osdep: Add qemu_lock_fd and qemu_unlock_fd

They are wrappers of POSIX fcntl "file private locking", with a
convenient "try lock" wrapper implemented with F_OFD_GETLK.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/qemu/osdep.h

@@ -341,6 +341,9 @@ int qemu_close(int fd);
 #ifndef _WIN32
 int qemu_dup(int fd);
 #endif
+int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive);
+int qemu_unlock_fd(int fd, int64_t start, int64_t len);
+int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);
 
 #if defined(__HAIKU__) && defined(__i386__)
 #define FMT_pid "%ld"

util/osdep.c

@@ -140,6 +140,54 @@ static int qemu_parse_fdset(const char *param)
 {
     return qemu_parse_fd(param);
 }
+
+static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int fl_type)
+{
+#ifdef F_OFD_SETLK
+    int ret;
+    struct flock fl = {
+        .l_whence = SEEK_SET,
+        .l_start  = start,
+        .l_len    = len,
+        .l_type   = fl_type,
+    };
+    ret = fcntl(fd, F_OFD_SETLK, &fl);
+    return ret == -1 ? -errno : 0;
+#else
+    return -ENOTSUP;
+#endif
+}
+
+int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive)
+{
+    return qemu_lock_fcntl(fd, start, len, exclusive ? F_WRLCK : F_RDLCK);
+}
+
+int qemu_unlock_fd(int fd, int64_t start, int64_t len)
+{
+    return qemu_lock_fcntl(fd, start, len, F_UNLCK);
+}
+
+int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive)
+{
+#ifdef F_OFD_SETLK
+    int ret;
+    struct flock fl = {
+        .l_whence = SEEK_SET,
+        .l_start  = start,
+        .l_len    = len,
+        .l_type   = exclusive ? F_WRLCK : F_RDLCK,
+    };
+    ret = fcntl(fd, F_OFD_GETLK, &fl);
+    if (ret == -1) {
+        return -errno;
+    } else {
+        return fl.l_type == F_UNLCK ? 0 : -EAGAIN;
+    }
+#else
+    return -ENOTSUP;
+#endif
+}
 #endif
 
 /*