Add flushonclose feature

README.md

@@ -248,7 +248,10 @@ These options are the same regardless of whether you use them with the
   to the same as the process thread count. (default: 0)
 * **pin-threads=STR**: Selects a strategy to pin threads to CPUs
   (default: unset)
-* **scheduling-priority=INT**: Set mergerfs' scheduling
+* **flush-on-close=never|always|opened-for-write**: Flush data cache
+  on file close. Mostly for when writeback is enabled or merging
+  network filesystems. (default: opened-for-write)
+  * **scheduling-priority=INT**: Set mergerfs' scheduling
   priority. Valid values range from -20 to 19. See `setpriority` man
   page for more details. (default: -10)
 * **fsname=STR**: Sets the name of the filesystem as seen in
@@ -926,6 +929,27 @@ The options `statfs` and `statfs_ignore` can be used to modify
 `statfs` behavior.
 
 
+#### flush-on-close
+
+https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il@gmail.com/T/
+
+By default FUSE would issue a flush before the release of a file
+descriptor. This was considered a bit aggressive and a feature added
+to give the FUSE server the ability to choose when that happens.
+
+Options: 
+* always
+* never
+* opened-for-write
+
+For now it defaults to "opened-for-write" which is less aggressive
+than the behavior before this feature was added. It should not be a
+problem because the flush is really only relevant when a file is
+written to. Given flush is irrelevant for many filesystems in the
+future a branch specific flag may be added so only files opened on a
+specific branch would be flushed on close.
+
+
 # ERROR HANDLING
 
 POSIX filesystem functions offer a single return code meaning that

libfuse/include/fuse_common.h

@@ -86,6 +86,8 @@ struct fuse_file_info_t
 
   uint32_t parallel_direct_writes:1;
 
+  uint32_t noflush:1;
+
   /** File handle.  May be filled in by filesystem in open().
       Available in all other file operations */
   uint64_t fh;

libfuse/lib/fuse_lowlevel.c

@@ -275,6 +275,8 @@ fill_open(struct fuse_open_out   *arg_,
     arg_->open_flags |= FOPEN_CACHE_DIR;
   if(ffi_->parallel_direct_writes)
     arg_->open_flags |= FOPEN_PARALLEL_DIRECT_WRITES;
+  if(ffi_->noflush)
+    arg_->open_flags |= FOPEN_NOFLUSH;
 }
 
 int

man/mergerfs.1

@@ -321,11 +321,18 @@ by the number of process threads plus read thread count.
 \f[B]pin-threads=STR\f[R]: Selects a strategy to pin threads to CPUs
 (default: unset)
 .IP \[bu] 2
+\f[B]flush-on-close=never|always|opened-for-write\f[R]: Flush data cache
+on file close.
+Mostly for when writeback is enabled or merging network filesystems.
+(default: opened-for-write)
+.RS 2
+.IP \[bu] 2
 \f[B]scheduling-priority=INT\f[R]: Set mergerfs\[cq] scheduling
 priority.
 Valid values range from -20 to 19.
 See \f[C]setpriority\f[R] man page for more details.
 (default: -10)
+.RE
 .IP \[bu] 2
 \f[B]fsname=STR\f[R]: Sets the name of the filesystem as seen in
 \f[B]mount\f[R], \f[B]df\f[R], etc.
@@ -1321,6 +1328,24 @@ be included when checking the mount\[cq]s stats.
 .PP
 The options \f[C]statfs\f[R] and \f[C]statfs_ignore\f[R] can be used to
 modify \f[C]statfs\f[R] behavior.
+.SS flush-on-close
+.PP
+https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il\[at]gmail.com/T/
+.PP
+By default FUSE would issue a flush before the release of a file
+descriptor.
+This was considered a bit aggressive and a feature added to give the
+FUSE server the ability to choose when that happens.
+.PP
+Options: * always * never * opened-for-write
+.PP
+For now it defaults to \[lq]opened-for-write\[rq] which is less
+aggressive than the behavior before this feature was added.
+It should not be a problem because the flush is really only relevant
+when a file is written to.
+Given flush is irrelevant for many filesystems in the future a branch
+specific flag may be added so only files opened on a specific branch
+would be flushed on close.
 .SH ERROR HANDLING
 .PP
 POSIX filesystem functions offer a single return code meaning that there

