fix!: stop merging "by-*" keys in util.templates.merge

BREAKING CHANGE: This can impact how `task-defaults` gets merged into
each task definition.

The default merge behaviour would mix the `by-<key>` key in with
non-keyed-by configs. This is a bug and certainly not what the user
would be expecting.

It is possible that the user intended to mix two `by-<key>` configs
together, but this case should be extremely rare.

This is a port of the following `gecko_taskgraph` patch:
https://hg.mozilla.org/mozilla-central/rev/0fe5ffc10ff8488d780c5c3cbcff1b43ca974cf7

Author: ahal

src/taskgraph/util/templates.py

@@ -2,7 +2,6 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-
 import copy
 
 
@@ -18,9 +17,19 @@ def merge_to(source, dest):
     """
 
     for key, value in source.items():
+        if (
+            isinstance(value, dict)
+            and len(value) == 1
+            and list(value)[0].startswith("by-")
+        ):
+            # Do not merge by-* values as it will almost certainly not do what
+            # the user expects.
+            dest[key] = value
+            continue
+
         # Override mismatching or empty types
         if type(value) != type(dest.get(key)):  # noqa
-            dest[key] = source[key]
+            dest[key] = value
             continue
 
         # Merge dict
@@ -29,10 +38,10 @@ def merge_to(source, dest):
             continue
 
         if isinstance(value, list):
-            dest[key] = dest[key] + source[key]
+            dest[key] = dest[key] + value
             continue
 
-        dest[key] = source[key]
+        dest[key] = value
 
     return dest
 

test/test_util_templates.py

@@ -2,7 +2,6 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-
 import unittest
 
 from taskgraph.util.templates import merge, merge_to
@@ -50,3 +49,24 @@ def test_merge(self):
         self.assertEqual(first, {"a": 1, "b": 2, "d": 11})
         self.assertEqual(second, {"b": 20, "c": 30})
         self.assertEqual(third, {"c": 300, "d": 400})
+
+    def test_merge_by(self):
+        source = {
+            "x": "abc",
+            "y": {"by-foo": {"quick": "fox", "default": ["a", "b", "c"]}},
+        }
+        dest = {"y": {"by-foo": {"purple": "rain", "default": ["x", "y", "z"]}}}
+        expected = {
+            "x": "abc",
+            "y": {"by-foo": {"quick": "fox", "default": ["a", "b", "c"]}},
+        }  # source wins
+        self.assertEqual(merge_to(source, dest), expected)
+        self.assertEqual(dest, expected)
+
+    def test_merge_multiple_by(self):
+        source = {"x": {"by-foo": {"quick": "fox", "default": ["a", "b", "c"]}}}
+        dest = {"x": {"by-bar": {"purple": "rain", "default": ["x", "y", "z"]}}}
+        expected = {
+            "x": {"by-foo": {"quick": "fox", "default": ["a", "b", "c"]}}
+        }  # source wins
+        self.assertEqual(merge_to(source, dest), expected)