[PATCH v3] file-posix: detect the lock using the real file

[Li Feng <fengli@smartx.com>]

This patch addresses this issue:
When accessing a volume on an NFS filesystem without supporting the file lock,
tools, like qemu-img, will complain "Failed to lock byte 100".

Add a new function 'qemu_has_file_lock' to detect if the filesystem supports locks
or not.
And when the drive is auto mode, use the 'qemu_has_file_lock' to set the toggle.

Signed-off-by: Li Feng <fengli@smartx.com>
---
v3: don't call the qemu_has_ofd_lock, use a new function instead.
v2: remove the refactoring.
---
 block/file-posix.c   | 30 +++++++++++++++++-------------
 include/qemu/osdep.h |  1 +
 util/osdep.c         | 29 +++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 13 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 806764f7e3..48f9a32de2 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -606,7 +606,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
         s->use_lock = false;
         break;
     case ON_OFF_AUTO_AUTO:
-        s->use_lock = qemu_has_ofd_lock();
+        s->use_lock = qemu_has_file_lock(filename);
         break;
     default:
         abort();
@@ -2388,6 +2388,7 @@ raw_co_create(BlockdevCreateOptions *options, Error **errp)
     int fd;
     uint64_t perm, shared;
     int result = 0;
+    bool use_lock;
 
     /* Validate options and set default values */
     assert(options->driver == BLOCKDEV_DRIVER_FILE);
@@ -2428,19 +2429,22 @@ raw_co_create(BlockdevCreateOptions *options, Error **errp)
     perm = BLK_PERM_WRITE | BLK_PERM_RESIZE;
     shared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
 
-    /* Step one: Take locks */
-    result = raw_apply_lock_bytes(NULL, fd, perm, ~shared, false, errp);
-    if (result < 0) {
-        goto out_close;
-    }
+    use_lock = qemu_has_file_lock(file_opts->filename);
+    if (use_lock) {
+        /* Step one: Take locks */
+        result = raw_apply_lock_bytes(NULL, fd, perm, ~shared, false, errp);
+        if (result < 0) {
+            goto out_close;
+        }
 
-    /* Step two: Check that nobody else has taken conflicting locks */
-    result = raw_check_lock_bytes(fd, perm, shared, errp);
-    if (result < 0) {
-        error_append_hint(errp,
-                          "Is another process using the image [%s]?\n",
-                          file_opts->filename);
-        goto out_unlock;
+        /* Step two: Check that nobody else has taken conflicting locks */
+        result = raw_check_lock_bytes(fd, perm, shared, errp);
+        if (result < 0) {
+            error_append_hint(errp,
+                              "Is another process using the image [%s]?\n",
+                              file_opts->filename);
+            goto out_unlock;
+        }
     }
 
     /* Clear the file by truncating it to 0 */
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index f9ec8c84e9..593ae0f4d2 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -513,6 +513,7 @@ 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);
 bool qemu_has_ofd_lock(void);
+bool qemu_has_file_lock(const char *filename);
 #endif
 
 #if defined(__HAIKU__) && defined(__i386__)
diff --git a/util/osdep.c b/util/osdep.c
index 66d01b9160..5d9961d261 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -225,6 +225,35 @@ static void qemu_probe_lock_ops(void)
     }
 }
 
+bool qemu_has_file_lock(const char *filename)
+{
+#ifdef F_OFD_SETLK
+    int cmd = F_OFD_GETLK;
+#else
+    int cmd = F_GETLK;
+#endif
+        int fd;
+        int ret;
+        struct flock fl = {
+            .l_whence = SEEK_SET,
+            .l_start  = 0,
+            .l_len    = 0,
+            .l_type   = F_WRLCK,
+        };
+
+        fd = open(filename, O_RDWR);
+        if (fd < 0) {
+            fprintf(stderr,
+                    "Failed to open %s for OFD lock probing: %s\n",
+                    filename,
+                    strerror(errno));
+            return false;
+        }
+        ret = fcntl(fd, cmd, &fl);
+        close(fd);
+        return ret == 0;
+}
+
 bool qemu_has_ofd_lock(void)
 {
     qemu_probe_lock_ops();
-- 
2.24.3