[PATCH] [PR gdb/23077] Fix error 'Couldn't get registers: Device busy' on FreeBSD

[Rajendra SY <rajendra dot sy at gmail dot com>]

<patch attached>

Problem:
GDB  "attach pid" command shows error message "Couldn't get registers:
Device busy" because of which below gdb tests fail.

Cause:
On FreeBSD ptrace() syscall requires LWP id to fetch info from the
inferior process.
The current implementation is every command is executed completely and
then it wait's for asynchronous events.

When attach command is executed it calls inf_ptrace_attach() function
which is currently doing just the ptrace(PT_ATTACH), it does not wait
for the process to stop to fetch LWP info.
The fbsd_wait() function is the one which fetches LWP info which is
being called after the command completes.

Command execution code flow -
gdb_do_one_event() => .. => handle_file_event() => .. =>
execute_command() => check_frame_language_change()

The async event handling code flow where LWP info gets updated -
gdb_one_event() => check_async_event_handlers() => ..
->do_target_wait() => .. => fbsd_wait()

The check, if the frame language changed via
check_frame_language_change() function is trying to read registers
using ptrace(pid) which fails because LWP id is not fetched.

The purposed patch tries to avoid this situation by avoiding calling
check_frame_language_change() function just for the "attach" command.

Tests:
FAIL: gdb.base/attach.exp: attach1, after setting file
FAIL: gdb.base/attach.exp: attach2, with no file
FAIL: gdb.base/attach.exp: load file manually, after attach2 (re-read)
(got interactive prompt)
FAIL: gdb.base/attach.exp: attach when process' a.out not in cwd

FAIL: gdb.base/dprintf-detach.exp: bai=on ds=gdb dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=on ds=gdb dd=off: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=on ds=call dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=on ds=call dd=off: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=on ds=agent dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=on ds=agent dd=off: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=gdb dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=gdb dd=off: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=call dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=call dd=off: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=agent dd=on: re-attach to inferior
FAIL: gdb.base/dprintf-detach.exp: bai=off ds=agent dd=off: re-attach
to inferior

These above currently failing tests will pass with this fix.

gdb/ChangeLog:
2018-04-20  Rajendra SY  <rajendra.sy@gmail.com>

        PR gdb/23077
        * gdb/top.c (execute_command):

Attachment: gdb-skip-check_frame_language_change.diff
Description: Binary data