[RFA] Fix internal error with 'set debug infrun 1' under high load

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

The test  gdb.threads/watchthreads-reorder.exp verifies that the
'set debug infrun 1' debug output does not crash GDB.

Under high load, the test can still cause a GDB internal error (see details
below).

This patch checks that ptid differs from null_ptid to call
pid_to_str so that it does not crash.
The change also outputs event_ptid.

  (top-gdb) bt
  #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
  #1  0x00007f3d54a0642a in __GI_abort () at abort.c:89
  #2  0x0000555c24c60e66 in dump_core () at ../../fixleaks/gdb/utils.c:201
  #3  0x0000555c24c63d49 in internal_vproblem(internal_problem *, const char *, int, const char *, typedef __va_list_tag __va_list_tag *) (problem=problem@entry=0x555c25338d40 <internal_error_problem>, file=<optimized out>, line=287,
      fmt=<optimized out>, ap=<optimized out>) at ../../fixleaks/gdb/utils.c:411
  #4  0x0000555c24c63eab in internal_verror (file=<optimized out>, line=<optimized out>, fmt=<optimized out>,
      ap=<optimized out>) at ../../fixleaks/gdb/utils.c:436
  #5  0x0000555c249e8c22 in internal_error (file=file@entry=0x555c24e0f2ad "../../fixleaks/gdb/inferior.c",
      line=line@entry=287, fmt=<optimized out>) at ../../fixleaks/gdb/common/errors.c:55
  #6  0x0000555c247d3f5c in find_inferior_pid (pid=<optimized out>) at ../../fixleaks/gdb/inferior.c:287
  #7  0x0000555c24ad2248 in find_inferior_pid (pid=<optimized out>) at ../../fixleaks/gdb/inferior.c:302
  #8  find_inferior_ptid (ptid=...) at ../../fixleaks/gdb/inferior.c:301
  #9  0x0000555c24c35f25 in find_thread_ptid (ptid=...) at ../../fixleaks/gdb/thread.c:522
  #10 0x0000555c24b0ab4d in thread_db_target::pid_to_str[abi:cxx11](ptid_t) (
      this=0x555c2532e3e0 <the_thread_db_target>, ptid=...) at ../../fixleaks/gdb/linux-thread-db.c:1637
  #11 0x0000555c24c2f420 in target_pid_to_str[abi:cxx11](ptid_t) (ptid=...) at ../../fixleaks/gdb/target.c:2083
  #12 0x0000555c24ad9cab in stop_all_threads () at ../../fixleaks/gdb/infrun.c:4373
  #13 0x0000555c24ada00f in stop_waiting (ecs=<optimized out>) at ../../fixleaks/gdb/infrun.c:7464
  #14 0x0000555c24adc401 in process_event_stop_test (ecs=ecs@entry=0x7ffc9402d9d0) at ../../fixleaks/gdb/infrun.c:6181
  ...
  (top-gdb) fr 12
  #12 0x0000555c24ad9cab in stop_all_threads () at ../../fixleaks/gdb/infrun.c:4373
  (top-gdb) p event_ptid
  $5 = {m_pid = 25419, m_lwp = 25427, m_tid = 0}
  (top-gdb) p ptid
  $6 = {m_pid = 0, m_lwp = 0, m_tid = 0}
  (top-gdb) p ws
  $7 = {kind = TARGET_WAITKIND_THREAD_EXITED, value = {integer = 0, sig = GDB_SIGNAL_0, related_pid = {m_pid = 0,
        m_lwp = 0, m_tid = 0}, execd_pathname = 0x0, syscall_number = 0}}
  (top-gdb)

