[PATCH v2 1/3] perf jevents: Name the metric when a MetricExpr is missing

[TAKANO Mitsuhiro <takano32@gmail.com>]

jevents.py reads each field it wants out of a JSON object by name, so a
MetricExpr key that is missing or spelled another way leaves the object
with a MetricName and no expression. Two different failures followed,
neither of which named the metric: the rewrite in _read_json_events_impl()
raised AttributeError on the None, and a metric that got that far reached
print_pending_metrics(), whose assert fired on it.

Raise in the reader instead, naming the metric, before either of them can
happen. The check is not inside the '-' test that decides whether a metric
takes part in the rewrite: a name with a dash in it can lose its
expression the same way.

Two smaller changes on the same path:

- An architecture standard table that does not parse now carries the
   reason in its message ("Failure processing '<file>' in '<dir>':
   <cause>").  The traceback used to end with the file name alone, and
   only the chained exception said why.

- A table that does not parse is reported on stderr with a "jevents:
   error:" prefix instead of on stdout, where it was mixed with the
   generated output.

And the output file is opened after the tables have been read, and not by
argparse, so a build that fails while reading them leaves a file the
caller named as it was instead of truncating it to the header.

Signed-off-by: TAKANO Mitsuhiro <takano32@gmail.com>
---
 tools/perf/pmu-events/jevents.py | 35 ++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
index 080e04b51b8c..86060a228b75 100755
--- a/tools/perf/pmu-events/jevents.py
+++ b/tools/perf/pmu-events/jevents.py
@@ -460,11 +460,17 @@ def _read_json_events_impl(path: str, topic: str) -> Sequence[JsonEvent]:
     with open(path, encoding='utf-8') as f:
       events = json.load(f, object_hook=JsonEvent)
   except BaseException:
-    print(f"Exception processing {path}")
+    print(f'jevents: error: Exception processing {path}', file=sys.stderr)
     raise
   metrics: list[Tuple[str, str, metric.Expression]] = []
   for event in events:
     event.topic = topic
+    if event.metric_name and event.metric_expr is None:
+      # A MetricName without a MetricExpr leaves nothing to put in the
+      # generated tables, whether or not the name has a dash, so the build
+      # must not go on.  Name the metric instead of letting the table printer
+      # assert on the None.
+      raise RuntimeError(f'Metric {event.metric_name!r} has no MetricExpr')
     if event.metric_name and '-' not in event.metric_name:
       metrics.append((event.pmu, event.metric_name, event.metric_expr))
   updates = metric.RewriteMetricsInTermsOfOthers(metrics)
@@ -493,7 +499,7 @@ def preprocess_arch_std_files(archpath: str) -> None:
         if event.metric_name:
           _arch_std_events[event.metric_name.lower()] = event
     except Exception as e:
-        raise RuntimeError(f'Failure processing \'{item.name}\' in \'{archpath}\'') from e
+      raise RuntimeError(f'Failure processing \'{item.name}\' in \'{archpath}\': {e}') from e
 
 
 def add_events_table_entries(item: os.DirEntry, topic: str) -> None:
@@ -1490,12 +1496,29 @@ such as "arm/cortex-a34".''',
       type=dir_path,
       help='Root of tree containing architecture directories containing json files'
   )
-  ap.add_argument(
-      'output_file', type=argparse.FileType('w', encoding='utf-8'), nargs='?', default=sys.stdout)
-  ap.add_argument(
-      'output_string_file', type=argparse.FileType('w', encoding='utf-8'), nargs='?', default=None)
+  ap.add_argument('output_file', nargs='?', default=None,
+                  help='file to write the tables to, default standard output')
+  ap.add_argument('output_string_file', nargs='?', default=None,
+                  help='file to write big_c_string to when it is to be kept '
+                       'apart from the tables')
   _args = ap.parse_args()
 
+  # Opened here, and not by argparse, so that a failure before the tables are
+  # written cannot empty a file the caller named.
+  if _args.output_file in (None, '-'):
+    _args.output_file = sys.stdout
+  else:
+    try:
+      _args.output_file = open(_args.output_file, 'w', encoding='utf-8')
+    except OSError as e:
+      ap.error(f'argument output_file: {e}')
+  if _args.output_string_file == '-':
+    _args.output_string_file = sys.stdout
+  elif _args.output_string_file is not None:
+    try:
+      _args.output_string_file = open(_args.output_string_file, 'w', encoding='utf-8')
+    except OSError as e:
+      ap.error(f'argument output_string_file: {e}')
   _args.output_file.write(f"""/* SPDX-License-Identifier: GPL-2.0 */
 /* THIS FILE WAS AUTOGENERATED BY `jevents.py arch={_args.arch} model={_args.model}` ! */
 """)
-- 
2.53.0