btrace: allow recording to be started (and stopped) for running threads

When recording is started for a running thread, GDB was able to start tracing
but then failed to read registers to insert the initial entry for the current
PC.  We don't really need that initial entry if we don't know where exactly we
started recording.  Skip that step to allow recording to be started while
threads are running.

If we do run into errors, we need to undo the tracing enable to not leak this
thread.  The operation did not complete so our caller won't clean up this
thread.

For the BTRACE_FORMAT_PT btrace format, we don't need that initial entry since
it will be recorded in the trace.  We can omit the call to btrace_add_pc.

gdb/
	* btrace.c (btrace_enable): Do not call btrace_add_pc for
	BTRACE_FORMAT_PT or if can_access_registers_ptid returns false.
	(btrace_fetch): Assert can_access_registers_ptid.
	* record-btrace.c (require_btrace_thread, record_btrace_info): Call
	validate_registers_access.

testsuite/
	* gdb.btrace/enable-running.c: New.
	* gdb.btrace/enable-running.exp: New.

2 files changed, 143 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.btrace/enable-running.c b/gdb/testsuite/gdb.btrace/enable-running.c
new file mode 100644
index 0000000..c2edf1e
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.c
@@ -0,0 +1,48 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 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/>.  */
+
+#include <pthread.h>
+#include <unistd.h>
+
+#define NTHREADS 3
+
+static void *
+test (void *arg)
+{
+  /* Let's hope this is long enough for GDB to enable tracing and check that
+     everything is working as expected.  */
+  sleep (10);
+
+  return arg;
+}
+
+int
+main (void)
+{
+  pthread_t th[NTHREADS];
+  int i;
+
+  for (i = 0; i < NTHREADS; ++i)
+    pthread_create (&th[i], NULL, test, NULL);
+
+  test (NULL); /* bp.1 */
+
+  for (i = 0; i < NTHREADS; ++i)
+    pthread_join (th[i], NULL);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.btrace/enable-running.exp b/gdb/testsuite/gdb.btrace/enable-running.exp
new file mode 100644
index 0000000..d549a40
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.exp
@@ -0,0 +1,95 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2017 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/>.
+
+if { [skip_btrace_tests] } { return -1 }
+
+standard_testfile
+if {[gdb_compile_pthreads "$srcdir/$subdir/$srcfile" "$binfile" executable {debug}] != "" } {
+    return -1
+}
+
+# We need to enable non-stop mode for the remote case.
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " -ex \"set non-stop on\""
+    clean_restart $testfile
+}
+
+if ![runto_main] {
+    return -1
+}
+
+set bp_1 [gdb_get_line_number "bp.1" $srcfile]
+
+gdb_breakpoint $bp_1
+gdb_continue_to_breakpoint "cont to $bp_1" ".*$bp_1\r\n.*"
+gdb_test "cont&" "Continuing\."
+
+# All threads are running.  Let's start recording.
+gdb_test_no_output "record btrace"
+
+proc check_tracing_enabled { thread } {
+    global gdb_prompt
+
+    with_test_prefix "thread $thread" {
+        gdb_test "thread $thread" "(running).*" "is running"
+
+        # We can't read the trace while the thread is running.
+        gdb_test "info record" "Selected thread is running\." \
+            "info record while running"
+
+        # Try various commands that try to read trace.
+        gdb_test "record instruction-history" "Selected thread is running\."
+        gdb_test "record function-call-history" "Selected thread is running\."
+
+        # Including reverse-stepping commands.
+        gdb_test "reverse-continue" "\[Ss\]elected thread is running\."
+        gdb_test "reverse-step" "\[Ss\]elected thread is running\."
+        gdb_test "reverse-next" "\[Ss\]elected thread is running\."
+        gdb_test "reverse-finish" "\[Ss\]elected thread is running\."
+
+        # Stop the thread before reading the trace.
+        gdb_test_multiple "interrupt" "interrupt" {
+            -re "interrupt\r\n$gdb_prompt " {
+                pass "interrupt"
+            }
+        }
+        # Wait until the thread actually stopped.
+        gdb_test_multiple "" "stopped" {
+            -re "Thread $thread.*stopped\." {
+                pass "stopped"
+            }
+        }
+        # We will consume the thread's current location as part of the
+        # "info record" output.
+        gdb_test "info record" [multi_line \
+            "Active record target: record-btrace" \
+            "Recording format: .*" \
+            "Recorded \[0-9\]+ instructions \[^\\\r\\\n\]*" \
+        ]
+
+        # Continue the thread again.
+        gdb_test "cont&" "Continuing\."
+    }
+}
+
+# Check that recording was started on each thread.
+foreach thread {1 2 3 4} {
+    check_tracing_enabled $thread
+}
+
+# Stop recording while all threads are running.
+gdb_test "record stop" "Process record is stopped \[^\\\r\\\n\]*"