Handle non-UTF8 diffs in the WPT upstream script

The output of `git diff` and `git show` can include non-UTF8 text or
binary data, so the WPT upstream script should handle this properly.

Fixes #29620.

4 files changed, 42 insertions, 6 deletions

diff --git a/etc/ci/upstream-wpt-changes/test.py b/etc/ci/upstream-wpt-changes/test.py
index c56831360ba..3ecf3afbb3c 100644
--- a/etc/ci/upstream-wpt-changes/test.py
+++ b/etc/ci/upstream-wpt-changes/test.py
@@ -229,7 +229,7 @@ class TestApplyCommitsToWPT(unittest.TestCase):
 
     def run_test(self, pr_number: int, commit_data: dict):
         def make_commit(data):
-            with open(os.path.join(TESTS_DIR, data[2]), encoding="utf-8") as file:
+            with open(os.path.join(TESTS_DIR, data[2]), "rb") as file:
                 return {"author": data[0], "message": data[1], "diff": file.read()}
 
         commits = [make_commit(data) for data in commit_data]
@@ -272,6 +272,15 @@ class TestApplyCommitsToWPT(unittest.TestCase):
             [
                 ["test author <test@author>", "test commit message", "18746.diff"],
                 ["another person <two@author>", "a different message", "wpt.diff"],
+                ["another person <two@author>", "adding some non-utf8 chaos", "add-non-utf8-file.diff"],
+            ],
+        )
+
+    def test_non_utf8_commit(self):
+        self.run_test(
+            100,
+            [
+                ["test author <nonutf8@author>", "adding some non-utf8 chaos", "add-non-utf8-file.diff"],
             ],
         )
 
@@ -455,6 +464,17 @@ class TestFullSyncRun(unittest.TestCase):
             ]
         )
 
+    def test_opened_upstreamable_pr_with_non_utf8_file_contents(self):
+        self.assertListEqual(
+            self.run_test("opened.json", ["add-non-utf8-file.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_18746→wpt/wpt#1",
+                "CommentStep:servo/servo#18746:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ],
+        )
+
     def test_open_new_upstreamable_pr_with_preexisting_upstream_pr_not_apply_cleanly_to_upstream(
         self,
     ):
diff --git a/etc/ci/upstream-wpt-changes/tests/add-non-utf8-file.diff b/etc/ci/upstream-wpt-changes/tests/add-non-utf8-file.diff
new file mode 100644
index 00000000000..bd3b755d87d
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/add-non-utf8-file.diff
@@ -0,0 +1,7 @@
+diff --git a/tests/wpt/web-platform-tests/non-utf8-file.txt b/tests/wpt/web-platform-tests/non-utf8-file.txt
+new file mode 100644
+index 0000000..5a992e0
+--- /dev/null
++++ b/tests/wpt/web-platform-tests/non-utf8-file.txt
+@@ -0,0 +1 @@
++foo bér ÿ "
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py b/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py
index 156bb108c5a..1ca65c56321 100644
--- a/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py
@@ -50,7 +50,7 @@ class LocalGitRepo:
         self.path = path
         self.sync = sync
 
-    def run(self, *args, env: dict = {}):
+    def run_without_encoding(self, *args, env: dict = {}):
         command_line = ["git"] + list(args)
         logging.info("  → Execution (cwd='%s'): %s",
                      self.path, " ".join(command_line))
@@ -63,12 +63,19 @@ class LocalGitRepo:
         try:
             return subprocess.check_output(
                 command_line, cwd=self.path, env=env, stderr=subprocess.STDOUT
-            ).decode("utf-8")
+            )
         except subprocess.CalledProcessError as exception:
             logging.warning("Process execution failed with output:\n%s",
-                            exception.output.decode("utf-8"))
+                            exception.output.decode("utf-8", errors="surrogateescape"))
             raise exception
 
+    def run(self, *args, env: dict = {}):
+        return (
+            self
+            .run_without_encoding(*args, env=env)
+            .decode("utf-8", errors="surrogateescape")
+        )
+
 
 @dataclasses.dataclass()
 class SyncRun:
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/step.py b/etc/ci/upstream-wpt-changes/wptupstreamer/step.py
index 8c9d5c9a93b..9781353ce15 100644
--- a/etc/ci/upstream-wpt-changes/wptupstreamer/step.py
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/step.py
@@ -114,7 +114,9 @@ class CreateOrUpdateBranchForPRStep(Step):
             # TODO: If we could cleverly parse and manipulate the full commit diff
             # we could avoid cloning the servo repository altogether and only
             # have to fetch the commit diffs from GitHub.
-            diff = local_servo_repo.run(
+            # NB: The output of git show might include binary files or non-UTF8 text,
+            # so store the content of the diff as a `bytes`.
+            diff = local_servo_repo.run_without_encoding(
                 "show", "--binary", "--format=%b", sha, "--", UPSTREAMABLE_PATH
             )
 
@@ -140,7 +142,7 @@ class CreateOrUpdateBranchForPRStep(Step):
         strip_count = UPSTREAMABLE_PATH.count("/") + 1
 
         try:
-            with open(patch_path, "w", encoding="utf-8") as file:
+            with open(patch_path, "wb") as file:
                 file.write(commit["diff"])
             run.sync.local_wpt_repo.run(
                 "apply", PATCH_FILE_NAME, "-p", str(strip_count)