gitlab: improve checking for project lookup during fork

The fork was being interrupted only when another fork, for a different
project, was found. However, there are other cases where a fork should fail:
(keep in mind the code is trying to find an identical _fork project_)

1) the fork matches the project being forked

	creates two remotes pointing to the exact same project.

2) the fork matches a non-forked project

	creates a remote pointing to a project, that just matches the
	namespace/name the user wants, but it isn't a fork.

	In general, it falls in case 1), however, lab allow the user to set a
	custom name, namespace and path for the fork, so we need this additional
	checking step.

3) (as said above) the fork matches another fork, but for a different project

	create a remote pointing to a project with same namespace/name but
	forked from another non-related project.

Signed-off-by: Bruno Meneguele <[email protected]>

internal/gitlab/gitlab.go

@@ -244,12 +244,27 @@ func Fork(project string, opts *gitlab.ForkProjectOptions, useHTTP bool, wait bo
 			opts.Name = gitlab.String(name)
 		}
 	}
+
 	target, err := FindProject(namespace + name)
 	if err == nil {
-		// Check if it was forked from the same project being requested
+		// Check if it isn't the same project being requested
+		if target.PathWithNamespace == project {
+			errMsg := "not possible to fork a project from the same namespace and name"
+			return "", errors.New(errMsg)
+		}
+
+		// Check if it isn't a non-fork project, meaning the user has
+		// access to a project with same namespace/name
+		if target.ForkedFromProject == nil {
+			errMsg := fmt.Sprintf("\"%s\" project already taken\n", target.PathWithNamespace)
+			return "", errors.New(errMsg)
+		}
+
+		// Check if it isn't already a fork for another project
 		if target.ForkedFromProject != nil &&
 			target.ForkedFromProject.PathWithNamespace != project {
-			errMsg := fmt.Sprintf("\"%s\" fork name already taken for a different project", name)
+			errMsg := fmt.Sprintf("\"%s\" fork already taken for a different project",
+				target.PathWithNamespace)
 			return "", errors.New(errMsg)
 		}