X-Git-Url: https://git.distorted.org.uk/~mdw/misc/blobdiff_plain/c9a198787de95321b2b8cf69bacf012834acfdf1..HEAD:/locking.c

diff --git a/locking.c b/locking.c
index 9435318..6317b2e 100644
--- a/locking.c
+++ b/locking.c
@@ -39,6 +39,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <sys/wait.h>
+#include <sys/stat.h>
 
 #include <mLib/mdwopt.h>
 #include <mLib/quis.h>
@@ -47,6 +48,7 @@
 /*----- Static variables --------------------------------------------------*/
 
 static jmp_buf jmp;
+static int err;
 
 /*----- Main code ---------------------------------------------------------*/
 
@@ -88,14 +90,16 @@ int main(int argc, char *argv[])
   const char *prog = 0;
   char *const *av;
   void (*oalrm)(int) = 0;
-  int fd;
+  int fd = -1;
   struct flock l;
   char *p;
   int t = -1;
+  int oflag;
   unsigned int ot = 0;
+  struct stat st, nst;
   time_t nt;
   pid_t kid;
-  int st;
+  int rc;
 
 #define f_bogus 1u
 #define f_wait 2u
@@ -167,24 +171,41 @@ doneopts:
 
   av = &argv[optind];
   if (!prog) prog = av[0];
-  if ((fd = open(file,
-		 ((f & f_create ? O_CREAT : 0) |
-		  (f & f_excl ? O_RDWR : O_RDONLY)), 0666)) < 0)
-    die(111, "error opening `%s': %s", file, strerror(errno));
+  oflag = f & f_excl ? O_RDWR : O_RDONLY;
+  if (f & f_create) oflag |= O_CREAT;
   l.l_type = f & f_excl ? F_WRLCK : F_RDLCK;
   l.l_whence = SEEK_SET;
   l.l_start = 0;
   l.l_len = 0;
   nt = time(0);
   if (setjmp(jmp)) {
-    errno = EAGAIN;
+    err = EAGAIN;
     nt = t;
-  } else {
-    ot = alarm(0);
-    oalrm = signal(SIGALRM, alrm);
-    if (t >= 0) alarm(t);
-    if (fcntl(fd, f & f_wait ? F_SETLKW : F_SETLK, &l) >= 0) errno = 0;
+    goto done;
+  }
+  ot = alarm(0);
+  oalrm = signal(SIGALRM, alrm);
+  if (t >= 0) alarm(t);
+again:
+  if ((fd = open(file, oflag, 0666)) < 0)
+    die(111, "error opening `%s': %s", file, strerror(errno));
+  if (fstat(fd, &st))
+    die(111, "error from fstat on `%s': %s", file, strerror(errno));
+  err = fcntl(fd, f & f_wait ? F_SETLKW : F_SETLK, &l) >= 0 ? 0 : errno;
+  /* It's tempting to `optimize' this code by opening a new file descriptor
+   * here so as to elide the additional call to fstat(2) above.  But this
+   * doesn't work: if we successfully acquire the lock, we then have two file
+   * descriptors open on the lock file, so we have to close one -- but, under
+   * the daft fcntl(2) rules, even closing `nfd' will release the lock
+   * immediately.
+   */
+  if (stat(file, &nst)) {
+    if (errno == ENOENT) { close(fd); goto again; }
+    else die(111, "error from stat on `%s': %s", file, strerror(errno));
   }
+  if (st.st_dev != nst.st_dev || st.st_ino != nst.st_ino)
+    { close(fd); goto again; }
+done:
   signal(SIGALRM, oalrm);
   if (!ot)
     alarm(0);
@@ -193,28 +214,33 @@ doneopts:
     if (nt > ot) raise(SIGALRM);
     else alarm(ot - nt);
   }
-  if (errno &&
-      ((errno != EAGAIN && errno != EWOULDBLOCK && errno != EACCES) ||
-       (f & f_fail)))
-    die(111, "error locking `%s': %s", file, strerror(errno));
-  if (errno) exit(0);
-
+  switch (err) {
+    case 0: break;
+    case EAGAIN:
+#if defined(EWOULDBLOCK) && EWOULDBLOCK != EAGAIN
+    case EWOULDBLOCK:
+#endif
+    case EACCES:
+      if (!(f & f_fail)) exit(0);
+    default:
+      die(111, "error locking `%s': %s", file, strerror(errno));
+  }
   if ((kid = fork()) < 0) die(111, "error from fork: %s", strerror(errno));
   if (!kid) {
     close(fd);
     execvp(prog, av);
     die(111, "couldn't exec `%s': %s", prog, strerror(errno));
   }
-  if (waitpid(kid, &st, 0) < 0)
+  if (waitpid(kid, &rc, 0) < 0)
     die(EXIT_FAILURE, "error from wait: %s", strerror(errno));
   l.l_type = F_UNLCK;
   l.l_whence = SEEK_SET;
   l.l_start = 0;
   l.l_len = 0;
   fcntl(fd, F_SETLK, &l);
-  close(fd);
-  if (WIFEXITED(st)) exit(WEXITSTATUS(st));
-  else exit(255);
+  if (fd >= 0) close(fd);
+  if (WIFEXITED(rc)) exit(WEXITSTATUS(rc));
+  else exit(128 + WTERMSIG(rc));
 }
 
 /*----- That's all, folks -------------------------------------------------*/