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Pedro Alves 3cada74087 Fix backwards compatibility with old GDBservers (PR remote/22597) 
At <https://sourceware.org/ml/gdb-patches/2017-12/msg00285.html>,
Maciej reported that commit:

  commit 5cd63fda03
  Date: Wed Oct 4 18:21:10 2017 +0100
  Subject: Fix "Remote 'g' packet reply is too long" problems with multiple inferiors

made GDB stop working with older stubs.  Any attempt to continue
execution after the initial connection fails with:

  [...]
  Process .../gdb/testsuite/outputs/gdb.base/advance/advance created; pid = 2670
  Listening on port 2346
  target remote [...]:2346
  Remote debugging using [...]:2346
  Reading symbols from .../lib64/ld.so.1...done.
  [Switching to Thread <main>]
  (gdb) continue
  Cannot execute this command without a live selected thread.
  (gdb)

The problem is:

  (gdb) c
  Cannot execute this command without a live selected thread.
  (gdb) info threads
    Id   Target Id         Frame
    1    Thread 14917      0x00007f341cd98ed0 in _start () from /lib64/ld-linux-x86-64.so.2

  The current thread <Thread ID 2> has terminated.  See `help thread'.
		      ^^^^^^^^^^^
  (gdb)

Note, thread _2_.  There's really only one thread in the inferior
(it's still at the entry point), but still GDB added a bogus second
thread.

The reason GDB started adding a second thread after 5cd63fda03 is
this hunk:

+                 if (event->ptid == null_ptid)
+                   {
+                     const char *thr = strstr (p1 + 1, ";thread:");
+                     if (thr != NULL)
+                       event->ptid = read_ptid (thr + strlen (";thread:"),
+                                                NULL);
+                     else
+                       event->ptid = magic_null_ptid;
+                   }

Note the else branch that falls back to magic_null_ptid.  We reach
that when we process the initial stop reply sent back in response to
the the "?" (status) packet early in the connection setup:

 Sending packet: $?#3f...Ack
 Packet received: T0506:0000000000000000;07:40a510f4fd7f0000;10:d0fe1201577f0000;

And note that that response does not include a ";thread:XXX" part.

This stop reply is processed after listing threads with qfThreadInfo /
qsThreadInfo :

 Sending packet: $qfThreadInfo#bb...Ack
 Packet received: m3915
 Sending packet: $qsThreadInfo#c8...Ack
 Packet received: l

meaning, when we process that stop reply, we treat the event as coming
from a thread with ptid == magic_null_ptid, which is not yet in the
thread list, so we add it then:

  (top-gdb) p ptid
  $1 = {m_pid = 42000, m_lwp = -1, m_tid = 1}
  (top-gdb) bt
  #0  0x0000000000840a8c in add_thread_silent(ptid_t) (ptid=...) at src/gdb/thread.c:269
  #1  0x00000000007ad61d in remote_add_thread(ptid_t, int, int) (ptid=..., running=0, executing=0)
      at src/gdb/remote.c:1838
  #2  0x00000000007ad8de in remote_notice_new_inferior(ptid_t, int) (currthread=..., executing=0)
      at src/gdb/remote.c:1921
  #3  0x00000000007b758b in process_stop_reply(stop_reply*, target_waitstatus*) (stop_reply=0x1158860, status=0x7fffffffcc00)
      at src/gdb/remote.c:7217
  #4  0x00000000007b7a38 in remote_wait_as(ptid_t, target_waitstatus*, int) (ptid=..., status=0x7fffffffcc00, options=0)
      at src/gdb/remote.c:7380
  #5  0x00000000007b7cd1 in remote_wait(target_ops*, ptid_t, target_waitstatus*, int) (ops=0x102fac0 <remote_ops>, ptid=..., status=0x7fffffffcc00, options=0) at src/gdb/remote.c:7446
  #6  0x000000000081587b in delegate_wait(target_ops*, ptid_t, target_waitstatus*, int) (self=0x102fac0 <remote_ops>, arg1=..., arg2=0x7fffffffcc00, arg3=0) at src/gdb/target-delegates.c:138
  #7  0x0000000000827d77 in target_wait(ptid_t, target_waitstatus*, int) (ptid=..., status=0x7fffffffcc00, options=0)
      at src/gdb/target.c:2179
  #8  0x0000000000715fda in do_target_wait(ptid_t, target_waitstatus*, int) (ptid=..., status=0x7fffffffcc00, options=0)
      at src/gdb/infrun.c:3589
  #9  0x0000000000716351 in wait_for_inferior() () at src/gdb/infrun.c:3707
  #10 0x0000000000715435 in start_remote(int) (from_tty=1) at src/gdb/infrun.c:3212

things go downhill from this.

We don't see the problem with current master gdbserver, because that
version always sends the ";thread:" part in the initial stop reply:

 Sending packet: $?#3f...Packet received: T0506:0000000000000000;07:a0d4ffffff7f0000;10:d05eddf7ff7f0000;thread:p3cea.3cea;core:3;

Years ago I had added a "--disable-packet=" command line option to
gdbserver which comes in handy for testing this, since the existing
"--disable-packet=Tthread" precisely makes gdbserver not send that
";thread:" part in stop replies.  The testcase added by this commit
emulates old gdbserver making use of that.

