Handle NO_OPEN/NO_OPENDIR support automatically (#949)

If the file system doesn't provide a ->open or an ->opendir, and the
kernel supports FUSE_CAP_NO_OPEN_SUPPORT or FUSE_CAP_NO_OPENDIR_SUPPORT,
allow the implementation to set FUSE_CAP_NO_OPEN*_SUPPORT on conn->want
in order to automatically get this behavior.  Expand the documentation
to be more explicit about the behavior of libfuse in the different cases
WRT this capability.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>

2 files changed, 22 insertions, 3 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index bafcda0..8803b68 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -325,8 +325,10 @@ struct fuse_loop_config_v1 {
  * kernel. (If this flag is not set, returning ENOSYS will be treated
  * as an error and signaled to the caller).
  *
- * Setting (or unsetting) this flag in the `want` field has *no
- * effect*.
+ * Setting this flag in the `want` field enables this behavior automatically
+ * within libfuse for low level API users. If non-low level users wish to have
+ * this behavior you must return `ENOSYS` from the open() handler on supporting
+ * kernels.
  */
 #define FUSE_CAP_NO_OPEN_SUPPORT	(1 << 17)
 
@@ -404,7 +406,10 @@ struct fuse_loop_config_v1 {
  * flag is not set, returning ENOSYS will be treated as an error and signalled
  * to the caller.)
  *
- * Setting (or unsetting) this flag in the `want` field has *no effect*.
+ * Setting this flag in the `want` field enables this behavior automatically
+ * within libfuse for low level API users.  If non-low level users with to have
+ * this behavior you must return `ENOSYS` from the opendir() handler on
+ * supporting kernels.
  */
 #define FUSE_CAP_NO_OPENDIR_SUPPORT    (1 << 24)
 
diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index 330d40e..8ff111a 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -532,6 +532,13 @@ struct fuse_lowlevel_ops {
 	 * future calls to open and release will also succeed without being
 	 * sent to the filesystem process.
 	 *
+	 * To get this behavior without providing an opendir handler, you may
+	 * set FUSE_CAP_NO_OPEN_SUPPORT in `fuse_conn_info.want` on supported
+	 * kernels to automatically get the zero message open().
+	 *
+	 * If this callback is not provided and FUSE_CAP_NO_OPEN_SUPPORT is not
+	 * set in `fuse_conn_info.want` then an empty reply will be sent.
+	 *
 	 * Valid replies:
 	 *   fuse_reply_open
 	 *   fuse_reply_err
@@ -705,6 +712,13 @@ struct fuse_lowlevel_ops {
 	 * process. In addition, the kernel will cache readdir results
 	 * as if opendir returned FOPEN_KEEP_CACHE | FOPEN_CACHE_DIR.
 	 *
+	 * To get this behavior without providing an opendir handler, you may
+	 * set FUSE_CAP_NO_OPENDIR_SUPPORT in `fuse_conn_info.want` on supported
+	 * kernels to automatically get the zero message opendir().
+	 *
+	 * If this callback is not provided and FUSE_CAP_NO_OPENDIR_SUPPORT is
+	 * not set in `fuse_conn_info.want` then an empty reply will be sent.
+	 *
 	 * Valid replies:
 	 *   fuse_reply_open
 	 *   fuse_reply_err