gdb: move all bfd_cache_close_all calls in gdb_bfd.c

In the following commit I ran into a problem.  The next commit aims to
improve GDB's handling of the main executable being a file on a remote
target (i.e. one with a 'target:' prefix).

To do this I have replaced a system 'stat' call with a bfd_stat call.

However, doing this caused a regression in gdb.base/attach.exp.

The problem is that the bfd library caches open FILE* handles for bfd
objects that it has accessed, which is great for short-lived, non
interactive programs (e.g. the assembler, or objcopy, etc), however,
for GDB this caching causes us a problem.

If we open the main executable as a bfd then the bfd library will
cache the open FILE*.  If some time passes, maybe just sat at the GDB
prompt, or with the inferior running, and then later we use bfd_stat
to check if the underlying, on-disk file has changed, then the bfd
library will actually use fstat on the underlying file descriptor.
This is of course slightly different than using system stat on with
the on-disk file name.

If the on-disk file has changed then system stat will give results for
the current on-disk file.  But, if the bfd cache is still holding open
the file descriptor for the original on-disk file (from before the
change) then fstat will return a result based on the original file,
and so show no change as having happened.

This is a known problem in GDB, and so far this has been solved by
scattering bfd_cache_close_all() calls throughout GDB.  But, as I
said, in the next commit I've made a change and run into a
problem (gdb.base/attach.exp) where we are apparently missing a
bfd_cache_close_all() call.

Now I could solve this problem by adding a bfd_cache_close_all() call
before the bfd_stat call that I plan to add in the next commit, that
would for sure solve the problem, but feels a little crude.

Better I think would be to track down where the bfd is being opened
and add a corresponding bfd_cache_close_all() call elsewhere in GDB
once we've finished doing whatever it is that caused us to open the
bfd in the first place.

This second solution felt like the better choice, so I tracked the
problem down to elf_locate_base and fixed that.  But that just exposed
another problem in gdb_bfd_map_section which was also re-opening the
bfd, so I fixed this (with another bfd_cache_close_all() call), and
that exposed another issue in gdbarch_lookup_osabi... and at this
point I wondered if I was approaching this problem the wrong way...

.... And so, I wonder, is there a _better_ way to handle these
bfd_cache_close_all() calls?

I see two problems with the current approach:

  1. It's fragile.  Folk aren't always aware that they need to clear
  the bfd cache, and this feels like something that is easy to
  overlook in review.  So adding new code to GDB can innocently touch
  a bfd, which populates the cache, which will then be a bug that can
  lie hidden until an on-disk file just happens to change at the wrong
  time ... and GDB fails to spot the change.  Additionally,

  2. It's in efficient.  The caching is intended to stop the bfd
  library from continually having to re-open the on-disk file.  If we
  have a function that touches a bfd then often that function is the
  obvious place to call bfd_cache_close_all.  But if a single GDB
  command calls multiple functions, each of which touch the bfd, then
  we will end up opening and closing the same on-disk file multiple
  times.  It feels like we would be better postponing the
  bfd_cache_close_all call until some later point, then we can benefit
  from the bfd cache.

So, in this commit I propose a new approach.  We now clear the bfd
cache in two places:

  (a) Just before we display a GDB prompt.  We display a prompt after
  completing a command, and GDB is about to enter an idle state
  waiting for further input from the user (or in async mode, for an
  inferior event).  If while we are in this idle state the user
  changes the on-disk file(s) then we would like GDB to notice this
  the next time it leaves its idle state, e.g. the next time the user
  executes a command, or when an inferior event arrives,

  (b) When we resume the inferior.  In synchronous mode, resuming the
  inferior is another time when GDB is blocked and sitting idle, but
  in this case we don't display a prompt.  As with (a) above, when an
  inferior event arrives we want GDB to notice any changes to on-disk
  files.

It turns out that there are existing observers for both of these
cases (before_prompt and target_resumed respectively), so my initial
thought was that I should attach to these observers in gdb_bfd.c, and
in both cases call bfd_cache_close_all().

And this does indeed solve the gdb.base/attach.exp problem that I see
with the following commit.

However, I see a problem with this solution.

Both of the observers I'm using are exposed through the Python API as
events that a user can hook into.  The user can potentially run any
GDB command (using gdb.execute), so Python code might end up causing
some bfds to be reopened, and inserted into the cache.

To solve this one solution would be to add a bfd_cache_close_all()
call into gdbpy_enter::~gdbpy_enter().  Unfortunately, there's no
similar enter/exit object for Guile, though right now Guile doesn't
offer the same event API, so maybe we could just ignore that
problem... but this doesn't feel great.

So instead, I think a better solution might be to not use observers
for the bfd_cache_close_all() calls.  Instead, I'll call
bfd_cache_close_all() directly from core GDB after we've notified the
before_prompt and target_resumed observers, this was we can be sure
that the cache is cleared after the observers have run, and before GDB
enters an idle state.

This commit also removes all of the other bfd_cache_close_all() calls
from GDB.  My claim is that these are no longer needed.

Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 0 insertions, 2 deletions

diff --git a/gdb/exec.c b/gdb/exec.c
index 5956012..59965b8 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -500,8 +500,6 @@ exec_file_attach (const char *filename, int from_tty)
 	(*deprecated_exec_file_display_hook) (filename);
     }
 
-  bfd_cache_close_all ();
-
   /* Are are loading the same executable?  */
   bfd *prev_bfd = exec_bfd_holder.get ();
   bfd *curr_bfd = current_program_space->exec_bfd ();