ci: cap build errors in the parser instead of piping into head

.claude/skills/fix-issue/scripts/lib/platform-apple.sh

@@ -223,7 +223,7 @@ apply_tally() {
 # Build errors with their file and line, which the result bundle keeps even when the log interleaves them.
 report_build_errors() {
     local errors
-    errors="$(python3 "$XCRESULT_CASES" errors "$1" | head -12)"
+    errors="$(python3 "$XCRESULT_CASES" errors --limit 12 "$1")"
     [ -n "$errors" ] || return 1
     note "errors:"
     note "$(sed 's/^/  /' <<< "$errors")"

scripts/lib/test_xcresult_cases.py

@@ -175,6 +175,34 @@ def test_no_build_results(self):
         self.assertEqual(list(parser.build_errors(None)), [])
 
 
+class ErrorsCommandTests(unittest.TestCase):
+    def setUp(self):
+        self.original = parser.xcresulttool
+        parser.xcresulttool = lambda bundle, *arguments: BROKEN_BUILD if arguments == ("build-results",) else None
+
+    def tearDown(self):
+        parser.xcresulttool = self.original
+
+    def errors(self, *arguments):
+        with redirect_stdout(io.StringIO()) as out:
+            code = parser.main(["errors", *arguments])
+        return code, out.getvalue().splitlines()
+
+    def test_prints_file_name_line_and_message(self):
+        self.assertEqual(self.errors("Build.xcresult"), (0, [
+            "Broken.swift:4: Cannot convert value of type 'String' to specified type 'Int'",
+            "Linker command failed with exit code 1",
+        ]))
+
+    def test_limit_stops_after_n_errors(self):
+        self.assertEqual(self.errors("--limit", "1", "Build.xcresult"),
+                         (0, ["Broken.swift:4: Cannot convert value of type 'String' to specified type 'Int'"]))
+
+    def test_a_bad_limit_is_a_usage_error(self):
+        with redirect_stderr(io.StringIO()):
+            self.assertEqual(self.errors("--limit", "x", "Build.xcresult")[0], 2)
+
+
 class XunitTests(unittest.TestCase):
     def setUp(self):
         self.directory = tempfile.TemporaryDirectory()

scripts/lib/xcresult-cases.py

@@ -4,8 +4,8 @@
 Usage:
   xcresult-cases.py tally [--filters <file>] <input>...
       verify.sh's notes, then "<executed> <failed> <filters that ran nothing> <runner failures>"
-  xcresult-cases.py errors <bundle.xcresult>
-      build errors as "<file>:<line>: <message>"
+  xcresult-cases.py errors [--limit <n>] <bundle.xcresult>
+      the first n build errors as "<file>:<line>: <message>"
   xcresult-cases.py summary <input>...
       the CI step summary; exits 1 when a bundle or a swift test run executed no test,
       which xcodebuild and swift test both call success
@@ -21,6 +21,7 @@
 import urllib.parse
 import xml.etree.ElementTree as ET
 from collections import Counter, namedtuple
+from itertools import islice
 from pathlib import Path
 
 Case = namedtuple("Case", "target suite name label result retried seconds message")
@@ -252,10 +253,15 @@ def main(arguments):
             filters, rest = read_filters(rest[1]), rest[2:]
         print("\n".join(tally([case for argument in rest for case in cases_of(argument)], filters)))
         return 0
-    if command == "errors" and len(rest) == 1:
-        for where, message in build_errors(xcresulttool(Path(rest[0]), "build-results")):
-            print(f"{Path(where).name}: {message}" if where else message)
-        return 0
+    if command == "errors":
+        limit = None
+        if rest[:1] == ["--limit"] and len(rest) > 1 and rest[1].isdigit():
+            limit, rest = int(rest[1]), rest[2:]
+        if len(rest) == 1:
+            errors = islice(build_errors(xcresulttool(Path(rest[0]), "build-results")), limit)
+            for where, message in errors:
+                print(f"{Path(where).name}: {message}" if where else message)
+            return 0
     if command == "summary":
         destination = os.environ.get("GITHUB_STEP_SUMMARY")
         if not destination: