Created attachment 424468 [details]
Test case.

Description of problem:

OCaml bindings take pointers to strings on the OCaml heap, and
then allow other threads to run.  For example:

  const char *filename = String_val (filenamev);
  int r;

  caml_enter_blocking_section ();
  r = guestfs_add_drive_ro (g, filename);

If other threads run (after caml_enter_blocking_section and while
waiting for the command to complete) then the GC could move or
reclaim the string pointed to by 'filename'.

Version-Release number of selected component (if applicable):

1.3.20

How reproducible:

Test case attached which reproduces the problem most of the time.

Steps to Reproduce:
1. Run test case with LIBGUESTFS_DEBUG=1
2.
3.
  
Actual results:

Strings are sometimes corrupted.

Expected results:

Strings should not be corrupted.

Additional info:

Should take a copy of strings before calling enter_blocking_section.