[PATCH, RFC] fix gdbserver channel leaks

[Pedro Alves]

On 01/16/2019 09:18 PM, Sandra Loosemore wrote:
> I've been trying to remove some ancient cruft in our local remote-target gdbserver test harness that has caused problems with some newer GDB tests.  This has exposed what I think is a problem in GDB's own gdbserver-support.exp:  having opened a channel to the gdbserver spawn in gdbserver_start, in many cases it is failing to close it cleanly, causing it to leak so many open channels that eventually TCL runs out and starts giving ERRORs.  Specifically, gdbserver_start is discarding any already-open server_spawn_id without closing it when opening a new channel, and the logic in gdbserver_gdb_exit that tries to clean up the gdbserver connection is failing to actually close the channel before discarding server_spawn_id.

When I read this, a question comes to mind:

How does it happen that we're starting a new gdbserver
without closing the original one?  It sounds like this may
be papering over a bug elsewhere?  Shouldn't that be
an error instead?  Why are we "failing to close it cleanly"?

I mean, default_gdb_spawn doesn't start a new gdb if
there's already one either.

Thanks,
Pedro Alves

> 
> I've thought it best to consolidate the logic to manage server_spawn_id in start_gdbserver and close_gdbserver.  It did seem plausible to put the logic to shut down any old server_spawn_id channel before opening a new one in gdbserver_run instead of in start_gdbserver, but there are places that bypass that function (e.g., in the mi test support).
> 
> The alternative to fixing this in GDB's own test support seems to be doing it at the DejaGnu board level instead, which is what I'm trying to get rid of in our local test harness.  There's no documentation about the expectations and it seems pretty hacky and a violation of abstractions to do it in the low-level board support.
> 
> I'm wondering, can other people who do remote-target gdbserver testing help try out this patch and see if it works for them?  I've gotten good results with my nios2-linux-gnu test setup but I don't know what kind of board support other people might be using.  Also, assuming the patch is OK, I don't know if it's inappropriate or too late for the 8.3 release.