gdbserver: hide fork child threads from GDB

gdbserver: hide fork child threads from GDB

This patch aims at fixing a bug where an inferior is unexpectedly
created when a fork happens at the same time as another event, and that
other event is reported to GDB first (and the fork event stays pending
in GDBserver).  This happens for example when we step a thread and
another thread forks at the same time.  The bug looks like (if I
reproduce the included test by hand):

    (gdb) show detach-on-fork
    Whether gdb will detach the child of a fork is on.
    (gdb) show follow-fork-mode
    Debugger response to a program call of fork or vfork is "parent".
    (gdb) si
    [New inferior 2]
    Reading /home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.threads/step-while-fork-in-other-thread/step-while-fork-in-other-thread from remote target...
    Reading /home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.threads/step-while-fork-in-other-thread/step-while-fork-in-other-thread from remote target...
    Reading symbols from target:/home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.threads/step-while-fork-in-other-thread/step-while-fork-in-other-thread...
    [New Thread 965190.965190]
    [Switching to Thread 965190.965190]
    Remote 'g' packet reply is too long (expected 560 bytes, got 816 bytes): ... <long series of bytes>

The sequence of events leading to the problem is:

 - We are using the all-stop user-visible mode as well as the
   synchronous / all-stop variant of the remote protocol
 - We have two threads, thread A that we single-step and thread B that
   calls fork at the same time
 - GDBserver's linux_process_target::wait pulls the "single step
   complete SIGTRAP" and the "fork" events from the kernel.  It
   arbitrarily choses one event to report, it happens to be the
   single-step SIGTRAP.  The fork stays pending in the thread_info.
 - GDBserver send that SIGTRAP as a stop reply to GDB
 - While in stop_all_threads, GDB calls update_thread_list, which ends
   up querying the remote thread list using qXfer:threads:read.
 - In the reply, GDBserver includes the fork child created as a result
   of thread B's fork.
 - GDB-side, the remote target sees the new PID, calls
   remote_notice_new_inferior, which ends up unexpectedly creating a new
   inferior, and things go downhill from there.

The problem here is that as long as GDB did not process the fork event,
it should pretend the fork child does not exist.  Ultimately, this event
will be reported, we'll go through follow_fork, and that process will be
detached.

The remote target (GDB-side), has some code to remove from the reported
thread list the threads that are the result of forks not processed by
GDB yet.  But that only works for fork events that have made their way
to the remote target (GDB-side), but haven't been consumed by the core
yet, so are still lingering as pending stop replies in the remote target
(see remove_new_fork_children in remote.c).  But in our case, the fork
event hasn't made its way to the GDB-side remote target.  We need to
implement the same kind of logic GDBserver-side: if there exists a
thread / inferior that is the result of a fork event GDBserver hasn't
reported yet, it should exclude that thread / inferior from the reported
thread list.

This was actually discussed a while ago, but not implemented AFAIK:

    https://pi.simark.ca/gdb-patches/1ad9f5a8-d00e-9a26-b0c9-3f4066af5142@redhat.com/#t
    https://sourceware.org/pipermail/gdb-patches/2016-June/133906.html

Implementation details-wise, the fix for this is all in GDBserver.  The
Linux layer of GDBserver already tracks unreported fork parent / child
relationships using the lwp_info::fork_relative, in order to avoid
wildcard actions resuming fork childs unknown to GDB.  This information
needs to be made available to the handle_qxfer_threads_worker function,
so it can filter the reported threads.  Add a new thread_pending_parent
target function that allows the Linux target to return the parent of an
eventual fork child.

Testing-wise, the test replicates pretty-much the sequence of events
shown above.  The setup of the test makes it such that the main thread
is about to fork.  We stepi the other thread, so that the step completes
very quickly, in a single event.  Meanwhile, the main thread is resumed,
so very likely has time to call fork.  This means that the bug may not
reproduce every time (if the main thread does not have time to call
fork), but it will reproduce more often than not.  The test fails
without the fix applied on the native-gdbserver and
native-extended-gdbserver boards.

At some point I suspected that which thread called fork and which thread
did the step influenced the order in which the events were reported, and
therefore the reproducibility of the bug.  So I made the test try  both
combinations: main thread forks while other thread steps, and vice
versa.  I'm not sure this is still necessary, but I left it there
anyway.  It doesn't hurt to test a few more combinations.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28288
Change-Id: I2158d5732fc7d7ca06b0eb01f88cf27bf527b990