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

ChangeLog.rst

@@ -8,6 +8,7 @@ libfuse 3.5.0 (UNRELEASED)
   on mountpoints within SMB 2.0 filesystems).
 * Added a new `cache_readdir` flag to `fuse_file_info` to enable
   caching of readdir results. Supported by kernels 4.20 and newer.  
+* Add support and documentation for FUSE_CAP_NO_OPENDIR_SUPPORT.
 
 libfuse 3.4.2 (2019-03-09)
 ==========================

example/printcap.c

@@ -77,6 +77,8 @@ static void pc_init(void *userdata,
 			printf("\tFUSE_CAP_PARALLEL_DIROPS\n");
 	if(conn->capable & FUSE_CAP_POSIX_ACL)
 			printf("\tFUSE_CAP_POSIX_ACL\n");
+	if(conn->capable & FUSE_CAP_NO_OPENDIR_SUPPORT)
+			printf("\tFUSE_CAP_NO_OPENDIR_SUPPORT\n");
 	fuse_session_exit(se);
 }
 

include/fuse_common.h

@@ -335,6 +335,18 @@ 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
  *

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

lib/fuse_lowlevel.c

@@ -1907,6 +1907,8 @@ static void do_init(fuse_req_t req, fuse_ino_t nodeid, const void *inarg)
 			se->conn.capable |= FUSE_CAP_POSIX_ACL;
 		if (arg->flags & FUSE_HANDLE_KILLPRIV)
 			se->conn.capable |= FUSE_CAP_HANDLE_KILLPRIV;
+		if (arg->flags & FUSE_NO_OPENDIR_SUPPORT)
+			se->conn.capable |= FUSE_CAP_NO_OPENDIR_SUPPORT;
 	} else {
 		se->conn.max_readahead = 0;
 	}