simpletrace: define exception and add handling

Define `SimpleException` to differentiate our exceptions from generic
exceptions (IOError, etc.). Adapted simpletrace to support this and
output to stderr.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230926103436.25700-8-mads@ynddal.dk
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/simpletrace.py

@@ -32,12 +32,15 @@
 log_header_fmt = '=QQQ'
 rec_header_fmt = '=QQII'
 
+class SimpleException(Exception):
+    pass
+
 def read_header(fobj, hfmt):
     '''Read a trace record header'''
     hlen = struct.calcsize(hfmt)
     hdr = fobj.read(hlen)
     if len(hdr) != hlen:
-        raise ValueError('Error reading header. Wrong filetype provided?')
+        raise SimpleException('Error reading header. Wrong filetype provided?')
     return struct.unpack(hfmt, hdr)
 
 def get_record(event_mapping, event_id_to_name, rechdr, fobj):
@@ -49,10 +52,10 @@ def get_record(event_mapping, event_id_to_name, rechdr, fobj):
         try:
             event = event_mapping[name]
         except KeyError as e:
-            sys.stderr.write(f'{e} event is logged but is not declared ' \
-                             'in the trace events file, try using ' \
-                             'trace-events-all instead.\n')
-            sys.exit(1)
+            raise SimpleException(
+                f'{e} event is logged but is not declared in the trace events'
+                'file, try using trace-events-all instead.'
+            )
 
         rec = (name, timestamp_ns, pid)
         for type, name in event.args:
@@ -247,8 +250,7 @@ def run(analyzer):
         *no_header, trace_event_path, trace_file_path = sys.argv[1:]
         assert no_header == [] or no_header == ['--no-header'], 'Invalid no-header argument'
     except (AssertionError, ValueError):
-        sys.stderr.write(f'usage: {sys.argv[0]} [--no-header] <trace-events> <trace-file>\n')
-        sys.exit(1)
+        raise SimpleException(f'usage: {sys.argv[0]} [--no-header] <trace-events> <trace-file>\n')
 
     with open(trace_event_path, 'r') as events_fobj, open(trace_file_path, 'rb') as log_fobj:
         process(events_fobj, log_fobj, analyzer, read_header=not no_header)
@@ -276,4 +278,8 @@ def catchall(self, event, rec):
                 i += 1
             print(' '.join(fields))
 
-    run(Formatter())
+    try:
+        run(Formatter())
+    except SimpleException as e:
+        sys.stderr.write(str(e) + "\n")
+        sys.exit(1)