Attempt to fix the sporadic uvar test failures on Linux

We identify when the universal variable file has changed out from under us by
comparing a bunch of fields from its stat: inode, device, size, high-precision
timestamp, generation. Linux aggressively reuses inodes, and the size may be
the same by coincidence (which is the case in the tests). Also, Linux
officially has nanosecond precision, but in practice it seems to only uses
millisecond precision for storing mtimes. Thus if there are three or more
updates within a millisecond, every field we check may be the same, and we are
vulnerable to the ABA problem. I believe this explains the occasional test
failures.

The solution is to manually set the nanosecond field of the mtime timestamp to
something unlikely to be duplicated, like a random number, or better yet, the
current time (with nanosecond precision). This is more in the spirit of the
timestamp, and it means we're around a million times less likely to collide.
This seems to fix the tests.

1 files changed, 1 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index f0c4d8b5..b5c16552 100644
--- a/configure.ac
+++ b/configure.ac
@@ -525,6 +525,7 @@ AC_CHECK_FUNCS( wcsdup wcsndup wcslen wcscasecmp wcsncasecmp fwprintf )
 AC_CHECK_FUNCS( futimes wcwidth wcswidth wcstok fputwc fgetwc )
 AC_CHECK_FUNCS( wcstol wcslcat wcslcpy lrand48_r killpg mkostemp )
 AC_CHECK_FUNCS( backtrace backtrace_symbols sysconf getifaddrs )
+AC_CHECK_FUNCS( futimens clock_gettime )
 
 if test x$local_gettext != xno; then
   AC_CHECK_FUNCS( gettext dcgettext )