windows: Added lockf function and lock PID file

If the PID file isn't locked then appctl.py detects it as stale and
bails out without doing anything. Because of this lots of Python tests fail.
Also this protects the PID file from being overwritten.

I used only shared lock, in order to be compatible with Python tests,
which try to acquire the lock exclusively. On Windows if the exclusive lock
is used, than the read access is denied too for other instances of this file.

Signed-off-by: Paul-Daniel Boca <pboca@cloudbasesolutions.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>

lib/daemon-windows.c

@@ -18,6 +18,7 @@
 #include "daemon.h"
 #include "daemon-private.h"
 #include <stdio.h>
+#include <io.h>
 #include <stdlib.h>
 #include "dirs.h"
 #include "ovs-thread.h"
@@ -26,6 +27,10 @@
 
 VLOG_DEFINE_THIS_MODULE(daemon_windows);
 
+/* Constants for flock function */
+#define	LOCK_SHARED	0x0                     /* Shared lock. */
+#define	LOCK_UNLOCK	0x80000000              /* Unlock. Custom value. */
+
 static bool service_create;          /* Was --service specified? */
 static bool service_started;         /* Have we dispatched service to start? */
 
@@ -403,10 +408,40 @@ detach_process(int argc, char *argv[])
     exit(0);
 }
 
+static void
+flock(FILE* fd, int operation)
+{
+    HANDLE hFile;
+    OVERLAPPED ov = {0};
+
+    hFile = (HANDLE)_get_osfhandle(fileno(fd));
+    if (hFile == INVALID_HANDLE_VALUE) {
+        VLOG_FATAL("Failed to get PID file handle (%s).",
+                   ovs_lasterror_to_string());
+    }
+
+    if (operation & LOCK_UNLOCK) {
+        if (UnlockFileEx(hFile, 0, 1, 0, &ov) == 0) {
+            VLOG_FATAL("Failed to unlock PID file (%s).",
+                       ovs_lasterror_to_string());
+        }
+    } else {
+       /* Use LOCKFILE_FAIL_IMMEDIATELY flag to avoid hang of another daemon that tries to
+           acquire exclusive lock over the same PID file */
+        if (LockFileEx(hFile, operation | LOCKFILE_FAIL_IMMEDIATELY,
+                       0, 1, 0, &ov) == FALSE) {
+            VLOG_FATAL("Failed to lock PID file (%s).",
+                       ovs_lasterror_to_string());
+        }
+    }
+}
+
 static void
 unlink_pidfile(void)
 {
     if (filep_pidfile) {
+        /* Remove the shared lock on file */
+        flock(filep_pidfile, LOCK_UNLOCK);
         fclose(filep_pidfile);
     }
     if (pidfile) {
@@ -437,13 +472,19 @@ make_pidfile(void)
         VLOG_FATAL("failed to open %s (%s)", pidfile, ovs_strerror(errno));
     }
 
+    flock(filep_pidfile, LOCKFILE_EXCLUSIVE_LOCK);
+
     fatal_signal_add_hook(unlink_pidfile, NULL, NULL, true);
 
     fprintf(filep_pidfile, "%d\n", _getpid());
     if (fflush(filep_pidfile) == EOF) {
         VLOG_FATAL("Failed to write into the pidfile %s", pidfile);
     }
 
+    flock(filep_pidfile, LOCK_SHARED);
+    /* This will remove the exclusive lock. The shared lock will remain */
+    flock(filep_pidfile, LOCK_UNLOCK);
+
     /* Don't close the pidfile till the process exits. */
 }