[gdb/remote] Fix invalid pointer in remote_async_serial_handler

On rare occasions, we run into this ERROR/UNRESOLVED on gdb-10-branch:
...
(gdb) PASS: gdb.multi/multi-target.exp: continue: non-stop=on: inferior 2
Remote debugging from host ::1, port 34088^M
Process outputs/gdb.multi/multi-target/multi-target created; pid = 8649^M
monitor exit^M
(gdb) Killing process(es): 8649^M
ERROR: GDB process no longer exists
GDB process exited with wait status 8627 exp14 0 0 CHILDKILLED SIGABRT SIGABRT
UNRESOLVED: gdb.multi/multi-target.exp: continue: non-stop=on: inferior 5
...

A trigger patch makes the crash happen all the time:
...
diff --git a/gdb/remote.c b/gdb/remote.c
index 71f814efb365..53ff8b63a1dc 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -14161,14 +14161,12 @@ remote_target::is_async_p ()
    will be able to delay notifying the client of an event until the
    point where an entire packet has been received.  */

-static serial_event_ftype remote_async_serial_handler;
-
 static void
 remote_async_serial_handler (struct serial *scb, void *context)
 {
-  /* Don't propogate error information up to the client.  Instead let
-     the client find out about the error by querying the target.  */
-  inferior_event_handler (INF_REG_EVENT);
+  remote_state *rs = (remote_state *) context;
+
+  mark_async_event_handler (rs->remote_async_inferior_event_token);
 }

 static void
...

And using -fsanitizer=address we can get a more elaborate error message:
...
==7196==ERROR: AddressSanitizer: heap-use-after-free on address \
  0x6170000bf258 at pc 0x000001481755 bp 0x7fff05b20840 sp 0x7fff05b20838
READ of size 8 at 0x6170000bf258 thread T0
    #0 0x1481754 in std::_Hashtable<gdbarch*, std::pair<gdbarch* const,
    remote_arch_state>, std::allocator<std::pair<gdbarch* const,
    remote_arch_state> >, std::__detail::_Select1st, std::equal_to<gdbarch*>,
    std::hash<gdbarch*>, std::__detail::_Mod_range_hashing,
    std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy,
    std::__detail::_Hashtable_traits<false, false, true>
    >::_M_bucket_index(unsigned long) const
    /usr/include/c++/11/bits/hashtable.h:719
    #1 0x147c8ab in std::_Hashtable<gdbarch*, std::pair<gdbarch* const,
    remote_arch_state>, std::allocator<std::pair<gdbarch* const,
    remote_arch_state> >, std::__detail::_Select1st, std::equal_to<gdbarch*>,
    std::hash<gdbarch*>, std::__detail::_Mod_range_hashing,
    std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy,
    std::__detail::_Hashtable_traits<false, false, true> >::find(gdbarch*
    const&) /usr/include/c++/11/bits/hashtable.h:1500
    #2 0x147852c in std::unordered_map<gdbarch*, remote_arch_state,
    std::hash<gdbarch*>, std::equal_to<gdbarch*>,
    std::allocator<std::pair<gdbarch* const, remote_arch_state> >
    >::find(gdbarch* const&) /usr/include/c++/11/bits/unordered_map.h:869
    #3 0x14306db in remote_state::get_remote_arch_state(gdbarch*)
    src/gdb/remote.c:1203
    #4 0x14309dc in remote_target::get_remote_state() src/gdb/remote.c:1232
    #5 0x1470c08 in remote_async_inferior_event_handler src/gdb/remote.c:14169
    #6 0xaa9f6b in check_async_event_handlers() src/gdb/async-event.c:295
    #7 0x1e93ab4 in gdb_do_one_event() src/gdbsupport/event-loop.cc:194
    #8 0x118f5f9 in start_event_loop src/gdb/main.c:356
    #9 0x118f8ed in captured_command_loop src/gdb/main.c:416
    #10 0x1192d6a in captured_main src/gdb/main.c:1253
    #11 0x1192dfa in gdb_main(captured_main_args*) src/gdb/main.c:1268
    #12 0x97b380 in main src/gdb/gdb.c:32
    #13 0x7f550c211349 in __libc_start_main ../csu/libc-start.c:308
    #14 0x97b199 in _start (build/gdb/gdb+0x97b199)

