gdbserver: switch to right process in find_one_thread

gdbserver: switch to right process in find_one_thread

New in this version: add a dedicated test.

When I do this:

    $ ./gdb -nx --data-directory=data-directory -q \
        /bin/sleep \
	-ex "maint set target-non-stop on" \
	-ex "tar ext :1234" \
	-ex "set remote exec-file /bin/sleep" \
	-ex "run 1231 &" \
	-ex add-inferior \
	-ex "inferior 2"
    Reading symbols from /bin/sleep...
    (No debugging symbols found in /bin/sleep)
    Remote debugging using :1234
    Starting program: /bin/sleep 1231
    Reading /lib64/ld-linux-x86-64.so.2 from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /lib64/ld-linux-x86-64.so.2 from remote target...
    Reading /usr/lib/debug/.build-id/a6/7a1408f18db3576757eea210d07ba3fc560dff.debug from remote target...
    [New inferior 2]
    Added inferior 2 on connection 1 (extended-remote :1234)
    [Switching to inferior 2 [<null>] (<noexec>)]
    (gdb) Reading /lib/x86_64-linux-gnu/libc.so.6 from remote target...
    attach 3659848
    Attaching to process 3659848
    /home/smarchi/src/binutils-gdb/gdb/thread.c:85: internal-error: inferior_thread: Assertion `current_thread_ != nullptr' failed.

Note the "attach" command just above.  When doing it on the command-line
with a -ex switch, the bug doesn't trigger.

The internal error of GDB is actually caused by GDBserver crashing, and
the error recovery of GDB is not on point.  This patch aims to fix just
the GDBserver crash, not the GDB problem.

GDBserver crashes with a segfault here:

    (gdb) bt
    #0  0x00005555557fb3f4 in find_one_thread (ptid=...) at /home/smarchi/src/binutils-gdb/gdbserver/thread-db.cc:177
    #1  0x00005555557fd5cf in thread_db_thread_handle (ptid=<error reading variable: Cannot access memory at address 0xffffffffffffffa0>, handle=0x7fffffffc400, handle_len=0x7fffffffc3f0)
        at /home/smarchi/src/binutils-gdb/gdbserver/thread-db.cc:461
    #2  0x000055555578a0b6 in linux_process_target::thread_handle (this=0x5555558a64c0 <the_x86_target>, ptid=<error reading variable: Cannot access memory at address 0xffffffffffffffa0>, handle=0x7fffffffc400,
        handle_len=0x7fffffffc3f0) at /home/smarchi/src/binutils-gdb/gdbserver/linux-low.cc:6905
    #3  0x00005555556dfcc6 in handle_qxfer_threads_worker (thread=0x60b000000510, buffer=0x7fffffffc8a0) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:1645
    #4  0x00005555556e00e6 in operator() (__closure=0x7fffffffc5e0, thread=0x60b000000510) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:1696
    #5  0x00005555556f54be in for_each_thread<handle_qxfer_threads_proper(buffer*)::<lambda(thread_info*)> >(struct {...}) (func=...) at /home/smarchi/src/binutils-gdb/gdbserver/gdbthread.h:159
    #6  0x00005555556e0242 in handle_qxfer_threads_proper (buffer=0x7fffffffc8a0) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:1694
    #7  0x00005555556e04ba in handle_qxfer_threads (annex=0x629000000213 "", readbuf=0x621000019100 '\276' <repeats 200 times>..., writebuf=0x0, offset=0, len=4097)
        at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:1732
    #8  0x00005555556e1989 in handle_qxfer (own_buf=0x629000000200 "qXfer:threads", packet_len=26, new_packet_len_p=0x7fffffffd630) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:2045
    #9  0x00005555556e720a in handle_query (own_buf=0x629000000200 "qXfer:threads", packet_len=26, new_packet_len_p=0x7fffffffd630) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:2685
    #10 0x00005555556f1a01 in process_serial_event () at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:4176
    #11 0x00005555556f4457 in handle_serial_event (err=0, client_data=0x0) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:4514
    #12 0x0000555555820f56 in handle_file_event (file_ptr=0x607000000250, ready_mask=1) at /home/smarchi/src/binutils-gdb/gdbsupport/event-loop.cc:573
    #13 0x0000555555821895 in gdb_wait_for_event (block=1) at /home/smarchi/src/binutils-gdb/gdbsupport/event-loop.cc:694
    #14 0x000055555581f533 in gdb_do_one_event (mstimeout=-1) at /home/smarchi/src/binutils-gdb/gdbsupport/event-loop.cc:264
    #15 0x00005555556ec9fb in start_event_loop () at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:3512
    #16 0x00005555556f0769 in captured_main (argc=4, argv=0x7fffffffe0d8) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:3992
    #17 0x00005555556f0e3f in main (argc=4, argv=0x7fffffffe0d8) at /home/smarchi/src/binutils-gdb/gdbserver/server.cc:4078

The reason is a wrong current process when find_one_thread is called.
The current process is the 2nd one, which was just attached.  It does
not yet have thread_db data (proc->priv->thread_db is nullptr).  As we
iterate on all threads of all process to fulfull the qxfer:threads:read
request, we get to a thread of process 1 for which we haven't read
thread_db information yet (lwp_info::thread_known is false), so we get
into find_one_thread.  find_one_thread uses
`current_process ()->priv->thread_db`, assuming the current process
matches the ptid passed as a parameter, which is wrong.  A segfault
happens when trying to dereference that thread_db pointer.

Fix this by making find_one_thread not assume what the current process /
current thread is.  If it needs to call into libthread_db, which we know
will try to read memory from the current process, then temporarily set
the current process.

In the case where the thread is already know and we return early, we
don't need to switch process.

Add a test to reproduce this specific situation.

Change-Id: I09b00883e8b73b7e5f89d0f47cb4e9c0f3d6caaa
Approved-By: Andrew Burgess <aburgess@redhat.com>