[RFA] Fix GDB 8.3 regression crash when registers cannot be modified.

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

This crash was detected when using GDB with the valgrind gdbserver.
To reproduce:

valgrind sleep 10000

In another window:
gdb
target remote | vgdb
p printf("make sleep print something\n")
=>
terminate called after throwing an instance of 'gdb_exception_RETURN_MASK_ERROR'
Aborted

The problem is that the valgrind gdbserver does not allow to change
registers when the inferior is blocked in a system call.
GDB then raises an exception.  The exception causes the destructor
of
 typedef std::unique_ptr<infcall_suspend_state, infcall_suspend_state_deleter>
    infcall_suspend_state_up;
to be called.  This destructor itself tries to restore the value of
the registers, and fails similarly.  We must catch the exception in
the destructor to avoid crashing GDB.
If the destructor encounters a problem, no warning is produced if
there is an uncaught exception, as in this case, the user will already
be informed of a problem via this exception.

With this change, no crash anymore, and all the valgrind 3.15 tests
pass succesfully.

Note: when this patch is approved, I will push an equivalent patch
on master, but with TRY/CATCH/e.message () replaced by
try/catch/e.what ().

gdb/ChangeLog

struct infcall_suspend_state_deleter
2019-04-13  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* inferior.h (struct infcall_suspend_state_deleter):
	Catch exception in destructor to avoid crash.
---
 gdb/inferior.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/gdb/inferior.h b/gdb/inferior.h
index 2d1bb97a28..4d84afac6a 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -68,7 +68,19 @@ struct infcall_suspend_state_deleter
 {
   void operator() (struct infcall_suspend_state *state) const
   {
-    restore_infcall_suspend_state (state);
+    TRY
+      {
+	restore_infcall_suspend_state (state);
+      }
+    CATCH (e, RETURN_MASK_ALL)
+      {
+	/* If we are restoring the inferior state due to an exception,
+	   some error message will be printed.  So, only warn the user
+	   when we cannot restore during normal execution.  */
+	if (!std::uncaught_exception ())
+	  warning (_("Failed to restore inferior state: %s"), e.message);
+      }
+    END_CATCH
   }
 };
 
-- 
2.20.1