Add path handling functions

There are several places throughout the code where we repeat the same
normalization patterns for paths. This change adds a couple of helper
functions and makes CheckSystemDriveAndRemoveDriveLetter() on all
platforms.

Additionally, all functions accept a platform flag that indicates the
target OS we're building the image for. Decissions are made based on
this flag instead of checking the platform on which buildkit is running.

Signed-off-by: Gabriel Adrian Samfira <[email protected]>

Author: gabriel-samfira

util/system/path.go

@@ -1,5 +1,14 @@
 package system
 
+import (
+	"path"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
 // DefaultPathEnvUnix is unix style list of directories to search for
 // executables. Each directory is separated from the next by a colon
 // ':' character .
@@ -16,3 +25,194 @@ func DefaultPathEnv(os string) string {
 	}
 	return DefaultPathEnvUnix
 }
+
+// NormalizePath cleans the path based on the operating system the path is meant for.
+// It takes into account a potential parent path, and will join the path to the parent
+// if the path is relative. Additionally, it will apply the folliwing rules:
+//   - always return an absolute path
+//   - always strip drive letters for Windows paths
+//   - optionally keep the trailing slashes on paths
+//   - paths are returned using forward slashes
+func NormalizePath(parent, newPath, inputOS string, keepSlash bool) (string, error) {
+	newPath = toSlash(newPath, inputOS)
+	parent = toSlash(parent, inputOS)
+	origPath := newPath
+
+	if parent == "" {
+		parent = "/"
+	}
+
+	var err error
+	parent, err = CheckSystemDriveAndRemoveDriveLetter(parent, inputOS)
+	if err != nil {
+		return "", errors.Wrap(err, "removing drive letter")
+	}
+
+	if !IsAbs(parent, inputOS) {
+		parent = path.Join("/", parent)
+	}
+
+	if newPath == "" {
+		// New workdir is empty. Use the "current" workdir. It should already
+		// be an absolute path.
+		newPath = parent
+	}
+
+	newPath, err = CheckSystemDriveAndRemoveDriveLetter(newPath, inputOS)
+	if err != nil {
+		return "", errors.Wrap(err, "removing drive letter")
+	}
+
+	if !IsAbs(newPath, inputOS) {
+		// The new WD is relative. Join it to the previous WD.
+		newPath = path.Join(parent, newPath)
+	}
+
+	if keepSlash {
+		if strings.HasSuffix(origPath, "/") && !strings.HasSuffix(newPath, "/") {
+			newPath += "/"
+		} else if strings.HasSuffix(origPath, "/.") {
+			if newPath != "/" {
+				newPath += "/"
+			}
+			newPath += "."
+		}
+	}
+
+	return toSlash(newPath, inputOS), nil
+}
+
+func toSlash(inputPath, inputOS string) string {
+	separator := "/"
+	if inputOS == "windows" {
+		separator = "\\"
+	}
+	return strings.Replace(inputPath, separator, "/", -1)
+}
+
+func fromSlash(inputPath, inputOS string) string {
+	separator := "/"
+	if inputOS == "windows" {
+		separator = "\\"
+	}
+	return strings.Replace(inputPath, "/", separator, -1)
+}
+
+// NormalizeWorkdir will return a normalized version of the new workdir, given
+// the currently configured workdir and the desired new workdir. When setting a
+// new relative workdir, it will be joined to the previous workdir or default to
+// the root folder.
+// On Windows we remove the drive letter and convert the path delimiter to "\".
+// Paths that begin with os.PathSeparator are considered absolute even on Windows.
+func NormalizeWorkdir(current, wd string, inputOS string) (string, error) {
+	wd, err := NormalizePath(current, wd, inputOS, false)
+	if err != nil {
+		return "", errors.Wrap(err, "normalizing working directory")
+	}
+
+	// Make sure we use the platform specific path separator. HCS does not like forward
+	// slashes in CWD.
+	return fromSlash(wd, inputOS), nil
+}
+
+// IsAbs returns a boolean value indicating whether or not the path
+// is absolute. On Linux, this is just a wrapper for filepath.IsAbs().
+// On Windows, we strip away the drive letter (if any), clean the path,
