[PATCH v6 1/3] perf jevents: Name the metric and the file 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 and the file it came from,
before either of them can happen.  The file matters because the tables are
read through a worker pool in main(), outside the directory walk, so an
exception there loses the name of the table on its way out.  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 the
  reader already knows the cause.

- The output files are opened after the tables have been read, so a tree
  that fails to parse leaves no half-written table behind.

Signed-off-by: TAKANO Mitsuhiro <takano32@gmail.com>
---
v6: patch 1 names the file as well as the metric when a MetricExpr is
    missing (the tables are read through a worker pool, outside the
    walk, so the path was lost).  The perf test's one-line description
    ends in "(no_mlock)" so perf test lists it whole and the test does
    not take a memlock slot.  No other change.

 tools/perf/pmu-events/jevents.py | 79 +++++++++++++++++++++-----------
 1 file changed, 52 insertions(+), 27 deletions(-)

diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
index 080e04b51b8c..843748915c12 100755
--- a/tools/perf/pmu-events/jevents.py
+++ b/tools/perf/pmu-events/jevents.py
@@ -460,11 +460,18 @@ 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'{path}: 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 +500,7 @@ 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:
@@ -1490,33 +1497,13 @@ 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()
 
-  _args.output_file.write(f"""/* SPDX-License-Identifier: GPL-2.0 */
-/* THIS FILE WAS AUTOGENERATED BY `jevents.py arch={_args.arch} model={_args.model}` ! */
-""")
-  _args.output_file.write("""
-#include <pmu-events/pmu-events.h>
-#include "util/header.h"
-#include "util/pmu.h"
-#include <string.h>
-#include <stddef.h>
-
-struct compact_pmu_event {
-\tint offset;
-};
-
-struct pmu_table_entry {
-\tconst struct compact_pmu_event *entries;
-\tuint32_t num_entries;
-\tstruct compact_pmu_event pmu_name;
-};
-
-""")
   archs = []
   for item in os.scandir(_args.starting_dir):
     if not item.is_dir():
@@ -1553,6 +1540,44 @@ struct pmu_table_entry {
 
   assert _bcs is not None
   _bcs.compute()
+
+  # Every table has been read by now, so opening the output here cannot truncate
+  # a file the caller named when a table turns out not to parse.
+  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}` ! */
+""")
+  _args.output_file.write("""
+#include <pmu-events/pmu-events.h>
+#include "util/header.h"
+#include "util/pmu.h"
+#include <string.h>
+#include <stddef.h>
+
+struct compact_pmu_event {
+\tint offset;
+};
+
+struct pmu_table_entry {
+\tconst struct compact_pmu_event *entries;
+\tuint32_t num_entries;
+\tstruct compact_pmu_event pmu_name;
+};
+
+""")
   _args.output_file.write('/* clang-format off */\n')
   if not _args.output_string_file:
     _args.output_file.write('static const char *const big_c_string =\n')
-- 
2.53.0