[source] tools: flock: add NFSv4 compatibility

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/?p=source.git;a=commitdiff;h=5d0b180f798ca53cc04be4cec82c7c56b6c25d97

commit 5d0b180f798ca53cc04be4cec82c7c56b6c25d97
Author: Mathias Kresin <dev at kresin.me>
AuthorDate: Tue Aug 9 08:22:24 2016 +0200

    tools: flock: add NFSv4 compatibility
    
    This patch fixes the LEDE build on mounted NFSv4 shares.
    
    The lock file cannot be opened in read-write mode by default, because
    then we cannot use flock(1) to lock executable files.
    
    The read-write mode for lock files is necessary on NFSv4 where
    flock(2) is emulated by by fcntl() -- this situation is possible to
    detect by flock(2) EBADF error.
    
    The patch consist of the following util-linux/flock commits
    
    http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=eb742a1f66d5e3a7c5b43efce741c113f51bef3b
    
    http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=caf1ba11a367ad702fb774653daf9ebdcca49d7b
    
    without including the pre kernel 3.4 support.
    
    Signed-off-by: Mathias Kresin <dev at kresin.me>
    Signed-off-by: Felix Fietkau <nbd at nbd.name> [minor cleanup]
---
 tools/flock/src/flock.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/flock/src/flock.c b/tools/flock/src/flock.c
index 3ac3a4b..37ab0ea 100644
--- a/tools/flock/src/flock.c
+++ b/tools/flock/src/flock.c
@@ -133,6 +133,7 @@ int main(int argc, char *argv[])
   int do_close = 0;
   int err;
   int status;
+  int open_flags = 0;
   char *eon;
   char **cmd_argv = NULL, *sh_c_argv[4];
   const char *filename = NULL;
@@ -266,6 +267,21 @@ int main(int argc, char *argv[])
       if ( timeout_expired )
 	exit(1);		/* -w option set and failed to lock */
       continue;			/* otherwise try again */
+    case EBADF:			/* since Linux 3.4 (commit 55725513) */
+      /* Probably NFSv4 where flock() is emulated by fcntl().
+       * Let's try to reopen in read-write mode.
+       */
+      if (!(open_flags & O_RDWR) &&
+          type != LOCK_SH &&
+          filename &&
+          access(filename, R_OK | W_OK) == 0) {
+
+              close(fd);
+              open_flags = O_RDWR;
+              fd = open(filename, open_flags);
+              break;
+      }
+      /* go through */
     default:			/* Other errors */
       if ( filename )
 	fprintf(stderr, "%s: %s: %s\n", program, filename, strerror(err));