storage: change logic of ArtifactExist method

Given that:

* The produced artifact as advertisted in the path should always
  be a regular file (including the exclusion of symlinks).
* The produced artifact should be readable, so any type of error
  should count as "does not exist".

We should use `os.Lstat` to not follow symlinks; return `false`
on any error we run in to, or return if the file mode information
reports a regular file.

Author: hiddeco

controllers/storage.go

@@ -120,12 +120,14 @@ func (s *Storage) RemoveAllButCurrent(artifact sourcev1.Artifact) error {
 	return nil
 }
 
-// ArtifactExist returns a boolean indicating whether the artifact file exists in storage
+// ArtifactExist returns a boolean indicating whether the artifact exists in storage and is a
+// regular file.
 func (s *Storage) ArtifactExist(artifact sourcev1.Artifact) bool {
-	if _, err := os.Stat(artifact.Path); os.IsNotExist(err) {
+	fi, err := os.Lstat(artifact.Path)
+	if err != nil {
 		return false
 	}
-	return true
+	return fi.Mode().IsRegular()
 }
 
 // Archive creates a tar.gz to the artifact path from the given dir excluding any VCS specific