0x6170000bf258 is located 600 bytes inside of 648-byte region \
  [0x6170000bf000,0x6170000bf288)
freed by thread T0 here:
    #0 0x7f550f516a57 in operator delete(void*, unsigned long)
    (/usr/lib64/libasan.so.6+0xaea57)
    #1 0x148b1fe in extended_remote_target::~extended_remote_target()
    src/gdb/remote.c:958
    #2 0x143b483 in remote_target::close() src/gdb/remote.c:4074
    #3 0x16cb90f in target_close(target_ops*) src/gdb/target.c:3230
    #4 0x16a2635 in decref_target(target_ops*) src/gdb/target.c:557
    #5 0x16a2abb in target_stack::unpush(target_ops*) src/gdb/target.c:645
    #6 0x16d01ef in inferior::unpush_target(target_ops*)
    src/gdb/inferior.h:356
    #7 0x16a2877 in unpush_target(target_ops*) src/gdb/target.c:607
    #8 0x16a2adf in unpush_target_and_assert src/gdb/target.c:655
    #9 0x16a2c57 in pop_all_targets_at_and_above(strata) src/gdb/target.c:678
    #10 0x1442749 in remote_unpush_target src/gdb/remote.c:5522
    #11 0x1458c16 in remote_target::readchar(int) src/gdb/remote.c:9137
    #12 0x145b25b in remote_target::getpkt_or_notif_sane_1(std::vector<char,
    gdb::default_init_allocator<char, std::allocator<char> > >*, int, int,
    int*) src/gdb/remote.c:9683
    #13 0x145bc9a in remote_target::getpkt_sane(std::vector<char,
    gdb::default_init_allocator<char, std::allocator<char> > >*, int)
    src/gdb/remote.c:9790
    #14 0x145b040 in remote_target::getpkt(std::vector<char,
    gdb::default_init_allocator<char, std::allocator<char> > >*, int)
    src/gdb/remote.c:9623
    #15 0x145780b in remote_target::remote_read_bytes_1(unsigned long,
    unsigned char*, unsigned long, int, unsigned long*) src/gdb/remote.c:8860
    #16 0x145805e in remote_target::remote_read_bytes(unsigned long,
    unsigned char*, unsigned long, int, unsigned long*) src/gdb/remote.c:8987
    #17 0x146113a in remote_target::xfer_partial(target_object, char const*,
    unsigned char*, unsigned char const*, unsigned long, unsigned long,
    unsigned long*) src/gdb/remote.c:10987
    #18 0x16a4004 in raw_memory_xfer_partial(target_ops*, unsigned char*,
    unsigned char const*, unsigned long, long, unsigned long*)
    src/gdb/target.c:918
    #19 0x16a4fcf in target_xfer_partial(target_ops*, target_object, char
    const*, unsigned char*, unsigned char const*, unsigned long, unsigned
    long, unsigned long*) src/gdb/target.c:1156
    #20 0x16a5d65 in target_read_partial src/gdb/target.c:1387
    #21 0x16a5f19 in target_read(target_ops*, target_object, char const*,
    unsigned char*, unsigned long, long) src/gdb/target.c:1427
    #22 0x16a5666 in target_read_raw_memory(unsigned long, unsigned char*,
    long) src/gdb/target.c:1260
    #23 0xd22f2a in dcache_read_line src/gdb/dcache.c:336
    #24 0xd232b7 in dcache_peek_byte src/gdb/dcache.c:403
    #25 0xd23845 in dcache_read_memory_partial(target_ops*, dcache_struct*,
    unsigned long, unsigned char*, unsigned long, unsigned long*) src/gdb/dcache.c:484
    #26 0x16a47da in memory_xfer_partial_1 src/gdb/target.c:1041
    #27 0x16a4a1e in memory_xfer_partial src/gdb/target.c:1084
    #28 0x16a4f44 in target_xfer_partial(target_ops*, target_object,
    char const*, unsigned char*, unsigned char const*, unsigned long,
    unsigned long, unsigned long*) src/gdb/target.c:1141
    #29 0x18203d4 in read_value_memory(value*, long, int, unsigned long,
    unsigned char*, unsigned long) src/gdb/valops.c:956

