Dropped support for writeback caching

As of kernel 4.14, the FUSE module's + writeback implementation is not
compatible with network filesystems, and there are no imminent plans
to change that.

For more details, see
https://marc.info/?l=fuse-devel&m=150592103107662&w=2 or

As a consequence, the -o unreliable_append option has become obsolete
as well.

Fixes: #93
Fixes: #88
Fixes: #81

ChangeLog.rst

@@ -1,6 +1,10 @@
 Unreleased Changes
 ------------------
 
+* Dropped support for writeback caching (and, as a consequence,
+  "unreliable append" operation). As of kernel 4.14, the FUSE module's
+  writeback implementation is not compatible with network filesystems
+  and there are no imminent plans to change that.
 * Add support for mounting from /etc/fstab
 * Dropped support for building with autotools.
 * Added missing options to man page.

sshfs.c

@@ -226,7 +226,6 @@ struct sshfs {
 	int nomap;
 	int disable_hardlink;
 	int dir_cache;
-	int writeback_cache;
 	int show_version;
 	int show_help;
 	int singlethread;
@@ -408,9 +407,6 @@ static struct fuse_opt sshfs_opts[] = {
 	SSHFS_OPT("disable_hardlink",  disable_hardlink, 1),
 	SSHFS_OPT("dir_cache=yes", dir_cache, 1),
 	SSHFS_OPT("dir_cache=no",  dir_cache, 0),
-	SSHFS_OPT("writeback_cache=yes", writeback_cache, 1),
-	SSHFS_OPT("writeback_cache=no", writeback_cache, 0),
-	SSHFS_OPT("unreliable_append", unrel_append, 1),
 
 	SSHFS_OPT("-h",		show_help, 1),
 	SSHFS_OPT("--help",	show_help, 1),
@@ -429,6 +425,10 @@ static struct fuse_opt sshfs_opts[] = {
 	SSHFS_OPT("cache=yes", dir_cache, 1),
 	SSHFS_OPT("cache=no",  dir_cache, 0),
 
+	FUSE_OPT_KEY("writeback_cache=no", FUSE_OPT_KEY_DISCARD),
+	FUSE_OPT_KEY("unreliable_append", FUSE_OPT_KEY_DISCARD),
+
+
 	FUSE_OPT_END
 
 };
@@ -1736,10 +1736,6 @@ static void *sshfs_init(struct fuse_conn_info *conn,
         // Lookup of . and .. is supported
         conn->capable |= FUSE_CAP_EXPORT_SUPPORT;
 
-	// Enable writeback cache if supported
-	if (sshfs.writeback_cache && (conn->capable & FUSE_CAP_WRITEBACK_CACHE))
-		conn->want |= FUSE_CAP_WRITEBACK_CACHE;
-
 	if (!sshfs.delay_connect)
 		start_processing_thread();
 
@@ -2502,24 +2498,6 @@ static int sshfs_open_common(const char *path, mode_t mode,
 	if (sshfs.dir_cache)
 		wrctr = cache_get_write_ctr();
 
-	/* With writeback cache, kernel may send read requests even
-	   when userspace opened write-only */
-	if (sshfs.writeback_cache &&
-	    (fi->flags & O_ACCMODE) == O_WRONLY) {
-		fi->flags &= ~O_ACCMODE;
-		fi->flags |= O_RDWR;
-	}
-
-	/* Having the kernel handle O_APPEND doesn't work reliably, if
-	   the file changes on the server at the wrong time, we will
-	   overwrite data instead of appending. */
-	if ((fi->flags & O_APPEND) && sshfs.writeback_cache) {
-		if(sshfs.unrel_append)
-			fi->flags &= ~O_APPEND;
-		else
-			return -EINVAL;
-	}
-
 	if ((fi->flags & O_ACCMODE) == O_RDONLY)
 		pflags = SSH_FXF_READ;
 	else if((fi->flags & O_ACCMODE) == O_WRONLY)
@@ -3364,9 +3342,7 @@ static void usage(const char *progname)
 "    -o sshfs_sync          synchronous writes\n"
 "    -o no_readahead        synchronous reads (no speculative readahead)\n"
 "    -o sync_readdir        synchronous readdir\n"
-"    -o unreliable_append   Enable (unreliable) O_APPEND support\n"
 "    -d, --debug            print some debugging information (implies -f)\n"
-"    -o writeback_cache=BOOL enable writeback cache {yes,no} (default: yes)\n"
 "    -o dir_cache=BOOL      enable caching of directory contents (names,\n"
 "                           attributes, symlink targets) {yes,no} (default: yes)\n"
 "    -o dcache_max_size=N   sets the maximum size of the directory cache (default: 10000)\n"
@@ -3876,8 +3852,6 @@ int main(int argc, char *argv[])
 	sshfs.wfd = -1;
 	sshfs.ptyfd = -1;
 	sshfs.dir_cache = 1;
-	sshfs.writeback_cache = 1;
-	sshfs.unrel_append = 0;
 	sshfs.show_help = 0;
 	sshfs.show_version = 0;
 	sshfs.singlethread = 0;

sshfs.rst

@@ -175,21 +175,6 @@ Options
 -o password_stdin
    read password from stdin (only for pam_mount!)
 
--o writeback_cache=BOOL
-   Enables (*yes*) or disables (*no*) the FUSE writeback cache. When
-   writeback caching is enabled, write operations are not immediately
-   sent to the SSH server but first collected and merged locally. This
-   can significantly improve performance. However, if the file is
-   changed on the server while there are unsent changes on the client,
-   data corruption is likely to result.
-
--o unreliable_append
-   When writeback caching is enabled, SSHFS cannot reliably support
-   the ``O_APPEND`` open flag and thus signals an error on open.  To
-   enable support for unreliable ``O_APPEND`` (which may overwrite
-   data if the file changes on the server at a bad time), mount the
-   file system with ``-o unreliable_append``.
-
 -o dir_cache=BOOL
    Enables (*yes*) or disables (*no*) the SSHFS directory cache.  The
    directory cache holds the names of directory entries. Enabling it

test/test_sshfs.py

@@ -33,9 +33,7 @@ def name_generator(__ctr=[0]):
 @pytest.mark.parametrize("debug", (False, True))
 @pytest.mark.parametrize("cache_timeout", (0,1))
 @pytest.mark.parametrize("sync_rd", (True, False))
-@pytest.mark.parametrize("writeback", (False, True))
-def test_sshfs(tmpdir, debug, cache_timeout, sync_rd,
-               writeback, capfd):
+def test_sshfs(tmpdir, debug, cache_timeout, sync_rd, capfd):
 
     # Avoid false positives from debug messages
     #if debug:
@@ -65,12 +63,6 @@ def test_sshfs(tmpdir, debug, cache_timeout, sync_rd,
     if sync_rd:
         cmdline += [ '-o', 'sync_readdir' ]
 
-    if writeback:
-        cmdline += [ '-o', 'writeback_cache=yes',
-                     '-o', 'unreliable_append' ]
-    else:
-        cmdline += [ '-o', 'writeback_cache=no' ]
-
     # SSHFS Cache
     if cache_timeout == 0:
         cmdline += [ '-o', 'dir_cache=no' ]