D2392: debugcommands: add debugwireproto command

[indygreg (Gregory Szorc)]

indygreg planned changes to this revision.
indygreg added inline comments.

INLINE COMMENTS

> debugcommands.py:2630
> +        # separation. This prevents a whole class of potential bugs around
> +        # shared state from interfering with server operation.
> +

I'm having second thoughts about this.

The reason is that from a testing perspective (which is the primary driver behind this work), `read()` isn't very reliable because of timing issues. Depending on operating system settings, system performance, etc, operations like `read(-1)` can return a variable number of bytes because they return only what's available on the wire.

`write()`, however, is more reliable. When you `write()` to something in Python, Python makes as many system calls as necessary to ensure all bytes are delivered. So a `write()` at the Python level is mostly deterministic.

I think the concerns around process separation here aren't that significant. So I think I'm going to rework this (yet again) to spawn the SSH server in process and to only monitor I/O operations that are deterministic. This may mean only monitoring `write()` calls on pipes and *possibly* monitoring `readline()` and `read(N)`. But if we monitor `write()` on both peers since they are both in-process, then `read()` monitoring is redundant. That could be useful to debug behavior. But for tests demonstrating the wire protocol exchange, it's less useful.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2392

To: indygreg, #hg-reviewers
Cc: sid0, mercurial-devel