[1.7] flock change breaks autotools 'make -j2'

[Eric Blake]

This change in cygwin 1.7:

- File locking is now advisory, not mandatory anymore.  The fcntl(2) and
  the new lockf(2) APIs create and maintain locks with POSIX semantics,
  the flock(2) API creates and maintains locks with BSD semantics.
  POSIX and BSD locks are independent of each other.

is causing parallel make bugs in autotooled projects:

http://lists.gnu.org/archive/html/bug-automake/2009-04/msg00048.html

The problem is that perl's flock is no longer locking files the way it used to, 
so parallel instances of autom4te (the engine behind both autoconf and 
automake) end up stomping on each others' output.  I don't know if it something 
that needs to be fixed in perl, in automake's XFile::lock wrapper around perl's 
flock, or even in cygwin itself.  But it seems like if two instances of the 
same perl program are using the same advisory locking mechanism, that they 
should keep each other out of the file properly (advisory means that processes 
not using flock can still stomp on the file, but not other processes using the 
same locking mechanism).

-- 
Eric Blake



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