Invalidate a register in cache when a remote target failed to write it.

Invalidate a register in cache when a remote target failed to write it.

As shown by the bug report, GDB crashes when the remote target was unable to
write to a register (the program counter) with the 'P' packet. This was reported
for AVR but can be reproduced on any architecture with a gdbserver that fails to
handle a 'P' packet.

Issue
=====

This GDB session was done with a custom gdbserver patched to send an error
packet when trying to set the program counter with a 'P' packet:

~~~
(gdb) file Debug/ATMega2560-simple-program.elf
Reading symbols from Debug/ATMega2560-simple-program.elf...done.
(gdb) target remote :51000
Remote debugging using :51000
0x00000000 in __vectors ()
(gdb) load
Loading section .text, size 0x1fc lma 0x0
Start address 0x0, load size 508
Transfer rate: 248 KB/sec, 169 bytes/write.
(gdb) b main
Breakpoint 1 at 0x164: file .././ATMega2560-simple-program.c, line 39.
(gdb) c
Continuing.

Program received signal SIGTRAP, Trace/breakpoint trap.
main () at .././ATMega2560-simple-program.c:42
42		DDRD |= LED0_MASK;// | LED1_MASK;
(gdb) info line 43
Line 43 of ".././ATMega2560-simple-program.c" is at address 0x178 <main+40> but contains no code.
(gdb) set $pc=0x178
Could not write register "PC2"; remote failure reply 'E00'
(gdb) info registers pc
pc             0x178	0x178 <main+40>
(gdb) s
../../unisrc-mainline/gdb/infrun.c:1978: internal-error: resume: Assertion `pc_in_thread_step_range (pc, tp)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)
../../unisrc-mainline/gdb/infrun.c:1978: internal-error: resume: Assertion `pc_in_thread_step_range (pc, tp)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Create a core file of GDB? (y or n)
~~~

We can see that even though GDB reports that writing to the register failed, the
register cache was updated:

~~~
(gdb) set $pc=0x178
Could not write register "PC2"; remote failure reply 'E00'
(gdb) info registers pc
pc             0x178	0x178 <main+40>
~~~

The root of the problem is of course in the gdbserver but I thought GDB should
keep a register cache consistent with the hardware even in case of a failure.

Changes
=======

This patch adds routines to add a regcache_invalidate cleanup to the current
chain.

We can then register one before calling target_store_registers. This way if the
target throws an error, the register we wanted to write to will be invalidated
in cache. If target_store_registers succeeds, we can discard the new cleanup.

2014-06-12  Pierre Langlois  <pierre.langlois@embecosm.com>

	* regcache.c (struct register_to_invalidate): New structure.
	(do_register_invalidate, make_cleanup_regcache_invalidate): New
	functions.
	(regcache_raw_write): Call make_cleanup_regcache_invalidate.