[Bug gdb/31521] New: Can't set watchpoints when current thread is running

["pedro at palves dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31521

            Bug ID: 31521
           Summary: Can't set watchpoints when current thread is running
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: pedro at palves dot net
  Target Milestone: ---

Currently, when the current thread is running, you can print global variables. 
However, if you try to set a watchpoint on the same globals, GDB errors out, 
complaining that the selected thread is running.  Like so:

 (gdb) c&
 Continuing.
 (gdb) p global
 $1 = 1098377287
 (gdb) watch global
 Selected thread is running.

I have a patch that makes setting the watchpoint work.  You'll get instead:

 (gdb) c&
 Continuing.
 (gdb) [New Thread 0x7ffff7d6e640 (LWP 434993)]
 [New Thread 0x7ffff756d640 (LWP 434994)]
 p global
 $1 = 88168
 (gdb) watch global
 Hardware watchpoint 2: global
 (gdb) [Switching to Thread 0x7ffff7d6e640 (LWP 434993)]

 Thread 2 "function0" hit Hardware watchpoint 2: global

 Old value = 185420
 New value = 185423
 int_return () at threads.c:39
 39      }

Software watchpoints require more fixing, as the succesfully set sw watchpoint
doesn't fire.  GDB needs to force the running-free thread to pause and switch
to single-stepping to make that work.  Software watchpoints have general design
issues with multi-threading, though, so that's just yet another issue for the
pile.

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