Fix re-runs of a second inferior (PR gdb/25410)

Fix re-runs of a second inferior (PR gdb/25410)

This fixes a latent bug exposed by the multi-target patch (5b6d1e4fa
"Multi-target support), and then fixes two other latent bugs exposed
by fixing that first latent bug.

The symptom described in the bug report is that starting a first
inferior, then trying to run a second (multi-threaded) inferior twice,
causes libthread_db to fail to load, along with other erratic
behavior:

 (gdb) run
 Starting program: /tmp/foo
 warning: td_ta_new failed: generic error

Going a bit deeply, I found that if the two inferiors have different
symbols, we can see that just after inferior 2 exits, we are left with
inferior 2 selected, which is correct, but the symbols in scope belong
to inferior 1, which is obviously incorrect...

This problem is that there's a path in
scoped_restore_current_thread::restore() that switches to no thread
selected, and switches the current inferior, but leaves the current
program space as is, resulting in leaving the program space pointing
to the wrong program space (the one of the other inferior).  This was
happening after handling TARGET_WAITKIND_NO_RESUMED, which is an event
that triggers after TARGET_WAITKIND_EXITED for the previous inferior
exit.  Subsequent symbol lookups find the symbols of the wrong
inferior.

The fix is to use switch_to_inferior_no_thread in that problem spot.
This function was recently added along with the multi-target work
exactly for these situations.

As for testing, this patch adds a new testcase that tests symbol
printing just after inferior exit, which exercises the root cause of
the problem more directly.  And then, to cover the use case described
in the bug too, it also exercises the lithread_db.so mis-loading, by
using TLS printing as a proxy for being sure that threaded debugging
was activated sucessfully.  The testcase fails without the fix like
this, for the "print symbol just after exit" bits:

 ...
 [Inferior 1 (process 8719) exited normally]
 (gdb) PASS: gdb.multi/multi-re-run.exp: re_run_inf=1: iter=1: continue until exit
 print re_run_var_1
 No symbol "re_run_var_1" in current context.
 (gdb) FAIL: gdb.multi/multi-re-run.exp: re_run_inf=1: iter=1: print re_run_var_1
 ...

And like this for the "libthread_db.so loading" bits:

 (gdb) run
 Starting program: /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run
 warning: td_ta_new failed: generic error
 [New LWP 27001]

 Thread 1.1 "multi-re-run" hit Breakpoint 3, all_started () at /home/pedro/gdb/binutils-gdb/build/../src/gdb/testsuite/gdb.multi/multi-re-run.c:44
 44      }
 (gdb) PASS: gdb.multi/multi-re-run.exp: re_run_inf=1: iter=2: running to all_started in runto
 print tls_var
 Cannot find thread-local storage for LWP 27000, executable file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run:
 Cannot find thread-local variables on this target
 (gdb) FAIL: gdb.multi/multi-re-run.exp: re_run_inf=1: iter=2: print tls_var

As mentioned, that fix above goes on to expose a couple other latent
bugs.  This commit fixes those as well.

The first latent bug exposed is in
infrun.c:handle_vfork_child_exec_or_exit.  The current code is leaving
inf->pspace == NULL while calling clone_program_space.  The idea was
to make it so that the breakpoints module doesn't use this inferior's
pspace to set breakpoints.  With that, any
scoped_restore_current_thread use from within clone_program_space
tries to restore a NULL program space, which hits an assertion:

 Attaching after Thread 0x7ffff74b8700 (LWP 27276) vfork to child process 27277]
 [New inferior 2 (process 27277)]
 [Thread debugging using libthread_db enabled]
 Using host libthread_db library "/lib64/libthread_db.so.1".
 /home/pedro/gdb/binutils-gdb/build/../src/gdb/progspace.c:243: internal-error: void set_current_program_space(program_space*): Assertion `pspace != NULL' faile
 d.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n) FAIL: gdb.threads/vfork-follow-child-exit.exp: detach-on-fork=off: continue (GDB internal error)

That NULL pspace idea was legitimate, but it's no longer necessary,
since commit b2e586e850db ("Defer breakpoint reset when cloning
progspace for fork child").  So the fix is to just set the inferior's
program space earlier.

The other latent bug exposed is in exec.c.  When exec_close is called
from the program_space destructor, it is purposedly called with a
current program space that is not the current inferior's program
space.  The problem is that the multi-target work added some code to
remove_target_sections that loops over all inferiors, and uses
scoped_restore_current_thread to save/restore the previous
thread/inferior/frame state.  This makes it so that exec_close returns
with the current program space set to the current inferior's program
space, which is exactly what we did not want.  Then the program_space
destructor continues into free_all_objfiles, but it is now running
that method on the wrong program space, resulting in:

 Reading symbols from /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.threads/fork-plus-threads/fork-plus-threads...
 Reading symbols from /usr/lib/debug/usr/lib64/libpthread-2.26.so.debug...
 Reading symbols from /usr/lib/debug/usr/lib64/libm-2.26.so.debug...
 Reading symbols from /usr/lib/debug/usr/lib64/libc-2.26.so.debug...
 Reading symbols from /usr/lib/debug/usr/lib64/ld-2.26.so.debug...
 [Inferior 3 (process 9583) exited normally]
 /home/pedro/gdb/binutils-gdb/build/../src/gdb/progspace.c:170: internal-error: void program_space::free_all_objfiles(): Assertion `so->objfile == NULL' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n) FAIL: gdb.threads/fork-plus-threads.exp: detach-on-fork=off: inferior 1 exited (GDB internal error)

The fix is to use scoped_restore_current_pspace_and_thread instead of
scoped_restore_current_thread.

gdb/ChangeLog:
2020-01-24  Pedro Alves  <palves@redhat.com>

	PR gdb/25410
	* thread.c (scoped_restore_current_thread::restore): Use
	switch_to_inferior_no_thread.
	* exec.c: Include "progspace-and-thread.h".
	(add_target_sections, remove_target_sections):
	scoped_restore_current_pspace_and_thread instead of
	scoped_restore_current_thread.
	* infrun.c (handle_vfork_child_exec_or_exit): Assign the pspace
	and aspace to the inferior before calling clone_program_space.
	Remove stale comment.

gdb/testsuite/ChangeLog:
2020-01-24  Pedro Alves  <palves@redhat.com>

	PR gdb/25410
	* gdb.multi/multi-re-run-1.c: New.
	* gdb.multi/multi-re-run-2.c: New.
	* gdb.multi/multi-re-run.exp: New.