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.

3 files changed, 103 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 1de6dc0..2f42ad1 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2014-04-22  Pedro Alves  <palves@redhat.com>
 
+	* gdb.base/consecutive-step-over.c: New file.
+	* gdb.base/consecutive-step-over.exp: New file.
+
+2014-04-22  Pedro Alves  <palves@redhat.com>
+
 	* lib/gdb.exp (gdb_continue_to_breakpoint): Use gdb_test_multiple
 	instead of send_gdb/gdb_expect.
 
diff --git a/gdb/testsuite/gdb.base/consecutive-step-over.c b/gdb/testsuite/gdb.base/consecutive-step-over.c
new file mode 100644
index 0000000..e7c8e9d
--- /dev/null
+++ b/gdb/testsuite/gdb.base/consecutive-step-over.c
@@ -0,0 +1,28 @@
+/* Copyright 2014 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+volatile int i;
+volatile int condition;
+
+int
+main (void)
+{
+  i = 0;
+  i = 1;
+  i = 2;
+  i = 3;
+
+  return 0; /* break here */
+}
diff --git a/gdb/testsuite/gdb.base/consecutive-step-over.exp b/gdb/testsuite/gdb.base/consecutive-step-over.exp
new file mode 100644
index 0000000..3f78042
--- /dev/null
+++ b/gdb/testsuite/gdb.base/consecutive-step-over.exp
@@ -0,0 +1,70 @@
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#
+# Regression test for a bug where GDB would internal error if a thread
+# runs into a breakpoint that needs stepping over, just after stepping
+# over another breakpoint, without a user visible stop in between.
+#
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+if ![runto_main] then {
+    fail "Can't run to main"
+    return 0
+}
+
+# Make sure the target doesn't hide the breakpoint hits (that don't
+# cause a user visible stop) from GDB.
+gdb_test_no_output "set breakpoint condition-evaluation host"
+
+set up_to_nl "\[^\r\n\]+\[\r\n\]+"
+
+# Number of consecutive breakpoints in a row to try.
+set n_insns 3
+
+# Extract addresses of a few consecutive instructions.
+set test "get breakpoint addresses"
+if { [gdb_test_multiple "x /[expr $n_insns + 1]i \$pc" $test {
+    -re "=> $hex${up_to_nl}   ($hex)${up_to_nl}   ($hex)${up_to_nl}   ($hex)${up_to_nl}$gdb_prompt $" {
+	for {set i 1} {$i <= $n_insns} {incr i} {
+	    set bp_addrs($i) $expect_out($i,string)
+	}
+	pass $test
+    }
+}] != 0 } {
+    # No use proceeding if bp_addrs wasn't set.
+    return
+}
+
+for {set i 1} {$i <= $n_insns} {incr i} {
+    with_test_prefix "insn $i" {
+	gdb_test "break \*$bp_addrs($i)" \
+	    "Breakpoint $decimal at $bp_addrs($i): file .*" \
+	    "set breakpoint"
+
+	# Give the breakpoint a condition that always fails, so that
+	# the thread is immediately re-resumed.
+	gdb_test_no_output "condition \$bpnum condition" \
+	    "set condition"
+    }
+}
+
+set lineno [gdb_get_line_number "break here"]
+gdb_breakpoint $lineno
+gdb_continue_to_breakpoint "break here" ".*break here.*"