[1/2] gdb: move all bfd_cache_close_all calls in gdb_bfd.c
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Commit Message
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.
---
gdb/corefile.c | 5 -----
gdb/event-top.c | 20 +++++++++++++++++---
gdb/exec.c | 2 --
gdb/infcmd.c | 1 -
gdb/inferior.c | 2 --
gdb/symfile.c | 1 -
gdb/target.c | 5 -----
gdb/thread.c | 5 +++++
8 files changed, 22 insertions(+), 19 deletions(-)
@@ -120,11 +120,6 @@ reopen_exec_file (void)
&& current_program_space->ebfd_mtime
&& current_program_space->ebfd_mtime != st.st_mtime)
exec_file_attach (filename.c_str (), 0);
- else
- /* If we accessed the file since last opening it, close it now;
- this stops GDB from holding the executable open after it
- exits. */
- bfd_cache_close_all ();
}
/* If we have both a core file and an exec file,
@@ -462,6 +462,22 @@ display_gdb_prompt (const char *new_prompt)
}
}
+/* Notify the 'before_prompt' observer, and run any additional actions
+ that must be done before we display the prompt. */
+static void
+notify_before_prompt (const char *prompt)
+{
+ /* Give observers a chance of changing the prompt. E.g., the python
+ `gdb.prompt_hook' is installed as an observer. */
+ gdb::observers::before_prompt.notify (prompt);
+
+ /* As we are about to display the prompt, and so GDB might be sitting
+ idle for some time, close all the cached BFDs. This ensures that
+ when we next start running a user command all BFDs will be reopened
+ as needed, and as a result, we will see any on-disk changes. */
+ bfd_cache_close_all ();
+}
+
/* Return the top level prompt, as specified by "set prompt", possibly
overridden by the python gdb.prompt_hook hook, and then composed
with the prompt prefix and suffix (annotations). */
@@ -469,9 +485,7 @@ display_gdb_prompt (const char *new_prompt)
static std::string
top_level_prompt (void)
{
- /* Give observers a chance of changing the prompt. E.g., the python
- `gdb.prompt_hook' is installed as an observer. */
- gdb::observers::before_prompt.notify (get_prompt ().c_str ());
+ notify_before_prompt (get_prompt ().c_str ());
const std::string &prompt = get_prompt ();
@@ -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 ();
@@ -2498,7 +2498,6 @@ kill_command (const char *arg, int from_tty)
int infnum = current_inferior ()->num;
target_kill ();
- bfd_cache_close_all ();
update_previous_thread ();
@@ -714,8 +714,6 @@ kill_inferior_command (const char *args, int from_tty)
target_kill ();
}
-
- bfd_cache_close_all ();
}
/* See inferior.h. */
@@ -1124,7 +1124,6 @@ symbol_file_add_with_addrs (const gdb_bfd_ref_ptr &abfd, const char *name,
gdb::observers::new_objfile.notify (objfile);
- bfd_cache_close_all ();
return objfile;
}
@@ -2729,11 +2729,6 @@ target_mourn_inferior (ptid_t ptid)
{
gdb_assert (ptid.pid () == inferior_ptid.pid ());
current_inferior ()->top_target ()->mourn_inferior ();
-
- /* We no longer need to keep handles on any of the object files.
- Make sure to release them to avoid unnecessarily locking any
- of them while we're not actually debugging. */
- bfd_cache_close_all ();
}
/* Look for a target which can describe architectural features, starting
@@ -876,6 +876,11 @@ notify_target_resumed (ptid_t ptid)
{
interps_notify_target_resumed (ptid);
gdb::observers::target_resumed.notify (ptid);
+
+ /* We are about to resume the inferior. Close all cached BFDs so that
+ when the inferior next stops, and GDB regains control, we will spot
+ any on-disk changes to the BFDs we are using. */
+ bfd_cache_close_all ();
}
/* See gdbthread.h. */