From d141ea44c1d901c27f61865808e8c5f9801b2606 Mon Sep 17 00:00:00 2001
From: Nikolaus Rath <Nikolaus@rath.org>
Date: Sun, 6 Aug 2017 11:06:42 +0200
Subject: Re-enabled writeback cache.

Fixes: #72.
---
 ChangeLog.rst      |  3 +--
 README.rst         | 10 ++++++++++
 sshfs.c            | 30 +++++++++++++++++++++++-------
 test/test_sshfs.py |  3 ++-
 4 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/ChangeLog.rst b/ChangeLog.rst
index 5285523..973d86b 100644
--- a/ChangeLog.rst
+++ b/ChangeLog.rst
@@ -1,8 +1,7 @@
 Unreleased Changes
 ------------------
 
-* Truly disable the writeback cache, instead of just adjusting the
-  default and printing a warning when explicitly enabled.
+* Re-enabled writeback cache.
 * SSHFS now supports O_APPEND.  
 
 Release 3.1.0 (2017-08-04)
diff --git a/README.rst b/README.rst
index 91c3ef5..1ec2d72 100644
--- a/README.rst
+++ b/README.rst
@@ -116,6 +116,16 @@ individual files when seen through an SSHFS mount, i.e. they will
 appear to have different inodes and an *st_nlink* value of 1.
 
 
+O_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``.
+
+
 Getting Help
 ------------
 
diff --git a/sshfs.c b/sshfs.c
index cc35577..a76ae8a 100644
--- a/sshfs.c
+++ b/sshfs.c
@@ -216,6 +216,7 @@ struct sshfs {
 	int rename_workaround;
 	int truncate_workaround;
 	int buflimit_workaround;
+	int unrel_append;
 	int fstat_workaround;
 	int transform_symlinks;
 	int follow_symlinks;
@@ -409,6 +410,7 @@ static struct fuse_opt sshfs_opts[] = {
 	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),
@@ -2500,6 +2502,24 @@ 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)
@@ -3344,6 +3364,7 @@ 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"
@@ -3855,7 +3876,8 @@ int main(int argc, char *argv[])
 	sshfs.wfd = -1;
 	sshfs.ptyfd = -1;
 	sshfs.dir_cache = 1;
-	sshfs.writeback_cache = 0;
+	sshfs.writeback_cache = 1;
+	sshfs.unrel_append = 0;
 	sshfs.show_help = 0;
 	sshfs.show_version = 0;
 	sshfs.singlethread = 0;
@@ -3904,12 +3926,6 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
-	if(sshfs.writeback_cache) {
-		printf("NOTICE: writeback cache is disabled in this release due to potential\n"
-		       "dataloss. It will be re-enabled in a future SSHFS release.\n");
-		sshfs.writeback_cache = 0;
-	}
-
 	if (sshfs.idmap == IDMAP_USER)
 		sshfs.detect_uid = 1;
 	else if (sshfs.idmap == IDMAP_FILE) {
diff --git a/test/test_sshfs.py b/test/test_sshfs.py
index 446a617..224179f 100755
--- a/test/test_sshfs.py
+++ b/test/test_sshfs.py
@@ -66,7 +66,8 @@ def test_sshfs(tmpdir, debug, cache_timeout, sync_rd,
         cmdline += [ '-o', 'sync_readdir' ]
 
     if writeback:
-        cmdline += [ '-o', 'writeback_cache=yes' ]
+        cmdline += [ '-o', 'writeback_cache=yes',
+                     '-o', 'unreliable_append' ]
     else:
         cmdline += [ '-o', 'writeback_cache=no' ]
     
-- 
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