trace: Forbid event format ending with newline character

Event format ending with newlines confuse the trace reports.
Forbid them.

Add a check to refuse new format added with trailing newline:

  $ make
  [...]
    GEN     hw/misc/trace.h
  Traceback (most recent call last):
    File "scripts/tracetool.py", line 152, in <module>
      main(sys.argv)
    File "scripts/tracetool.py", line 143, in main
      events.extend(tracetool.read_events(fh, arg))
    File "scripts/tracetool/__init__.py", line 367, in read_events
      event = Event.build(line)
    File "scripts/tracetool/__init__.py", line 281, in build
      raise ValueError("Event format can not end with a newline character")
  ValueError: Error at hw/misc/trace-events:121: Event format can not end with a newline character

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190916095121.29506-3-philmd@redhat.com
Message-Id: <20190916095121.29506-3-philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/devel/tracing.txt

@@ -112,6 +112,8 @@ Trace events should use types as follows:
 Format strings should reflect the types defined in the trace event.  Take
 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
 respectively.  This ensures portability between 32- and 64-bit platforms.
+Format strings must not end with a newline character.  It is the responsibility
+of backends to adapt line ending for proper logging.
 
 Each event declaration will start with the event name, then its arguments,
 finally a format string for pretty-printing. For example:

scripts/tracetool/__init__.py

@@ -277,6 +277,9 @@ def build(line_str):
         if fmt.find("%m") != -1 or fmt_trans.find("%m") != -1:
             raise ValueError("Event format '%m' is forbidden, pass the error "
                              "as an explicit trace argument")
+        if fmt.endswith(r'\n"'):
+            raise ValueError("Event format must not end with a newline "
+                             "character")
 
         if len(fmt_trans) > 0:
             fmt = [fmt_trans, fmt]