fsutil: Add Dir() and Clean() functions

From the code:

  These functions exists because we work with slash terminated
  directory paths that come from deb package tarballs but standard
  library path functions treat slash terminated paths as unclean.

We use ad-hoc code to make filepath.Dir() slash terminated in two places
right now. For example this code

  targetDir := filepath.Dir(strings.TrimRight(targetPath, "/")) + "/"

is not strictly correct as "/a/b/.///" will be turned into "/a/b/./"
which is still "/a/b".

Since we deal with slash terminated directory paths everywhere, create
and use dedicated helper functions to process them.

internal/fsutil/path.go

@@ -0,0 +1,47 @@
+package fsutil
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// These functions exists because we work with slash terminated directory paths
+// that come from deb package tarballs but standard library path functions
+// treat slash terminated paths as unclean.
+
+// Dir is like filepath.Dir() but trailing slash on input is ignored and the
+// return value always includes trailing slash.
+//
+// Comparison of filepath.Dir() with fsutil.Dir():
+//
+//	filepath.Dir("/foo/bar/")	== "/foo/bar"
+//	filepath.Dir("/foo/bar")	== "/foo"
+//
+//	fsutil.Dir("/foo/bar")		== "/foo/"
+//	fsutil.Dir("/foo/bar/")		== "/foo/"
+func Dir(path string) string {
+	parent := filepath.Dir(filepath.Clean(path))
+	if parent != "/" {
+		parent += "/"
+	}
+	return parent
+}
+
+// Clean is like filepath.Clean() but trailing slash is kept.
+//
+// Comparison of filepath.Clean() with fsutil.Clean():
+//
+//	filepath.Clean("/foo/bar")	== "/foo/bar"
+//	filepath.Clean("/foo/bar/")	== "/foo/bar"
+//	filepath.Clean("/foo/bar/.//)	== "/foo/bar"
+//
+//	fsutil.Clean("/foo/bar")	== "/foo/bar"
+//	fsutil.Clean("/foo/bar/")	== "/foo/bar/"
+//	fsutil.Clean("/foo/bar/.//")	== "/foo/bar/"
+func Clean(path string) string {
+	clean := filepath.Clean(path)
+	if strings.HasSuffix(path, "/") {
+		clean += "/"
+	}
+	return clean
+}

internal/fsutil/path_test.go

@@ -0,0 +1,52 @@
+package fsutil_test
+
+import (
+	. "gopkg.in/check.v1"
+
+	"github.com/canonical/chisel/internal/fsutil"
+)
+
+var dirTests = []struct {
+	input  string
+	output string
+}{
+	{"/a/b/c", "/a/b/"},
+	{"/a/b/c/", "/a/b/"},
+	{"/a/b/c//", "/a/b/"},
+	{"/a/b/c/././", "/a/b/"},
+	{"/a/b/c/.././", "/a/"},
+	{"/a/b//c", "/a/b/"},
+	{"/a/b/./c", "/a/b/"},
+	{"a/b/./c", "a/b/"},
+	{"./a/b/./c", "a/b/"},
+	{"/", "/"},
+	{"///.///", "/"},
+	{".", "./"},
+	{"", "./"},
+}
+
+var cleanTests = []struct {
+	input  string
+	output string
+}{
+	{"/a/b/c", "/a/b/c"},
+	{"/a/b/c/", "/a/b/c/"},
+	{"/a/b/c/.//", "/a/b/c/"},
+	{"/a/b//./c", "/a/b/c"},
+	{"/a/b//./c/", "/a/b/c/"},
+	{"/a/b/.././c/", "/a/c/"},
+}
+
+func (s *S) TestDir(c *C) {
+	for _, t := range dirTests {
+		c.Logf("%s => %s", t.input, t.output)
+		c.Assert(fsutil.Dir(t.input), Equals, t.output)
+	}
+}
+
+func (s *S) TestClean(c *C) {
+	for _, t := range cleanTests {
+		c.Logf("%s => %s", t.input, t.output)
+		c.Assert(fsutil.Clean(t.input), Equals, t.output)
+	}
+}

internal/slicer/slicer.go

@@ -32,25 +32,21 @@ func Run(options *RunOptions) error {
 
 	knownPaths["/"] = true
 
+	// addKnownPath path adds path and all its directory parent paths into
+	// knownPaths set.
 	addKnownPath := func(path string) {
 		if path[0] != '/' {
 			panic("bug: tried to add relative path to known paths")
 		}
-		cleanPath := filepath.Clean(path)
-		slashPath := cleanPath
-		if path[len(path)-1] == '/' && cleanPath != "/" {
-			slashPath += "/"
-		}
+		path = fsutil.Clean(path)
 		for {
-			if _, ok := knownPaths[slashPath]; ok {
+			if _, ok := knownPaths[path]; ok {
 				break
 			}
-			knownPaths[slashPath] = true
-			cleanPath = filepath.Dir(cleanPath)
-			if cleanPath == "/" {
+			knownPaths[path] = true
+			if path = fsutil.Dir(path); path == "/" {
 				break
 			}
-			slashPath = cleanPath + "/"
 		}
 	}
 
@@ -113,7 +109,7 @@ func Run(options *RunOptions) error {
 					hasCopyright = true
 				}
 			} else {
-				targetDir := filepath.Dir(strings.TrimRight(targetPath, "/")) + "/"
+				targetDir := fsutil.Dir(targetPath)
 				if targetDir == "" || targetDir == "/" {
 					continue
 				}