daemon: Fix behavior of read_pidfile() for our own pidfile.

Opening a file descriptor and then closing it always discards any locks
held on the underlying file, even if the file is still open as another file
descriptor.  This meant that calling read_pidfile() on the process's own
pidfile would discard the lock and make other OVS processes think that the
process had died.  This commit fixes the problem.

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/daemon.c b/lib/daemon.c
index 6b61879d..bbcfe6af 100644
--- a/lib/daemon.c
+++ b/lib/daemon.c
@@ -42,6 +42,10 @@ static bool detach;
 /* --pidfile: Name of pidfile (null if none). */
 static char *pidfile;
 
+/* Device and inode of pidfile, so we can avoid reopening it. */
+static dev_t pidfile_dev;
+static ino_t pidfile_ino;
+
 /* --overwrite-pidfile: Create pidfile even if one already exists and is
    locked? */
 static bool overwrite_pidfile;
@@ -208,6 +212,15 @@ make_pidfile(void)
                         close(fd);
                     } else {
                         /* Keep 'fd' open to retain the lock. */
+                        struct stat s;
+
+                        if (!fstat(fd, &s)) {
+                            pidfile_dev = s.st_dev;
+                            pidfile_ino = s.st_ino;
+                        } else {
+                            VLOG_ERR("%s: fstat failed: %s",
+                                     pidfile, strerror(errno));
+                        }
                     }
                     free(text);
                 } else {
@@ -494,9 +507,21 @@ read_pidfile(const char *pidfile)
 {
     char line[128];
     struct flock lck;
+    struct stat s;
     FILE *file;
     int error;
 
+    if ((pidfile_ino || pidfile_dev)
+        && !stat(pidfile, &s)
+        && s.st_ino == pidfile_ino && s.st_dev == pidfile_dev) {
+        /* It's our own pidfile.  We can't afford to open it, because closing
+         * *any* fd for a file that a process has locked also releases all the
+         * locks on that file.
+         *
+         * Fortunately, we know the associated pid anyhow: */
+        return getpid();
+    }
+
     file = fopen(pidfile, "r");
     if (!file) {
         error = errno;