[PATCH][gdb] Fix stepping over fork with follow-fork-mode child and gcc-8

[Simon Marchi]

On 2020-05-07 1:04 a.m., Tom de Vries wrote:
> Hi,
> 
> When running test-case gdb.threads/fork-child-threads.exp with gcc-8 instead
> of gcc-7, we have:
> ...
>  (gdb) next^M
>  [Attaching after Thread 0x7ffff7fae740 (LWP 27574) fork to child process \
>    27578]^M
>  [New inferior 2 (process 27578)]^M
>  [Detaching after fork from parent process 27574]^M
>  [Inferior 1 (process 27574) detached]^M
>  [Thread debugging using libthread_db enabled]^M
>  Using host libthread_db library "/lib64/libthread_db.so.1".^M
>  [Switching to Thread 0x7ffff7fae740 (LWP 27578)]^M
> -main () at src/gdb/testsuite/gdb.threads/fork-child-threads.c:41^M
> +main () at src/gdb/testsuite/gdb.threads/fork-child-threads.c:34^M
> -41            i = pthread_create (&thread, NULL, start, NULL);^M
> +34        switch (fork ())^M
> -(gdb) PASS: gdb.threads/fork-child-threads.exp: next over fork
> +(gdb) FAIL: gdb.threads/fork-child-threads.exp: next over fork
> ...
> 
> This is due to the fact that gcc-8 generates more precise line info, making
> the instruction after the call to fork a "recommended breakpoint location".
> However, it is a bug because next is supposed to move to the next source
> line.
> 
> The problem is that in process_event_stop_test we hit this code:
> ...
>   if ((ecs->event_thread->suspend.stop_pc == stop_pc_sal.pc)
>       && (ecs->event_thread->current_line != stop_pc_sal.line
> 	  || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
>     {
>       if (stop_pc_sal.is_stmt)
> 	{
> 	  /* We are at the start of a different line.  So stop.  Note that
> 	     we don't stop if we step into the middle of a different line.
> 	     That is said to make things like for (;;) statements work
> 	     better.  */
> 	  if (debug_infrun)
> 	    fprintf_unfiltered (gdb_stdlog,
> 				"infrun: stepped to a different line\n");
> 	  end_stepping_range (ecs);
> 	  return;
> 	}
> ...
> because current_line and current_symtab have initial values:
> ...
> (gdb) p ecs->event_thread->current_line
> $8 = 0
> (gdb) p ecs->event_thread->current_symtab
> $9 = (symtab *) 0x0
> ...
> 
> Fix this in follow_fork by copying current_line and current_symtab from
> parent thread to child thread.
> 
> Tested on x86_64-linux, with gcc 7.5.0 and gcc 10.0.1.
> 
> OK for trunk?

Hi Tom,

The change makes sense to me, although I don't know this code in depth (things
related to lines and SaL).  But I confirm that it fixes on my machine the same
FAIL that you see.

Simon