[PATCH v2 2/4] Unregister the last inferior from the event loop

["Maciej W. Rozycki" <macro@wdc.com>]

Fix an issue with GDB starting to effectively busy-loop (though still 
accepting and interpreting commands) using the full processing power 
available when a remote connection is forcibly terminated while the 
target is running.

This was observed with RISC-V/Linux `gdbserver' modified to hang after a 
successful acceptance of a `vCont' packet, as follows:

(gdb) set debug infrun 2
(gdb) set debug remote 2
(gdb) continue
Continuing.
infrun: clear_proceed_status_thread (Thread 22854.22854)
infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
Sending packet: $Z0,10430,2#0c...Packet received:
Packet Z0 (software-breakpoint) is NOT supported
Sending packet: $m10430,2#c3...Packet received: 8147
Sending packet: $X10430,0:#e6...Packet received: OK
binary downloading supported by target
Sending packet: $X10430,2:\002\220#7a...Packet received: OK
Sending packet: $m15555607b8,2#07...Packet received: 8280
Sending packet: $X15555607b8,2:\002\220#be...Packet received: OK
infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 22854.22854] at 0x1555556ef0
Sending packet: $QPassSignals:e;10;14;17;1a;1b;1c;21;24;25;2c;4c;97;#0a...Packet received: OK
Sending packet: $vCont?#49...Packet received: vCont;c;C;t
Packet vCont (verbose-resume) is supported
Sending packet: $vCont;c:p5946.-1#b6...infrun: infrun_async(1)
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: handle_inferior_event status->kind = ignore
infrun: prepare_to_wait
^Cremote_pass_ctrlc called
remote_interrupt called
^Cremote_pass_ctrlc called
infrun: infrun_async(0)
The target is not responding to interrupt requests.
Stop debugging it? (y or n) y
infrun: infrun_async(1)
Disconnected from target.
(gdb) infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: handle_inferior_event status->kind = ignore
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: handle_inferior_event status->kind = ignore
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: handle_inferior_event status->kind = ignore
infrun: prepare_to_wait
[...]

This is because `remote_target::resume' enables the asynchronous event 
loop, as indicated by the first `infrun: infrun_async(1)' record above, 
and then the confirmation dialogue temporarily disables it and then 
reenables, as indicated by the second `infrun: infrun_async(1)' record 
above.  The problem with that approach is that the reenabling also marks 
the handler for the `infrun_async_inferior_event_token' event ready, 
even though it was not before the temporary disabling, by calling 
`mark_async_event_handler' on it, and that triggers the infinite loop as 
there's no inferior anymore and consequently no event waiting that would 
stop it.

Unregister the `infrun_async_inferior_event_token' event from the loop 
then when the last inferior has gone.  The final part of the session now 
looks like:

^Cremote_pass_ctrlc called
remote_interrupt called
^Cremote_pass_ctrlc called
infrun: infrun_async(0)
The target is not responding to interrupt requests.
Stop debugging it? (y or n) y
infrun: infrun_async(1)
infrun: infrun_async(0)
Disconnected from target.
(gdb) 

and the looping does not happen anymore.

	gdb/
	* target.c (target_stack::unpush): Unregister the last inferior 
	from the event loop.
---
New change in v2.
---
 gdb/target.c |    6 ++++++
 1 file changed, 6 insertions(+)

gdb-unpush-target-async.diff
Index: binutils-gdb/gdb/target.c
===================================================================
--- binutils-gdb.orig/gdb/target.c
+++ binutils-gdb/gdb/target.c
@@ -634,6 +634,12 @@ target_stack::unpush (target_ops *t)
      implementation don't end up in T anymore.  */
   target_close (t);
 
+  /* If this was the last inferior, then make sure it's been unregistered
+     from the event loop so that we don't get distracted by spurious
+     inferior output.  */
+  if (!have_live_inferiors ())
+    infrun_async (0);
+
   return true;
 }