add ValidateContextDirectory to utils/utils.go

This commit adds a function which can be used to ensure all contents of
a directory can be accessed.

This function doesn't follow symlinks to check if they're pointing to
files which exist. Such symlinks can be useful later.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

utils/utils.go

@@ -1067,3 +1067,40 @@ func TreeSize(dir string) (size int64, err error) {
 	})
 	return
 }
+
+// ValidateContextDirectory checks if all the contents of the directory
+// can be read and returns an error if some files can't be read
+// symlinks which point to non-existing files don't trigger an error
+func ValidateContextDirectory(srcPath string) error {
+	var finalError error
+
+	filepath.Walk(filepath.Join(srcPath, "."), func(filePath string, f os.FileInfo, err error) error {
+		// skip this directory/file if it's not in the path, it won't get added to the context
+		_, err = filepath.Rel(srcPath, filePath)
+		if err != nil && os.IsPermission(err) {
+			return nil
+		}
+
+		if _, err := os.Stat(filePath); err != nil && os.IsPermission(err) {
+			finalError = fmt.Errorf("can't stat '%s'", filePath)
+			return err
+		}
+		// skip checking if symlinks point to non-existing files, such symlinks can be useful
+		lstat, _ := os.Lstat(filePath)
+		if lstat.Mode()&os.ModeSymlink == os.ModeSymlink {
+			return err
+		}
+
+		if !f.IsDir() {
+			currentFile, err := os.Open(filePath)
+			if err != nil && os.IsPermission(err) {
+				finalError = fmt.Errorf("no permission to read from '%s'", filePath)
+				return err
+			} else {
+				currentFile.Close()
+			}
+		}
+		return nil
+	})
+	return finalError
+}