add flock fallback

Import the flock compatibility wrapper from NetBSD.

Work on #3340.

src/fallback.cpp

@@ -464,3 +464,76 @@ static int mk_wcswidth(const wchar_t *pwcs, size_t n) {
     return width;
 }
 #endif  // HAVE_BROKEN_WCWIDTH
+
+#ifndef HAVE_FLOCK
+/*	$NetBSD: flock.c,v 1.6 2008/04/28 20:24:12 martin Exp $	*/
+
+/*-
+ * Copyright (c) 2001 The NetBSD Foundation, Inc.
+ * All rights reserved.
+ *
+ * This code is derived from software contributed to The NetBSD Foundation
+ * by Todd Vierling.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/*
+ * Emulate flock() with fcntl(), where available.
+ * Otherwise, don't do locking; just pretend success.
+ */
+
+int flock(int fd, int op) {
+    int rc = 0;
+
+#if defined(F_SETLK) && defined(F_SETLKW)
+    struct flock fl = {0};
+
+    switch (op & (LOCK_EX|LOCK_SH|LOCK_UN)) {
+        case LOCK_EX:
+            fl.l_type = F_WRLCK;
+            break;
+
+        case LOCK_SH:
+            fl.l_type = F_RDLCK;
+            break;
+
+        case LOCK_UN:
+            fl.l_type = F_UNLCK;
+            break;
+
+        default:
+            errno = EINVAL;
+            return -1;
+    }
+
+    fl.l_whence = SEEK_SET;
+    rc = fcntl(fd, op & LOCK_NB ? F_SETLK : F_SETLKW, &fl);
+
+    if (rc && (errno == EAGAIN))
+        errno = EWOULDBLOCK;
+#endif
+
+    return rc;
+}
+
+#endif // HAVE_FLOCK