Add a note about REPLICATE requiring a single space

[turt2live]

Otherwise errors happen server-side. A more correct fix might be to just remove the requirement, though given this is the only command which shows up bare it's probably fine to just document it instead.

The error, for reference:

2020-04-22 02:41:58,808 - twisted - 192 - CRITICAL -  - Unhandled Error
Traceback (most recent call last):
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/python/log.py", line 103, in callWithLogger
    return callWithContext({"system": lp}, func, *args, **kw)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/python/log.py", line 86, in callWithContext
    return context.call({ILogContext: newCtx}, func, *args, **kw)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/python/context.py", line 122, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/python/context.py", line 85, in callWithContext
    return func(*args,**kw)
--- <exception caught here> ---
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/internet/posixbase.py", line 614, in _doReadOrWrite
    why = selectable.doRead()
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/internet/tcp.py", line 243, in doRead
    return self._dataReceived(data)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/internet/tcp.py", line 249, in _dataReceived
    rval = self.protocol.dataReceived(data)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/twisted/protocols/basic.py", line 454, in dataReceived
    self.lineReceived(line)
  File "/home/matrix/.synapse/local/lib/python3.6/site-packages/synapse/replication/tcp/protocol.py", line 214, in lineReceived
    idx = linestr.index(" ")
builtins.ValueError: substring not found

This behaviour was added in #7024

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

Done with a community hat on:

Signed-off-by: Travis Ralston <travis@t2bot.io>

[richvdh]

what a massive idiot I am. I tried to fix this in e13c6c7, but was too lazy to write tests, and messed it up. I'll have another go.

[richvdh]

going to close this in favour of #7326. Thanks though!

[turt2live]

Looks like the fix was easier than anticipated too - thanks for taking a look :D