gdb, btrace: diagnose double and failed enable

GDB silently ignores attempts to enable branch tracing on a thread that is
already recorded.  This shouldn't happen as recording is enabled exactly
once:

  - when the btrace record target is opened for existing threads
  - when a new thread is added while the btrace record target is pushed

GDB also silently ignores if recording is disabled on threads that were not
recorded.  This shouldn't happen, either, since when stopping recording,
we only disable recording on threads that were recorded.

GDB further silently ignores if recording was not enabled by the
corresponding target method.  Also this shouldn't happen since the target
is supposed to already throw an error if recording cannot be enabled.
This new error in btrace_enable catches cases where the target silently
failed to enable recording.

Throw an error in those cases.

This allows us to detect an actual issue more easily.  It will be
addressed in the next patch.

gdb/ChangeLog:

2020-03-19  Markus Metzger  <markus.t.metzger@intel.com>

	* btrace.c (btrace_enable): Throw an error on double enables and
	when enabling recording fails.
	(btrace_disable): Throw an error if the thread is not recorded.

1 files changed, 6 insertions, 4 deletions

diff --git a/gdb/btrace.c b/gdb/btrace.c
index 9f90d59e2b5..d41e3c4f8f9 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -1592,7 +1592,8 @@ void
 btrace_enable (struct thread_info *tp, const struct btrace_config *conf)
 {
   if (tp->btrace.target != NULL)
-    return;
+    error (_("Recording already enabled on thread %s (%s)."),
+	   print_thread_id (tp), target_pid_to_str (tp->ptid).c_str ());
 
 #if !defined (HAVE_LIBIPT)
   if (conf->format == BTRACE_FORMAT_PT)
@@ -1604,9 +1605,9 @@ btrace_enable (struct thread_info *tp, const struct btrace_config *conf)
 
   tp->btrace.target = target_enable_btrace (tp->ptid, conf);
 
-  /* We're done if we failed to enable tracing.  */
   if (tp->btrace.target == NULL)
-    return;
+    error (_("Failed to enable recording on thread %s (%s)."),
+	   print_thread_id (tp), target_pid_to_str (tp->ptid).c_str ());
 
   /* We need to undo the enable in case of errors.  */
   try
@@ -1651,7 +1652,8 @@ btrace_disable (struct thread_info *tp)
   struct btrace_thread_info *btp = &tp->btrace;
 
   if (btp->target == NULL)
-    return;
+    error (_("Recording not enabled on thread %s (%s)."),
+	   print_thread_id (tp), target_pid_to_str (tp->ptid).c_str ());
 
   DEBUG ("disable thread %s (%s)", print_thread_id (tp),
 	 target_pid_to_str (tp->ptid).c_str ());