sshx protocol from MS Windows occasionally cuts off results in M-x rgrep

[Jim Porter]

This is an odd issue, and I'm not sure what's happening yet: sometimes
when calling `M-x rgrep' or other similar commands, I don't see all
the results I'd expect. I'm consistently able to reproduce the bug but
it only appears rarely, so it usually requires refreshing the rgrep
buffer repeatedly until I get unlucky:

  * Start emacs from an MS Windows client
  * Open tramp/info/dir remotely:
    `C-x C-f /sshx:server:~/src/tramp/info/dir'
  * rgrep for "info" in all files of tramp/info:
    `M-x rgrep RET info RET RET RET'
  * There should be 72 results (as of commit f00e5853)
  * Switch to the rgrep buffer and hit g to refresh it repeatedly
  * Every so often (maybe 5% of the time), fewer than 72 results are
    returned(!)

I can reproduce the issue on both Emacs 27.2 and the Emacs 28 snapshot
from 2021-01-15, using both the built-in Tramp and v2.5.0.4 from GNU
ELPA. However, I've tested this on a Linux client and everything is
ok. I've also tested from MS Windows using plinkx and everything is ok
then too. Manually running the same commands in a command prompt
appears to work as well. Strangely, this seems to happen most often
when there are approximately 70 results; I haven't seen it when the
expected number of results is very large or small.

Here are some other details about my setup that may be affecting this:

  * I'm connecting to a Linux VM in VirtualBox using bridged networking
  * ssh -V on the MS Windows client gives:
    OpenSSH_for_Windows_7.7p1, LibreSSL 2.6.5
  * ssh -V on the Linux VM gives:
    OpenSSH_8.2p1 Ubuntu-4ubuntu0.2, OpenSSL 1.1.1f  31 Mar 2020

I can provide a debug log, but with `tramp-verbose' set to 6, I
couldn't see any difference in the logs between good and bad runs of
rgrep. If there are other steps I can take to diagnose this, just let
me know; hopefully the steps above will work on other systems though.

Other than this (which I only just noticed) Tramp v2.5.0.4 has been
working very well, and I was able to remove about 40 lines of
tweaks/fixes from my .emacs!

Thanks,
- Jim