Replace error handling package with emperror; add better reporting for errors escaping server root

server/filesystem/errors.go

@@ -1,16 +1,43 @@
 package filesystem
 
 import (
+	"emperror.dev/errors"
+	"fmt"
 	"github.com/apex/log"
-	"github.com/pkg/errors"
 	"os"
 	"path/filepath"
 )
 
-var ErrIsDirectory = errors.New("filesystem: is a directory")
-var ErrNotEnoughDiskSpace = errors.New("filesystem: not enough disk space")
-var ErrBadPathResolution = errors.New("filesystem: invalid path resolution")
-var ErrUnknownArchiveFormat = errors.New("filesystem: unknown archive format")
+var ErrIsDirectory = errors.Sentinel("filesystem: is a directory")
+var ErrNotEnoughDiskSpace = errors.Sentinel("filesystem: not enough disk space")
+var ErrUnknownArchiveFormat = errors.Sentinel("filesystem: unknown archive format")
+
+type BadPathResolutionError struct {
+	path     string
+	resolved string
+}
+
+// Returns the specific error for a bad path resolution.
+func (b *BadPathResolutionError) Error() string {
+	r := b.resolved
+	if r == "" {
+		r = "<empty>"
+	}
+	return fmt.Sprintf("filesystem: server path [%s] resolves to a location outside the server root: %s", b.path, r)
+}
+
+// Returns a new BadPathResolution error.
+func NewBadPathResolution(path string, resolved string) *BadPathResolutionError {
+	return &BadPathResolutionError{path, resolved}
+}
+
+// Determines if the given error is a bad path resolution error.
+func IsBadPathResolutionError(err error) bool {
+	if _, ok := err.(*BadPathResolutionError); ok {
+		return true
+	}
+	return false
+}
 
 // Generates an error logger instance with some basic information.
 func (fs *Filesystem) error(err error) *log.Entry {
@@ -23,8 +50,8 @@ func (fs *Filesystem) error(err error) *log.Entry {
 // directory, otherwise return nil. Returning this error for a file will stop the walking
 // for the remainder of the directory. This is assuming an os.FileInfo struct was even returned.
 func (fs *Filesystem) handleWalkerError(err error, f os.FileInfo) error {
-	if !errors.Is(err, ErrBadPathResolution) {
-		return err
+	if !IsBadPathResolutionError(err) {
+		return errors.WithStackIf(err)
 	}
 
 	if f != nil && f.IsDir() {
@@ -32,4 +59,4 @@ func (fs *Filesystem) handleWalkerError(err error, f os.FileInfo) error {
 	}
 
 	return nil
-}
+}