pkg/idtools: mkdirAs(): fix infinite loops and repeated "chown"

This fixes an inifinite loop in mkdirAs(), used by `MkdirAllAndChown`,
`MkdirAndChown`, and `MkdirAllAndChownNew`, as well as directories being
chown'd multiple times when relative paths are used.

The for loop in this function was incorrectly assuming that;

1. `filepath.Dir()` would always return the parent directory of any given path
2. traversing any given path to ultimately result in "/"

While this is correct for absolute and "cleaned" paths, both assumptions are
incorrect in some variations of "path";

1. for paths with a trailing path-separator ("some/path/"), or dot ("."),
   `filepath.Dir()` considers the (implicit) "." to be a location _within_ the
   directory, and returns "some/path" as ("parent") directory. This resulted
   in the path itself to be included _twice_ in the list of paths to chown.
2. for relative paths ("./some-path", "../some-path"), "traversing" the path
   would never end in "/", causing the for loop to run indefinitely:

    ```go
    // walk back to "/" looking for directories which do not exist
    // and add them to the paths array for chown after creation
    dirPath := path
    for {
        dirPath = filepath.Dir(dirPath)
        if dirPath == "/" {
            break
        }
        if _, err := os.Stat(dirPath); err != nil && os.IsNotExist(err) {
            paths = append(paths, dirPath)
        }
    }
    ```

A _partial_ mitigation for this would be to use `filepath.Clean()` before using
the path (while `filepath.Dir()` _does_ call `filepath.Clean()`, it only does so
_after_ some processing, so only cleans the result). Doing so would prevent the
double chown from happening, but would not prevent the "final" path to be "."
or ".." (in the relative path case), still causing an infinite loop, or
additional checks for "." / ".." to be needed.

| path           | filepath.Dir(path) | filepath.Dir(filepath.Clean(path)) |
|----------------|--------------------|------------------------------------|
| some-path      | .                  | .                                  |
| ./some-path    | .                  | .                                  |
| ../some-path   | ..                 | ..                                 |
| some/path/     | some/path          | some                               |
| ./some/path/   | some/path          | some                               |
| ../some/path/  | ../some/path       | ../some                            |
| some/path/.    | some/path          | some                               |
| ./some/path/.  | some/path          | some                               |
| ../some/path/. | ../some/path       | ../some                            |
| /some/path/    | /some/path         | /some                              |
| /some/path/.   | /some/path         | /some                              |

Instead, this patch adds a `filepath.Abs()` to the function, so make sure that
paths are both cleaned, and not resulting in an infinite loop.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/idtools/idtools_unix.go

@@ -30,6 +30,10 @@ func mkdirAs(path string, mode os.FileMode, owner Identity, mkAll, chownExisting
 	// chown the full directory path if it exists
 
 	var paths []string
+	path, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
 
 	stat, err := system.Stat(path)
 	if err == nil {

pkg/idtools/idtools_unix_test.go

@@ -127,6 +127,98 @@ func TestMkdirAllAndChownNew(t *testing.T) {
 	assert.NilError(t, compareTrees(testTree, verifyTree))
 }
 
+func TestMkdirAllAndChownNewRelative(t *testing.T) {
+	RequiresRoot(t)
+
+	tests := []struct {
+		in  string
+		out []string
+	}{
+		{
+			in:  "dir1",
+			out: []string{"dir1"},
+		},
+		{
+			in:  "dir2/subdir2",
+			out: []string{"dir2", "dir2/subdir2"},
+		},
+		{
+			in:  "dir3/subdir3/",
+			out: []string{"dir3", "dir3/subdir3"},
+		},
+		{
+			in:  "dir4/subdir4/.",
+			out: []string{"dir4", "dir4/subdir4"},
+		},
+		{
+			in:  "dir5/././subdir5/",
+			out: []string{"dir5", "dir5/subdir5"},
+		},
+		{
+			in:  "./dir6",
+			out: []string{"dir6"},
+		},
+		{
+			in:  "./dir7/subdir7",
+			out: []string{"dir7", "dir7/subdir7"},
+		},
+		{
+			in:  "./dir8/subdir8/",
+			out: []string{"dir8", "dir8/subdir8"},
+		},
+		{
+			in:  "./dir9/subdir9/.",
+			out: []string{"dir9", "dir9/subdir9"},
+		},
+		{
+			in:  "./dir10/././subdir10/",
+			out: []string{"dir10", "dir10/subdir10"},
+		},
+	}
+
+	// Set the current working directory to the temp-dir, as we're
+	// testing relative paths.
+	tmpDir := t.TempDir()
+	setWorkingDirectory(t, tmpDir)
+
+	const expectedUIDGID = 101
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.in, func(t *testing.T) {
+			for _, p := range tc.out {
+				_, err := os.Stat(p)
+				assert.ErrorIs(t, err, os.ErrNotExist)
+			}
+
+			err := MkdirAllAndChownNew(tc.in, 0755, Identity{UID: expectedUIDGID, GID: expectedUIDGID})
+			assert.Check(t, err)
+
+			for _, p := range tc.out {
+				s := &unix.Stat_t{}
+				err = unix.Stat(p, s)
+				if assert.Check(t, err) {
+					assert.Check(t, is.Equal(uint64(s.Uid), uint64(expectedUIDGID)))
+					assert.Check(t, is.Equal(uint64(s.Gid), uint64(expectedUIDGID)))
+				}
+			}
+		})
+	}
+}
+
+// Change the current working directory for the duration of the test. This may
+// break if tests are run in parallel.
+func setWorkingDirectory(t *testing.T, dir string) {
+	t.Helper()
+	cwd, err := os.Getwd()
+	assert.NilError(t, err)
+	t.Cleanup(func() {
+		assert.NilError(t, os.Chdir(cwd))
+	})
+	err = os.Chdir(dir)
+	assert.NilError(t, err)
+}
+
 func TestMkdirAndChown(t *testing.T) {
 	RequiresRoot(t)
 	dirName, err := os.MkdirTemp("", "mkdir")