Print current thread after loading a core file

downstream Fedora request:
	Please make it easier to find the backtrace of the crashing thread
	https://bugzilla.redhat.com/show_bug.cgi?id=1024504

Currently after loading a core file GDB prints:

Core was generated by `./threadcrash1'.
Program terminated with signal SIGSEGV, Segmentation fault.
8       *(volatile int *)0=0;
(gdb) _

there is nowhere seen which of the threads had crashed.  In reality GDB always
numbers that thread as #1 and it is the current thread that time.  But after
dumping all the info into a file for later analysis it is no longer obvious.
'thread apply all bt' even puts the thread #1 to the _end_ of the output!!!

Should GDB always print after loading a core file what "thread" command would
print?
[Current thread is 1 (Thread 0x7fcbe28fe700 (LWP 15453))]

BTW I think it will print the thread even when loading single/non-threaded
core file when other inferior(s) exist.  But that currently crashes
	[Bug threads/12074] multi-inferior internal error
	https://sourceware.org/bugzilla/show_bug.cgi?id=12074
plus I think that would be a correct behavior anyway.

gdb/ChangeLog
2015-01-22  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* corelow.c (core_open): Call also thread_command.
	* gdbthread.h (thread_command): New prototype moved from ...
	* thread.c (thread_command): ... here.
	(thread_command): Make it global.

4 files changed, 23 insertions, 2 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 6e3e258..8d9e367 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2015-01-22  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+	* corelow.c (core_open): Call also thread_command.
+	* gdbthread.h (thread_command): New prototype moved from ...
+	* thread.c (thread_command): ... here.
+	(thread_command): Make it global.
+
 2015-01-22  Pedro Alves  <palves@redhat.com>
 
 	* configure.ac [*mingw32*]: Check $curses_found instead of
diff --git a/gdb/corelow.c b/gdb/corelow.c
index a9eadd5..6a67a98 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -456,6 +456,19 @@ core_open (const char *arg, int from_tty)
   /* Now, set up the frame cache, and print the top of stack.  */
   reinit_frame_cache ();
   print_stack_frame (get_selected_frame (NULL), 1, SRC_AND_LOC, 1);
+
+  /* Current thread should be NUM 1 but the user does not know that.
+     If a program is single threaded gdb in general does not mention
+     anything about threads.  That is why the test is >= 2.  */
+  if (thread_count () >= 2)
+    {
+      TRY_CATCH (except, RETURN_MASK_ERROR)
+	{
+	  thread_command (NULL, from_tty);
+	}
+      if (except.reason < 0)
+	exception_print (gdb_stderr, except);
+    }
 }
 
 static void
diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index 15a979b..a2f378a 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -455,6 +455,8 @@ extern void finish_thread_state_cleanup (void *ptid_p);
 /* Commands with a prefix of `thread'.  */
 extern struct cmd_list_element *thread_cmd_list;
 
+extern void thread_command (char *tidstr, int from_tty);
+
 /* Print notices on thread events (attach, detach, etc.), set with
    `set print thread-events'.  */
 extern int print_thread_events;
diff --git a/gdb/thread.c b/gdb/thread.c
index ed20fbe..4bce212 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -62,7 +62,6 @@ static int highest_thread_num;
    spawned new threads we haven't heard of yet.  */
 static int threads_executing;
 
-static void thread_command (char *tidstr, int from_tty);
 static void thread_apply_all_command (char *, int);
 static int thread_alive (struct thread_info *);
 static void info_threads_command (char *, int);
@@ -1506,7 +1505,7 @@ thread_apply_command (char *tidlist, int from_tty)
 /* Switch to the specified thread.  Will dispatch off to thread_apply_command
    if prefix of arg is `apply'.  */
 
-static void
+void
 thread_command (char *tidstr, int from_tty)
 {
   if (!tidstr)