Consecutive step-overs trigger internal error.

Consecutive step-overs trigger internal error.

If a thread trips on a breakpoint that needs stepping over just after
finishing a step over, GDB currently fails an assertion.  This is a
regression caused by the "Handle multiple step-overs." patch
(99619beac6252113fed212fdb9e1ab97bface423) at
https://sourceware.org/ml/gdb-patches/2014-02/msg00765.html.

 (gdb) x /4i $pc
 => 0x400540 <main+4>:   movl   $0x0,0x2003da(%rip)        # 0x600924 <i>
    0x40054a <main+14>:  movl   $0x1,0x2003d0(%rip)        # 0x600924 <i>
    0x400554 <main+24>:  movl   $0x2,0x2003c6(%rip)        # 0x600924 <i>
    0x40055e <main+34>:  movl   $0x3,0x2003bc(%rip)        # 0x600924 <i>
 (gdb) PASS: gdb.base/consecutive-step-over.exp: get breakpoint addresses
 break *0x40054a
 Breakpoint 2 at 0x40054a: file ../../../src/gdb/testsuite/gdb.base/consecutive-step-over.c, line 23.
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 1: set breakpoint
 condition $bpnum condition
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 1: set condition
 break *0x400554
 Breakpoint 3 at 0x400554: file ../../../src/gdb/testsuite/gdb.base/consecutive-step-over.c, line 24.
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 2: set breakpoint
 condition $bpnum condition
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 2: set condition
 break *0x40055e
 Breakpoint 4 at 0x40055e: file ../../../src/gdb/testsuite/gdb.base/consecutive-step-over.c, line 25.
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 3: set breakpoint
 condition $bpnum condition
 (gdb) PASS: gdb.base/consecutive-step-over.exp: insn 3: set condition
 break 27
 Breakpoint 5 at 0x400568: file ../../../src/gdb/testsuite/gdb.base/consecutive-step-over.c, line 27.
 (gdb) continue
 Continuing.
 ../../src/gdb/infrun.c:5200: internal-error: switch_back_to_stepped_thread: Assertion `!tp->control.trap_expected' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 FAIL: gdb.base/consecutive-step-over.exp: continue to breakpoint: break here (GDB internal error)

The assertion fails, because the code is not expecting that the event
thread itself might need another step over.  IOW, not expecting that
TP in:

     tp = find_thread_needs_step_over (stepping_thread != NULL,
                                      stepping_thread);

could be the event thread.

A small fix for this would be to clear the event thread's
trap_expected earlier, before asserting.  But looking deeper, although
currently_stepping_or_nexting_callback's intention is finding the
thread that is doing a step/next, it also returns the thread that is
doing a step-over dance, with trap_expected set.  If there ever was a
reason for that (it was I who added
currently_stepping_or_nexting_callback , but I can't recall why I put
trap_expected there in the first place), the only remaining reason
nowadays is to aid in implementing switch_back_to_stepped_thread's
assertion that is now triggering, by piggybacking on the walk over all
threads, thus avoiding a separate walk.  This is quite obscure, and I
think we can do even better, by merging the walks that look for the
stepping thread, and the walk that looks for some thread that might
need a step over.

Tested on x86_64 Fedora 17, native and gdbserver, and also native on
top of my "software single-step on x86_64" series.

gdb/
2014-04-22  Pedro Alves  <palves@redhat.com>

	* infrun.c (schedlock_applies): New function, factored out from
	find_thread_needs_step_over.
	(find_thread_needs_step_over): Use it.
	(switch_back_to_stepped_thread): Always clear trap_expected if the
	step over is finished.  Return early if scheduler locking applies.
	Look for the stepping thread and a potential step-over thread with
	a single loop.
	(currently_stepping_or_nexting_callback): Delete.

2014-04-22  Pedro Alves  <palves@redhat.com>

	* gdb.base/consecutive-step-over.c: New file.
	* gdb.base/consecutive-step-over.exp: New file.