[GHC] #13945: 'ghc-pkg update' fails due to bad file descriptor error

[GHC]

#13945: 'ghc-pkg update' fails due to bad file descriptor error
---------------------------------+----------------------------------------
        Reporter:  mpickering    |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  high          |            Milestone:  8.2.2
       Component:  Compiler      |              Version:  8.2.1-rc3
      Resolution:                |             Keywords:
Operating System:  Linux         |         Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by bgamari):

 I am able to reproduce this locally. Here is a small reproducer,
 {{{#!c
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/file.h>

 int main () {
   int fd = open("libraries/bootstrapping.conf/package.cache.lock",
 O_RDONLY|O_NOCTTY|O_NONBLOCK);
   int res;

   struct stat stat;
   res = fstat(fd, &stat);
   printf("stat: %s\n", strerror(errno));

   res = flock(fd, LOCK_EX);
   printf("flock: %s\n", strerror(errno));

   res = flock(fd, LOCK_UN);
   printf("funlock: %s\n", strerror(errno));

   close(fd);
   return 0;
 }
 }}}

 When run in an NFS-mounted GHC tree with an existing lockfile this will
 fail with,
 {{{
 $ gcc test.c && ./a.out
 stat: Success
 flock: Bad file descriptor
 funlock: Bad file descriptor
 }}}

 Strangely if one changes `O_RDONLY` to `O_RDWR` the failure becomes,
 {{{
 $ gcc test.c && ./a.out
 stat: Success
 flock: No locks available
 funlock: No locks available
 }}}
 So I think this may be in part due to the read-only nature of the fd, but
 there may be more at play.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13945#comment:10>
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