[gdb/testsuite] Fix silent timeout in gdb.multi/multi-target.exp

While running test-case gdb.multi/multi-target.exp, I observed a silent
timeout related to "monitor exit".

By making the timeout explicit in an expect clause in gdbserver_gdb_exit:
...
+  timeout {
+    warning "Timed out waiting for EOF in server after $monitor_exit"
+  }
...
we get in the log:
...
monitor exit^M
"monitor" command not supported by this target.^M
(gdb) WARNING: Timed out waiting for EOF in server after monitor exit
...

What happens is the following:
- the inferior 5 is selected
- a breakpoint is set in inferior 1
- the breakpoint triggers and we switch to inferior 1
- setup is called by test_continue, which calls clean_restart, which calls
  gdbserver_gdb_exit (due to load_lib gdbserver-support.exp)
- gdbserver_gdb_exit issues "monitor exit"
- gdb responds with "not supported by this target" because inferior 1 is
  native

Fix this by keeping a list of server_spawn_id, and cleaning those up before
calling gdbserver_gdb_exit.

This reduces testing time from 1m22s to 32s.

gdb/testsuite/ChangeLog:

2020-04-02  Tom de Vries  <tdevries@suse.de>

	* lib/gdbserver-support.exp (gdbserver_exit): Factor out of ...
	(gdbserver_gdb_exit): ... here.  Add timeout warning.
	* gdb.multi/multi-target.exp (server_spawn_ids): New global var.
	(connect_target_extended_remote): Append new server_spawn_id to
	server_spawn_ids.
	(cleanup): New proc.
	(setup, <toplevel>): Call cleanup.

1 files changed, 27 insertions, 13 deletions

diff --git a/gdb/testsuite/lib/gdbserver-support.exp b/gdb/testsuite/lib/gdbserver-support.exp
index 706bbeb..a2cc80f 100644
--- a/gdb/testsuite/lib/gdbserver-support.exp
+++ b/gdb/testsuite/lib/gdbserver-support.exp
@@ -431,21 +431,11 @@ if { [info procs gdbserver_orig_gdb_exit] == "" } {
     rename mi_gdb_exit gdbserver_orig_mi_gdb_exit
 }
 
-proc gdbserver_gdb_exit { is_mi } {
+# Cleanup gdbserver $server_spawn_id
+
+proc gdbserver_exit { is_mi } {
     global gdb_spawn_id server_spawn_id
     global gdb_prompt
-    global gdbserver_reconnect_p
-
-    # Leave GDBserver running if we're exiting GDB in order to
-    # reconnect to the same instance of GDBserver again.
-    if {[info exists gdbserver_reconnect_p] && $gdbserver_reconnect_p} {
-	if { $is_mi } {
-	    gdbserver_orig_mi_gdb_exit
-	} else {
-	    gdbserver_orig_gdb_exit
-	}
-	return
-    }
 
     if {[info exists gdb_spawn_id] && [info exists server_spawn_id]} {
 	# GDB may be terminated in an expected way or an unexpected way,
@@ -469,10 +459,34 @@ proc gdbserver_gdb_exit { is_mi } {
 		    wait -i $expect_out(spawn_id)
 		    unset server_spawn_id
 		}
+               timeout {
+                   warning "Timed out waiting for EOF in server after $monitor_exit"
+               }
 	    }
 	}
     }
     close_gdbserver
+}
+
+# Local version of gdb_exit that also cleans up gdbserver $server_spawn_id.
+
+proc gdbserver_gdb_exit { is_mi } {
+    global gdb_spawn_id server_spawn_id
+    global gdb_prompt
+    global gdbserver_reconnect_p
+
+    # Leave GDBserver running if we're exiting GDB in order to
+    # reconnect to the same instance of GDBserver again.
+    if {[info exists gdbserver_reconnect_p] && $gdbserver_reconnect_p} {
+	if { $is_mi } {
+	    gdbserver_orig_mi_gdb_exit
+	} else {
+	    gdbserver_orig_gdb_exit
+	}
+	return
+    }
+
+    gdbserver_exit $is_mi
 
     if { $is_mi } {
 	gdbserver_orig_mi_gdb_exit