github-labeler: check if the label exists before adding it

The GitHub API automatically creates a label when applying it (if it doesn't
exist already), so we need to check if the label exists in the repo before we go
to apply (and create) it. This way, the humans have control over which labels
are created and assigned, which is how the bot should work.

github-labeler.py

@@ -31,22 +31,32 @@
 
 # ==============================================================================
 
-# Given a pullRequest object, the function checks what labels are currently on
-# the pull request, removes any matching the form "Target: {branch}" (if
-# {branch} is not the current target branch), and adds the correct label.
-def ensureLabels(pullRequest):
+# Given a pullRequest object and list of existing labels, the function checks
+# what labels are currently on the pull request, removes any matching the form
+# "Target: {branch}" (if {branch} is not the current target branch), and adds
+# the correct label.
+#
+# Because the GH API automatically creates a label when applying it (if it
+# doesn't exist), we try to get a label object to check if the label has been
+# created before we add (and create) it ourselves.
+def ensureLabels(pullRequest, repo):
     needsLabel = True
     targetPrefix = "Target: "
     targetLabel = f"{targetPrefix}{GITHUB_BASE_REF}"
+    try:
+        repo.get_label(targetLabel)
+    except:
+        print(f"Label '{targetLabel}' not found")
+        return None
     for label in pullRequest.get_labels():
         if label.name.startswith(targetPrefix):
             if label.name == targetLabel:
                 needsLabel = False
             else:
-                print(f"Removing label '{label.name}")
+                print(f"Removing label '{label.name}'")
                 pullRequest.remove_from_labels(label)
     if needsLabel:
-        print(f"Adding label '{targetLabel}")
+        print(f"Adding label '{targetLabel}'")
         pullRequest.add_to_labels(targetLabel)
     return None
 
@@ -56,4 +66,4 @@ def ensureLabels(pullRequest):
 repo = g.get_repo(GITHUB_REPOSITORY)
 prNum = int(PR_NUM)
 pr = repo.get_pull(prNum)
-ensureLabels(pr)
+ensureLabels(pr, repo)