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.

1 files changed, 40 insertions, 3 deletions

diff --git a/gdb/regcache.c b/gdb/regcache.c
index 8b588c6..5ee90b0 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -267,6 +267,32 @@ make_cleanup_regcache_xfree (struct regcache *regcache)
   return make_cleanup (do_regcache_xfree, regcache);
 }
 
+/* Cleanup routines for invalidating a register.  */
+
+struct register_to_invalidate
+{
+  struct regcache *regcache;
+  int regnum;
+};
+
+static void
+do_regcache_invalidate (void *data)
+{
+  struct register_to_invalidate *reg = data;
+
+  regcache_invalidate (reg->regcache, reg->regnum);
+}
+
+static struct cleanup *
+make_cleanup_regcache_invalidate (struct regcache *regcache, int regnum)
+{
+  struct register_to_invalidate* reg = XNEW (struct register_to_invalidate);
+
+  reg->regcache = regcache;
+  reg->regnum = regnum;
+  return make_cleanup_dtor (do_regcache_invalidate, (void *) reg, xfree);
+}
+
 /* Return REGCACHE's architecture.  */
 
 struct gdbarch *
@@ -846,7 +872,8 @@ void
 regcache_raw_write (struct regcache *regcache, int regnum,
 		    const gdb_byte *buf)
 {
-  struct cleanup *old_chain;
+  struct cleanup *chain_before_save_inferior;
+  struct cleanup *chain_before_invalidate_register;
 
   gdb_assert (regcache != NULL && buf != NULL);
   gdb_assert (regnum >= 0 && regnum < regcache->descr->nr_raw_registers);
@@ -864,16 +891,26 @@ regcache_raw_write (struct regcache *regcache, int regnum,
 		  regcache->descr->sizeof_register[regnum]) == 0))
     return;
 
-  old_chain = save_inferior_ptid ();
+  chain_before_save_inferior = save_inferior_ptid ();
   inferior_ptid = regcache->ptid;
 
   target_prepare_to_store (regcache);
   memcpy (register_buffer (regcache, regnum), buf,
 	  regcache->descr->sizeof_register[regnum]);
   regcache->register_status[regnum] = REG_VALID;
+
+  /* Register a cleanup function for invalidating the register after it is
+     written, in case of a failure.  */
+  chain_before_invalidate_register
+    = make_cleanup_regcache_invalidate (regcache, regnum);
+
   target_store_registers (regcache, regnum);
 
-  do_cleanups (old_chain);
+  /* The target did not throw an error so we can discard invalidating the
+     register and restore the cleanup chain to what it was.  */
+  discard_cleanups (chain_before_invalidate_register);
+
+  do_cleanups (chain_before_save_inferior);
 }
 
 void