[bug #24867] `define' should be thread-safe

[Linas Vepstas <INVALID.NOREPLY@gnu.org>]

Follow-up Comment #3, bug #24867 (project guile):

Note that the attached test case shows 4 distinct behaviours:

1) runs fine, exiting normally, w/o any errors

2) Spewing messages such as:
ERROR: Unbound variable: x2-347
ERROR: Unbound variable: x2-347
ERROR: Unbound variable: x2-347
ERROR: Unbound variable: define
ERROR: Unbound variable: x1-2525
ERROR: Unbound variable: x1-2525
ERROR: Unbound variable: x1-2525
ERROR: Unbound variable: x1-2525
ERROR: Unbound variable: x1-2525
ERROR: Unbound variable: x1-2525

  the numbers being different each time, of course, but then exiting
normally.

3) Deadlocking in garbage collection, with all four threads
   stuck in 
   #5  0xf7f01f6b in scm_gc_for_newcell (freelist=0xf7f8c8ec,
    free_cells=0x90c810c) at gc.c:484

  (this may be preceeded by the prints above)

4) Segfault:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xf74c5b90 (LWP 30649)]
0xf7f426ad in scm_assert_smob_type (tag=639, val=0x0) at smob.c:63
63	  if (!SCM_SMOB_PREDICATE (tag, val))
(gdb) bt
#0  0xf7f426ad in scm_assert_smob_type (tag=639, val=0x0) at smob.c:63
#1  0xf7f0b4c5 in scm_make_dynamic_state (parent=0x0) at fluids.c:508
#2  0xf7f62bdf in guilify_self_2 (parent=0x0) at threads.c:508
#3  0xf7f64992 in scm_i_init_thread_for_guile (base=0xf74c5388, parent=0x0)
    at threads.c:611
#4  0xf7f649c5 in scm_i_with_guile_and_parent (
    func=0x8048754 <guile_mode_definer>, data=0xffaf0cbc, parent=0x0)
    at threads.c:743
#5  0xf7f64ace in scm_with_guile (func=0x8048754 <guile_mode_definer>, 
    data=0xffaf0cbc) at threads.c:732
#6  0x080488a8 in definer ()
#7  0xf7fa14fb in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#8  0xf7e45e5e in clone () from /lib/tls/i686/cmov/libc.so.6


I'm guessing that the use of immutable vlists, as proposed by Ludo, for use
in the module-obarray per mail discussion, would solve all of the above.  


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