err := os.Readlink(b.String() + p)
+		if err != nil {
+			return "", err
+		}
+		if IsAbs(dest) {
+			b.Reset()
+		}
+		path = dest + SeparatorString + path
+	}
+	return Clean(b.String()), nil
+}
+
 // Visitor methods are invoked for corresponding file tree entries
 // visited by Walk. The parameter path is the full path of f relative
 // to root.
@@ -267,7 +329,7 @@ func Base(path string) string {
 	}
 	// If empty now, it had only slashes.
 	if path == "" {
-		return string(Separator)
+		return SeparatorString
 	}
 	return path
 }

src/pkg/path/filepath/path_test.go

@@ -414,3 +414,63 @@ func TestIsAbs(t *testing.T) {
 		}
 	}
 }
+
+type EvalSymlinksTest struct {
+	path, dest string
+}
+
+var EvalSymlinksTestDirs = []EvalSymlinksTest{
+	{"test", ""},
+	{"test/dir", ""},
+	{"test/dir/link3", "../../"},
+	{"test/link1", "../test"},
+	{"test/link2", "dir"},
+}
+
+var EvalSymlinksTests = []EvalSymlinksTest{
+	{"test", "test"},
+	{"test/dir", "test/dir"},
+	{"test/dir/../..", "."},
+	{"test/link1", "test"},
+	{"test/link2", "test/dir"},
+	{"test/link1/dir", "test/dir"},
+	{"test/link2/..", "test"},
+	{"test/dir/link3", "."},
+	{"test/link2/link3/test", "test"},
+}
+
+func TestEvalSymlinks(t *testing.T) {
+	defer os.RemoveAll("test")
+	for _, d := range EvalSymlinksTestDirs {
+		var err os.Error
+		if d.dest == "" {
+			err = os.Mkdir(d.path, 0755)
+		} else {
+			err = os.Symlink(d.dest, d.path)
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	// relative
+	for _, d := range EvalSymlinksTests {
+		if p, err := filepath.EvalSymlinks(d.path); err != nil {
+			t.Errorf("EvalSymlinks(%v) error: %v", d.path, err)
+		} else if p != d.dest {
+			t.Errorf("EvalSymlinks(%v)=%v, want %v", d.path, p, d.dest)
+		}
+	}
+	// absolute
+	testroot := filepath.Join(os.Getenv("GOROOT"), "src", "pkg", "path", "filepath")
+	for _, d := range EvalSymlinksTests {
+		a := EvalSymlinksTest{
+			filepath.Join(testroot, d.path),
+			filepath.Join(testroot, d.dest),
+		}
+		if p, err := filepath.EvalSymlinks(a.path); err != nil {
+			t.Errorf("EvalSymlinks(%v) error: %v", a.path, err)
+		} else if p != a.dest {
+			t.Errorf("EvalSymlinks(%v)=%v, want %v", a.path, p, a.dest)
+		}
+	}
+}