otherwise we explode with:
```
Traceback (most recent call last):
File /usr/lib/python2.7/logging/handlers.py, line 78, in emit
logging.FileHandler.emit(self, record)
File /usr/lib/python2.7/logging/__init__.py, line 950, in emit
StreamHandler.emit(self, record)
File /usr/lib/python2.7/logging/__init__.py, line 887, in emit
self.handleError(record)
File /usr/lib/python2.7/logging/__init__.py, line 810, in handleError
None, sys.stderr)
File /usr/lib/python2.7/traceback.py, line 124, in print_exception
_print(file, 'Traceback (most recent call last):')
File /usr/lib/python2.7/traceback.py, line 13, in _print
file.write(str+terminator)
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_io.py, line 170, in write
self.log.emit(self.level, format=u{log_io}, log_io=line)
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_logger.py, line 144, in emit
self.observer(event)
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_observer.py, line 136, in __call__
errorLogger = self._errorLoggerForObserver(brokenObserver)
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_observer.py, line 156, in _errorLoggerForObserver
if obs is not observer
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_observer.py, line 81, in __init__
self.log = Logger(observer=self)
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_logger.py, line 64, in __init__
namespace = self._namespaceFromCallingContext()
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/logger/_logger.py, line 42, in _namespaceFromCallingContext
return currentframe(2).f_globals[__name__]
File /home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/python/compat.py, line 93, in currentframe
for x in range(n + 1):
RuntimeError: maximum recursion depth exceeded while calling a Python object
Logged from file site.py, line 129
File /usr/lib/python2.7/logging/__init__.py, line 859, in emit
msg = self.format(record)
File /usr/lib/python2.7/logging/__init__.py, line 732, in format
return fmt.format(record)
File /usr/lib/python2.7/logging/__init__.py, line 471, in format
record.message = record.getMessage()
File /usr/lib/python2.7/logging/__init__.py, line 335, in getMessage
msg = msg % self.args
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 4: ordinal not in range(128)
Logged from file site.py, line 129
```
...where the logger apparently recurses whilst trying to log the error, hitting the
maximum recursion depth and killing everything badly.
When we finish processing a request, log the number of events we fetched from
the database to handle it.
[I'm trying to figure out which requests are responsible for large amounts of
event cache churn. It may turn out to be more helpful to add counts to the
prometheus per-request/block metrics, but that is an extension to this code
anyway.]
(instead of everywhere that writes a response. Or rather, the subset of places
which write responses where we haven't forgotten it).
This also means that we don't have to have the mysterious version_string
attribute in anything with a request handler.
Unfortunately it does mean that we have to pass the version string wherever we
instantiate a SynapseSite, which has been c&ped 150 times, but that is code
that ought to be cleaned up anyway really.
For each request, track the amount of time spent waiting for a db
connection. This entails adding it to the LoggingContext and we may as well add
metrics for it while we are passing.
