Add documentation for opting out of opendir and releasedir (#391)

2 files changed, 19 insertions, 0 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index 823a37d..0481aac 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -336,6 +336,18 @@ struct fuse_loop_config {
 #define FUSE_CAP_HANDLE_KILLPRIV         (1 << 20)
 
 /**
+ * Indicates support for zero-message opendirs. If this flag is set in
+ * the `capable` field of the `fuse_conn_info` structure, then the filesystem
+ * may return `ENOSYS` from the opendir() handler to indicate success. Further
+ * opendir and releasedir messages will be handled in the kernel. (If this
+ * 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*.
+ */
+#define FUSE_CAP_NO_OPENDIR_SUPPORT    (1 << 24)
+
+/**
  * Ioctl flags
  *
  * FUSE_IOCTL_COMPAT: 32bit compat ioctl on 64bit machine
diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index 87c362a..b7afe16 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -665,6 +665,13 @@ struct fuse_lowlevel_ops {
 	 * etc) in fi->fh, and use this in other all other directory
 	 * stream operations (readdir, releasedir, fsyncdir).
 	 *
+	 * If this request is answered with an error code of ENOSYS and
+	 * FUSE_CAP_NO_OPENDIR_SUPPORT is set in `fuse_conn_info.capable`,
+	 * this is treated as success and future calls to opendir and
+	 * releasedir will also succeed without being sent to the filesystem
+	 * process. In addition, the kernel will cache readdir results
+	 * as if opendir returned FOPEN_KEEP_CACHE | FOPEN_CACHE_DIR.
+	 *
 	 * Valid replies:
 	 *   fuse_reply_open
 	 *   fuse_reply_err