previously allocated by thread T0 here:
    #0 0x7f550f515c37 in operator new(unsigned long)
    (/usr/lib64/libasan.so.6+0xadc37)
    #1 0x14429f0 in remote_target::open_1(char const*, int, int)
    src/gdb/remote.c:5562
    #2 0x14405e6 in extended_remote_target::open(char const*, int)
    src/gdb/remote.c:4907
    #3 0x16a0f3c in open_target src/gdb/target.c:242
    #4 0xc19ff5 in do_sfunc src/gdb/cli/cli-decode.c:111
    #5 0xc221db in cmd_func(cmd_list_element*, char const*, int)
    src/gdb/cli/cli-decode.c:2181
    #6 0x16feda6 in execute_command(char const*, int) src/gdb/top.c:668
    #7 0xee9dc9 in command_handler(char const*) src/gdb/event-top.c:588
    #8 0xeea6a8 in command_line_handler(std::unique_ptr<char,
    gdb::xfree_deleter<char> >&&) src/gdb/event-top.c:773
    #9 0xee8a12 in gdb_rl_callback_handler src/gdb/event-top.c:219
    #10 0x7f550f24aead in rl_callback_read_char
    (/lib64/libreadline.so.7+0x31ead)
...

The problem is here in remote_async_inferior_event_handler:
...
static void
remote_async_inferior_event_handler (gdb_client_data data)
{
  inferior_event_handler (INF_REG_EVENT);

  remote_target *remote = (remote_target *) data;
  remote_state *rs = remote->get_remote_state ();
...

The remote target (passed in the data argument) can be destroyed during the
call to inferior_event_handler.  If so, the call to remote->get_remote_state
() is done using a dangling pointer.

Fix this by increasing the reference count on the remote target before calling
inferior_event_handler, such that it won't get destroyed until right before
returning from remote_async_inferior_event_handler.

Tested on x86_64-linux.

Intended for gdb-10-branch.

The problem has stopped reproducing with the trigger patch since master commit
79952e6 "Make scoped_restore_current_thread's cdtors exception free
(RFC)".  We could still apply this to master though.

gdb/ChangeLog:

2021-01-07  Pedro Alves  <[email protected]>
	    Simon Marchi  <[email protected]>
	    Tom de Vries  <[email protected]>

	PR remote/26614
	* remote.c (remote_async_inferior_event_handler):  Hold a strong
	reference to the remote target while handling an event.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2021-01-07  Pedro Alves  <[email protected]>
+	    Simon Marchi  <[email protected]>
+	    Tom de Vries  <[email protected]>
+
+	PR remote/26614
+	* remote.c (remote_async_inferior_event_handler):  Hold a strong
+	reference to the remote target while handling an event.
+
 2021-01-01  Joel Brobecker  <[email protected]>
 
 	* gdbarch.sh: Update copyright year range.

gdb/remote.c

@@ -14163,9 +14163,13 @@ remote_async_serial_handler (struct serial *scb, void *context)
 static void
 remote_async_inferior_event_handler (gdb_client_data data)
 {
+  remote_target *remote = (remote_target *) data;
+  /* Hold a strong reference to the remote target while handling an
+     event, since that could result in closing the connection.  */
+  auto remote_ref = target_ops_ref::new_reference (remote);
+
   inferior_event_handler (INF_REG_EVENT);
 
-  remote_target *remote = (remote_target *) data;
   remote_state *rs = remote->get_remote_state ();
 
   /* inferior_event_handler may have consumed an event pending on the