+// and check whether or not the path starts with a filepath.Separator.
+// This function is meant to check if a path is absolute, in the context
+// of a COPY, ADD or WORKDIR, which have their root set in the mount point
+// of the writable layer we are mutating. The filepath.IsAbs() function on
+// Windows will not work in these scenatios, as it will return true for paths
+// that:
+//   - Begin with drive letter (DOS style paths)
+//   - Are volume paths \\?\Volume{UUID}
+//   - Are UNC paths
+func IsAbs(pth, inputOS string) bool {
+	if inputOS == "" {
+		inputOS = runtime.GOOS
+	}
+	cleanedPath, err := CheckSystemDriveAndRemoveDriveLetter(pth, inputOS)
+	if err != nil {
+		return false
+	}
+	cleanedPath = toSlash(cleanedPath, inputOS)
+	// We stripped any potential drive letter and converted any backslashes to
+	// forward slashes. We can safely use path.IsAbs() for both Windows and Linux.
+	return path.IsAbs(cleanedPath)
+}
+
+// CheckSystemDriveAndRemoveDriveLetter verifies and manipulates a Windows path.
+// For linux, this is a no-op.
+//
+// This is used, for example, when validating a user provided path in docker cp.
+// If a drive letter is supplied, it must be the system drive. The drive letter
+// is always removed. It also converts any backslash to forward slash. The conversion
+// to OS specific separator should happen as late as possible (ie: before passing the
+// value to the function that will actually use it). Paths are parsed and code paths are
+// triggered starting with the client and all the way down to calling into the runtime
+// environment. The client may run on a foreign OS from the one the build will be triggered
+// (Windows clients connecting to Linux or vice versa).
+// Keeping the file separator consistent until the last moment is desirable.
+//
+// We need the Windows path without the drive letter so that it can ultimately be concatenated with
+// a Windows long-path which doesn't support drive-letters. Examples:
+// C:			--> Fail
+// C:somepath   --> somepath // This is a relative path to the CWD set for that drive letter
+// C:\			--> \
+// a			--> a
+// /a			--> \a
+// d:\			--> Fail
+//
+// UNC paths can refer to multiple types of paths. From local filesystem paths,
+// to remote filesystems like SMB or named pipes.
+// There is no sane way to support this without adding a lot of complexity
+// which I am not sure is worth it.
+// \\.\C$\a     --> Fail
+func CheckSystemDriveAndRemoveDriveLetter(path string, inputOS string) (string, error) {
+	if inputOS == "" {
+		inputOS = runtime.GOOS
+	}
+
+	if inputOS != "windows" {
+		return path, nil
+	}
+
+	if len(path) == 2 && string(path[1]) == ":" {
+		return "", errors.Errorf("No relative path specified in %q", path)
+	}
+
+	// UNC paths should error out
+	if len(path) >= 2 && toSlash(path[:2], inputOS) == "//" {
+		return "", errors.Errorf("UNC paths are not supported")
+	}
+
+	parts := strings.SplitN(path, ":", 2)
+	// Path does not have a drive letter. Just return it.
+	if len(parts) < 2 {
+		return toSlash(filepath.Clean(path), inputOS), nil
+	}
+
+	// We expect all paths to be in C:
+	if !strings.EqualFold(parts[0], "c") {
+		return "", errors.New("The specified path is not on the system drive (C:)")
+	}
+
+	// A path of the form F:somepath, is a path that is relative CWD set for a particular
+	// drive letter. See:
+	// https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#fully-qualified-vs-relative-paths
+	//
+	// C:\>mkdir F:somepath
+	// C:\>dir F:\
+	// Volume in drive F is New Volume
+	// Volume Serial Number is 86E5-AB64
+	//
+	// Directory of F:\
+	//
+	// 11/27/2022  02:22 PM    <DIR>          somepath
+	// 			0 File(s)              0 bytes
+	// 			1 Dir(s)   1,052,876,800 bytes free
+	//
+	// We must return the second element of the split path, as is, without attempting to convert
+	// it to an absolute path. We have no knowledge of the CWD; that is treated elsewhere.
+	return toSlash(filepath.Clean(parts[1]), inputOS), nil
+}