I've compared a testrun at 5cd63fda035d^ (before regression) with
'current master+patch', against old gdbserver at f8b73d13b7ca^.  I
hacked out --once, and "monitor exit" to be able to test.  The results
are a bit too unstable to tell accurately, but it looked like there
were no regressions.  Maciej confirmed this worked for him as well.

No regressions on master (against master gdbserver).

gdb/ChangeLog:
2018-01-11  Pedro Alves  <palves@redhat.com>

	PR remote/22597
	* remote.c (remote_parse_stop_reply): Default to the last-set
	general thread instead of to 'magic_null_ptid'.

gdb/testsuite/ChangeLog:
2018-01-11  Pedro Alves  <palves@redhat.com>

	PR remote/22597
	* gdb.server/stop-reply-no-thread.c: New file.
	* gdb.server/stop-reply-no-thread.exp: New file.
2018-01-11 00:23:04 +00:00
bfd Automatic date update in version.in 2018-01-11 00:00:23 +00:00
binutils Update top level configure files by synchronizing them with gcc. 2018-01-10 15:29:21 +00:00
config Update top level configure files by synchronizing them with gcc. 2018-01-10 15:29:21 +00:00
cpu Update the openrisc previous program counter (ppc) when running code in the cgen based simulator. 2017-03-20 15:33:51 +00:00
elfcpp Update year range in copyright notice of binutils files 2018-01-03 17:49:56 +10:30
etc Update year range in copyright notice of binutils files 2018-01-03 17:49:56 +10:30
gas Remove VL variants for 4FMAPS and 4VNNIW insns. 2018-01-11 03:09:47 +03:00
gdb Fix backwards compatibility with old GDBservers (PR remote/22597) 2018-01-11 00:23:04 +00:00
gold Update year range in copyright notice of binutils files 2018-01-03 17:49:56 +10:30
gprof Update year range in copyright notice of binutils files 2018-01-03 17:49:56 +10:30
include RISC-V: Add 2 missing privileged registers. 2018-01-04 14:17:53 -08:00
intl Require ngettext in test of system gettext implementation 2017-11-07 15:56:44 +10:30
ld Add explicit shared check to eh-frame-hdr test. 2018-01-09 10:08:47 -08:00
libdecnumber Remove leading/trailing white spaces in ChangeLog 2015-07-24 04:16:47 -07:00
libiberty Update the libiberty sources with the latest patches found in the master sources. 2018-01-10 13:57:48 +00:00
opcodes Remove VL variants for 4FMAPS and 4VNNIW insns. 2018-01-11 03:09:47 +03:00
readline Avoid MinGW compilation warning in readline/input.c 2017-05-19 11:05:59 +03:00
sim Fix compile time warning (in the ARM simulator) about a print statement with insufficient arguments. 2018-01-02 17:15:16 +00:00
texinfo
zlib Sync ZLIB with FSF GCC sources, bringing in version 1.2.11. 2017-02-20 12:52:22 +00:00
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ChangeLog oops - updates to ChangeLog omitted by accident from previous delta. 2018-01-10 15:35:34 +00:00
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config-ml.in Update top level configure files by synchronizing them with gcc. 2018-01-10 15:29:21 +00:00
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COPYING
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COPYING.LIBGLOSS Update the address of the FSF in the copyright notice of files which were using the old address. 2017-12-14 12:48:55 +00:00
COPYING.NEWLIB
depcomp
djunpack.bat
install-sh
libtool.m4 Sync top-level btool.m4 with GCC 2016-01-12 08:44:52 -08:00
lt~obsolete.m4
ltgcc.m4
ltmain.sh
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ltsugar.m4
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MAINTAINERS
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README 

		   README for GNU development tools

This directory contains various GNU compilers, assemblers, linkers, 
debuggers, etc., plus their support routines, definitions, and documentation.

If you are receiving this as part of a GDB release, see the file gdb/README.
If with a binutils release, see binutils/README;  if with a libg++ release,
see libg++/README, etc.  That'll give you info about this
package -- supported targets, how to use it, how to report bugs, etc.

It is now possible to automatically configure and build a variety of
tools with one command.  To build all of the tools contained herein,
run the ``configure'' script here, e.g.:

	./configure 
	make

To install them (by default in /usr/local/bin, /usr/local/lib, etc),
then do:
	make install

(If the configure script can't determine your type of computer, give it
the name as an argument, for instance ``./configure sun4''.  You can
use the script ``config.sub'' to test whether a name is recognized; if
it is, config.sub translates it to a triplet specifying CPU, vendor,
and OS.)

If you have more than one compiler on your system, it is often best to
explicitly set CC in the environment before running configure, and to
also set CC when running make.  For example (assuming sh/bash/ksh):

	CC=gcc ./configure
	make

A similar example using csh:

	setenv CC gcc
	./configure
	make

Much of the code and documentation enclosed is copyright by
the Free Software Foundation, Inc.  See the file COPYING or
COPYING.LIB in the various directories, for a description of the
GNU General Public License terms under which you can copy the files.

REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info
on where and how to report problems.