[PATCHv2 2/3] gdb/regcache: When saving, ignore registers that can't be read

[Andrew Burgess <andrew dot burgess at embecosm dot com>]

The previous commit addressed an assertion that could trigger if a
target threw an error while saving state ahead of an inferior function
call.

The specific case that highlighted this issue was a RISC-V target that
claimed to support floating point registers, but when GDB tried to
read a floating point register the remote sent back an error.

With the previous commit we no longer see an assertion for this
target, now GDB abandons the inferior function call.

Although this is slightly better, it feels like for this specific case
GDB could do even better.  If during a call to reg_buffer::save GDB
encounters an error trying to read a register then GDB should simply
mark the register as unavailable and carry on.  The consequence of
marking the register unavailable is that GDB will not then try to
restore the register once the inferior function call is complete.

What I haven't done in this commit is provide any user feedback that
GDB would like to backup a particular register, but can't.  Right now
I figure that if the user cares about this they would probably try 'p
$reg_name' themselves, at which point it becomes obvious that the
register can't be read.  That said, I'm open to adding a warning that
the register failed to save if that is thought important.

I've tested this using on X86-64/Linux native, and for
native-gdbserver with no regressions.  Against my miss-behaving target
I can now make inferior calls without any problems.

gdb/ChangeLog:

	* regcache.c (reg_buffer::save): When saving the current register
	state, ignore registers that can't be read.
---
 gdb/ChangeLog  |  5 +++++
 gdb/regcache.c | 12 +++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/gdb/regcache.c b/gdb/regcache.c
index 6e0e8c3e7e0..c9503295f59 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -277,7 +277,17 @@ reg_buffer::save (register_read_ftype cooked_read)
       if (gdbarch_register_reggroup_p (gdbarch, regnum, save_reggroup))
 	{
 	  gdb_byte *dst_buf = register_buffer (regnum);
-	  enum register_status status = cooked_read (regnum, dst_buf);
+	  enum register_status status;
+
+	  TRY
+	    {
+	      status = cooked_read (regnum, dst_buf);
+	    }
+	  CATCH (ex, RETURN_MASK_ERROR)
+	    {
+	      status = REG_UNAVAILABLE;
+	    }
+	  END_CATCH
 
 	  gdb_assert (status != REG_UNKNOWN);
 
-- 
2.14.5