annotate lock.cpp @ 498:342822c2d49a

Merge api-inversion branch (-r656:771, but I don't expect to return to
that branch) into the trunk.

I expect there to be minor performance regressions (e.g. in the SOAP
server index cacheing, which I have forcibly removed) and minor
unplugged memory leaks (e.g. in audioDB::query(), where I don't free up
the datum).  I hope that these leaks and performance regressions can be
plugged in short order.  I also expect that some (but maybe not all) of
the issues currently addressed in the memory-leaks branch are superseded
or fixed by this merge.

There remains much work to be done; go forth and do it.

#include "audioDB.h"

int acquire_lock(int fd, bool exclusive) {
  struct flock lock;
  int status;
  
  lock.l_type = exclusive ? F_WRLCK : F_RDLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 0;
  lock.l_len = 0; /* "the whole file" */

 retry:
  do {
    status = fcntl(fd, F_SETLKW, &lock);
  } while (status != 0 && errno == EINTR);
  
  if (status) {
    if (errno == EAGAIN) {
      sleep(1);
      goto retry;
    } else {
      return status;
    }
  }
  return 0;
}

int divest_lock(int fd) {
  struct flock lock;

  lock.l_type = F_UNLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 0;
  lock.l_len = 0;

  return fcntl(fd, F_SETLKW, &lock);
}