The gdb.log corresponding to the above crash is:
  (gdb) PASS: gdb.threads/watchthreads-reorder.exp: reorder1: set debug infrun 1
  continue
  Continuing.
  infrun: clear_proceed_status_thread (Thread 0x7ffff7fcfb40 (LWP 25419))
  infrun: clear_proceed_status_thread (Thread 0x7ffff7310700 (LWP 25427))
  infrun: clear_proceed_status_thread (Thread 0x7ffff6b0f700 (LWP 25428))
  infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
  infrun: proceed: resuming Thread 0x7ffff7fcfb40 (LWP 25419)
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff7fcfb40 (LWP 25419)] at 0x7ffff7344317
  infrun: infrun_async(1)
  infrun: prepare_to_wait
  infrun: proceed: resuming Thread 0x7ffff7310700 (LWP 25427)
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff7310700 (LWP 25427)] at 0x5555555553d7
  infrun: prepare_to_wait
  infrun: proceed: resuming Thread 0x7ffff6b0f700 (LWP 25428)
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff6b0f700 (LWP 25428)] at 0x5555555554c8
  infrun: prepare_to_wait
  infrun: target_wait (-1.0.0, status) =
  infrun:   -1.0.0 [process -1],
  infrun:   status->kind = ignore
  infrun: TARGET_WAITKIND_IGNORE
  infrun: prepare_to_wait
  Joining the threads.
  [Thread 0x7ffff6b0f700 (LWP 25428) exited]
  infrun: target_wait (-1.0.0, status) =
  infrun:   -1.0.0 [process -1],
  infrun:   status->kind = ignore
  infrun: TARGET_WAITKIND_IGNORE
  infrun: prepare_to_wait
  infrun: target_wait (-1.0.0, status) =
  infrun:   25419.25419.0 [Thread 0x7ffff7fcfb40 (LWP 25419)],
  infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
  infrun: TARGET_WAITKIND_STOPPED
  infrun: stop_pc = 0x555555555e50
  infrun: context switch
  infrun: Switching context from Thread 0x7ffff6b0f700 (LWP 25428) to Thread 0x7ffff7fcfb40 (LWP 25419)
  infrun: BPSTAT_WHAT_STOP_NOISY
  infrun: stop_waiting
  infrun: stop_all_threads
  infrun: stop_all_threads, pass=0, iterations=0
  infrun:   Thread 0x7ffff7fcfb40 (LWP 25419) not executing
  infrun:   Thread 0x7ffff7310700 (LWP 25427) executing, need stop
  [Thread 0x7ffff7310700 (LWP 25427) exited]
  infrun: target_wait (-1.0.0, status) =
  infrun:   25419.25427.0 [LWP 25427],
  infrun:   status->kind = thread exited, status = 0
  infrun: infrun_async(0)
  ../../fixleaks/gdb/inferior.c:287: internal-error: inferior* find_inferior_pid(int): Assertion `pid != 0' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) FAIL: gdb.threads/watchthreads-reorder.exp: reorder1: continue to breakpoint: break-at-exit (GDB internal error)
  Resyncing due to internal error.
  n
  infrun: infrun_async(1)

  This is a bug, please report it.  For instructions, see:
  <http://www.gnu.org/software/gdb/bugs/>.

  infrun: infrun_async(0)
  ../../fixleaks/gdb/inferior.c:287: internal-error: inferior* find_inferior_pid(int): Assertion `pid != 0' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Create a core file of GDB? (y or n) y
---
 gdb/infrun.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index ad7892105a..111879c318 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -4368,9 +4368,13 @@ stop_all_threads (void)
 		  ptid_t ptid = ptid_t (ws.value.integer);
 
 		  fprintf_unfiltered (gdb_stdlog,
-				      "infrun: %s exited while "
+				      "infrun: %s "
+				      "(event_ptid %s) exited while "
 				      "stopping threads\n",
-				      target_pid_to_str (ptid).c_str ());
+				      ptid == null_ptid
+				      ? "<null thread>"
+				      : target_pid_to_str (ptid).c_str (),
+				      target_pid_to_str (event_ptid).c_str ());
 		}
 	    }
 	  else
-- 
2.20.1