btrace: no replay without history

When using a reverse execution command without execution history, GDB
might end up in a state where replaying has been started but remains
at the current instruction.  This state is illegal.

Do not step if there is no execution history to avoid this.

2014-05-20  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_step_thread): Check for empty history.

testsuite/
	* gdb.btrace/nohist.exp: New.

4 files changed, 58 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 22ca25c8..6221225 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2014-05-20  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* record-btrace.c (record_btrace_step_thread): Check for empty history.
+
 2014-05-20  Hui Zhu  <hui@codesourcery.com>
 	    Yao Qi  <yao@codesourcery.com>
 
diff --git a/gdb/record-btrace.c b/gdb/record-btrace.c
index bcac165..d768225 100644
--- a/gdb/record-btrace.c
+++ b/gdb/record-btrace.c
@@ -1577,6 +1577,10 @@ record_btrace_step_thread (struct thread_info *tp)
   enum btrace_thread_flag flags;
   unsigned int steps;
 
+  /* We can't step without an execution history.  */
+  if (btrace_is_empty (tp))
+    return btrace_step_no_history ();
+
   btinfo = &tp->btrace;
   replay = btinfo->replay;
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 01a6b92..6767997 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2014-05-20  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* gdb.btrace/nohist.exp: New.
+
 2014-05-20  Yao Qi  <yao@codesourcery.com>
 
 	* lib/gdb.exp (gdb_init): Set timeout if test file is under
diff --git a/gdb/testsuite/gdb.btrace/nohist.exp b/gdb/testsuite/gdb.btrace/nohist.exp
new file mode 100644
index 0000000..c0a1a87
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/nohist.exp
@@ -0,0 +1,46 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2014 Free Software Foundation, Inc.
+#
+# Contributed by Intel Corp. <markus.t.metzger@intel.com>
+#
+# 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/>.
+
+# check for btrace support
+if { [skip_btrace_tests] } { return -1 }
+
+# start inferior
+standard_testfile x86-record_goto.S
+if [prepare_for_testing nohist.exp $testfile $srcfile] {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+proc check_not_replaying {} {
+  gdb_test "info record" [join [list \
+    "Active record target: record-btrace" \
+	"Recorded 0 instructions in 0 functions for \[^\\\r\\\n\]*" \
+    ] "\r\n"]
+}
+
+gdb_test_no_output "record btrace"
+
+check_not_replaying
+
+gdb_test "reverse-continue" "No more reverse-execution history\.\r\n.*"
+
+check_not_replaying