gdb, btrace: forward fetch_registers for unknown threads

In the record-btrace target, while replaying, we can only provide the PC
register.  The btrace state is stored in the thread_info.  So, when trying
to determine whether we are currently replaying, GDB calls
find_thread_ptid() to obtain the thread_info.  It also asserts that we do
have a thread_info.

For new threads, libthread-db may fetch registers before the thread is
known to GDB.  In this case, find_thread_ptid() returns nullptr and the
assertion fails.

Forward the fetch_registers request to the target beneath in that case.

gdb/ChangeLog:

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

	* record-btrace.c (record_btrace_target::fetch_registers): Forward
	request if we do not have a thread_info.

gdb/testsuite/ChangeLog:

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

	* gdb.btrace/enable-new-thread.c: New test.
	* gdb.btrace/enable-new-thread.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-21  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* record-btrace.c (record_btrace_target::fetch_registers): Forward
+	request if we do not have a thread_info.
+
 2020-04-21  Tom de Vries  <tdevries@suse.de>
 
 	PR gdb/25471

gdb/record-btrace.c

@@ -1532,11 +1532,16 @@ record_btrace_target::remove_breakpoint (struct gdbarch *gdbarch,
 void
 record_btrace_target::fetch_registers (struct regcache *regcache, int regno)
 {
-  thread_info *tp = find_thread_ptid (regcache->target (), regcache->ptid ());
-  gdb_assert (tp != NULL);
+  btrace_insn_iterator *replay = nullptr;
 
-  btrace_insn_iterator *replay = tp->btrace.replay;
-  if (replay != NULL && !record_btrace_generating_corefile)
+  /* Thread-db may ask for a thread's registers before GDB knows about the
+     thread.  We forward the request to the target beneath in this
+     case.  */
+  thread_info *tp = find_thread_ptid (regcache->target (), regcache->ptid ());
+  if (tp != nullptr)
+    replay =  tp->btrace.replay;
+
+  if (replay != nullptr && !record_btrace_generating_corefile)
     {
       const struct btrace_insn *insn;
       struct gdbarch *gdbarch;

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-21  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* gdb.btrace/enable-new-thread.c: New test.
+	* gdb.btrace/enable-new-thread.exp: New file.
+
 2020-04-21  Tom de Vries  <tdevries@suse.de>
 
 	PR gdb/25471

gdb/testsuite/gdb.btrace/enable-new-thread.c

@@ -0,0 +1,36 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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>
+
+static void *
+test (void *arg)
+{
+  return arg; /* bp.1 */
+}
+
+int
+main (void)
+{
+  pthread_t th;
+
+  pthread_create (&th, NULL, test, NULL);
+  pthread_join (th, NULL);
+
+  return 0;
+}

gdb/testsuite/gdb.btrace/enable-new-thread.exp

@@ -0,0 +1,57 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2020 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/>.
+
+# Test that new threads of recorded inferiors also get recorded.
+
+if { [skip_btrace_tests] } {
+    unsupported "target does not support record-btrace"
+    return -1
+}
+
+standard_testfile
+if [prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] {
+    return -1
+}
+
+if ![runto_main] {
+    untested "failed to run to main"
+    return -1
+}
+
+# Record the main thread.  Recording will automatically be enabled for the
+# other thread.
+gdb_test "record btrace"
+
+gdb_breakpoint [gdb_get_line_number "bp.1" $srcfile]
+gdb_continue_to_breakpoint "cont to bp.1" ".*/\\* bp\.1 \\*/.*"
+
+proc check_thread_recorded { num } {
+    global decimal
+
+    with_test_prefix "thread $num" {
+	gdb_test "thread $num" "Switching to thread $num.*"
+
+	gdb_test "info record" [multi_line \
+	    "Active record target: record-btrace" \
+	    ".*" \
+	    "Recorded $decimal instructions in $decimal functions \[^\\\r\\\n\]*" \
+       ]
+    }
+}
+
+check_thread_recorded 1
+check_thread_recorded 2