fix: ensure taskgraph full -J --tasks <regex> shows all dependencies

Overwriting `task.dependencies` with the graph edges doesn't play nicely
with `get_filtered_taskgraph` (e.g `--tasks <regex>`) because in this
case the full graph has already been trimmed. The result is that when
using `--tasks`, the dependencies displayed will be a subset of the real
dependencies. This is very misleading and can easily lead one to false
conclusions while debugging. Recently this caused me to waste an hour of
my time.

I traced this comment back through history and it's been like this since
it was originally implemented. Likely the intent was that if we a have a
subset of the graph, then we should modify the task definition to not
display any dependencies that aren't part of it.

But I disagree with this. I'd argue that `Task.dependencies` is a very
important part of the task definition, and not at all relevant to the
higher level DAG that the task happens to be a part of. I believe
mutating the task definition just because we did some filtering is a
bug.

Testing the full graph from the Firefox repo, I can confirm this change
results in identical graphs (when not using `--tasks`).

Author: ahal

src/taskgraph/taskgraph.py

@@ -44,13 +44,10 @@ def __iter__(self):
 
     def to_json(self):
         "Return a JSON-able object representing the task graph, as documented"
-        named_links_dict = self.graph.named_links_dict()
         # this dictionary may be keyed by label or by taskid, so let's just call it 'key'
         tasks = {}
         for key in self.graph.visit_postorder():
             tasks[key] = self.tasks[key].to_json()
-            # overwrite dependencies with the information in the taskgraph's edges.
-            tasks[key]["dependencies"] = named_links_dict.get(key, {})
         return tasks
 
     @classmethod

test/test_main.py

@@ -165,6 +165,26 @@ def test_output_file(run_taskgraph, tmpdir):
             },
             id="no-op",
         ),
+        pytest.param(
+            "^a",
+            None,
+            {
+                "a": {
+                    "attributes": {"kind": "task"},
+                    "dependencies": {"dep": "b"},
+                    "description": "",
+                    "kind": "task",
+                    "label": "a",
+                    "optimization": None,
+                    "soft_dependencies": [],
+                    "if_dependencies": [],
+                    "task": {
+                        "foo": {"bar": 1},
+                    },
+                },
+            },
+            id="regex-a-only",
+        ),
         pytest.param(
             "^b",
             None,
@@ -226,7 +246,13 @@ def test_output_file(run_taskgraph, tmpdir):
 )
 def test_get_filtered_taskgraph(regex, exclude, expected):
     tasks = {
-        "a": Task(kind="task", label="a", attributes={}, task={"foo": {"bar": 1}}),
+        "a": Task(
+            kind="task",
+            label="a",
+            attributes={},
+            dependencies={"dep": "b"},
+            task={"foo": {"bar": 1}},
+        ),
         "b": Task(
             kind="task",
             label="b",

test/test_taskgraph.py

@@ -20,15 +20,14 @@ def test_taskgraph_to_json(self):
                 label="a",
                 attributes={"attr": "a-task"},
                 task={"taskdef": True},
+                dependencies={"edgelabel": "b"},
             ),
             "b": Task(
                 kind="test",
                 label="b",
                 attributes={},
                 task={"task": "def"},
                 optimization={"seta": None},
-                # note that this dep is ignored, superseded by that
-                # from the taskgraph's edges
                 dependencies={"first": "a"},
             ),
         }
@@ -56,7 +55,7 @@ def test_taskgraph_to_json(self):
                     "label": "b",
                     "attributes": {"kind": "test"},
                     "task": {"task": "def"},
-                    "dependencies": {},
+                    "dependencies": {"first": "a"},
                     "description": "",
                     "soft_dependencies": [],
                     "if_dependencies": [],