src/config.cpp

@@ -94,8 +94,9 @@ Config::Config()
     category(func),
     direct_io(false),
     dropcacheonclose(false),
-    fsname(),
+    flushonclose(FlushOnClose::ENUM::ALWAYS),
     follow_symlinks(FollowSymlinks::ENUM::NEVER),
+    fsname(),
     func(),
     fuse_msg_size(FUSE_MAX_MAX_PAGES),
     ignorepponrename(false),
@@ -149,6 +150,7 @@ Config::Config()
   _map["category.search"]        = &category.search;
   _map["direct_io"]              = &direct_io;
   _map["dropcacheonclose"]       = &dropcacheonclose;
+  _map["flush-on-close"]         = &flushonclose;
   _map["follow-symlinks"]        = &follow_symlinks;
   _map["fsname"]                 = &fsname;
   _map["func.access"]            = &func.access;

src/config.hpp

@@ -19,6 +19,7 @@
 #include "branches.hpp"
 #include "category.hpp"
 #include "config_cachefiles.hpp"
+#include "config_flushonclose.hpp"
 #include "config_follow_symlinks.hpp"
 #include "config_inodecalc.hpp"
 #include "config_link_exdev.hpp"
@@ -116,8 +117,9 @@ public:
   Categories     category;
   ConfigBOOL     direct_io;
   ConfigBOOL     dropcacheonclose;
-  ConfigSTR      fsname;
+  FlushOnClose   flushonclose;
   FollowSymlinks follow_symlinks;
+  ConfigSTR      fsname;
   Funcs          func;
   ConfigUINT64   fuse_msg_size;
   ConfigBOOL     ignorepponrename;

src/config_flushonclose.cpp

@@ -0,0 +1,54 @@
+/*
+  ISC License
+
+  Copyright (c) 2023, Antonio SJ Musumeci <trapexit@spawn.link>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#include "config_flushonclose.hpp"
+#include "ef.hpp"
+#include "errno.hpp"
+
+template<>
+std::string
+FlushOnClose::to_string() const
+{
+  switch(_data)
+    {
+    case FlushOnClose::ENUM::NEVER:
+      return "never";
+    case FlushOnClose::ENUM::OPENED_FOR_WRITE:
+      return "opened-for-write";
+    case FlushOnClose::ENUM::ALWAYS:
+      return "always";
+    }
+
+  return {};
+}
+
+template<>
+int
+FlushOnClose::from_string(const std::string &s_)
+{
+  if(s_ == "never")
+    _data = FlushOnClose::ENUM::NEVER;
+  ef(s_ == "opened-for-write")
+    _data = FlushOnClose::ENUM::OPENED_FOR_WRITE;
+  ef(s_ == "always")
+    _data = FlushOnClose::ENUM::ALWAYS;
+  else
+    return -EINVAL;
+
+  return 0;
+}

src/config_flushonclose.hpp

@@ -0,0 +1,31 @@
+/*
+  ISC License
+
+  Copyright (c) 2023, Antonio SJ Musumeci <trapexit@spawn.link>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#pragma once
+
+#include "enum.hpp"
+
+
+enum class FlushOnCloseEnum
+  {
+    NEVER,
+    OPENED_FOR_WRITE,
+    ALWAYS
+  };
+
+typedef Enum<FlushOnCloseEnum> FlushOnClose;

src/fuse_create.cpp

@@ -51,6 +51,31 @@ namespace l
       *flags_ &= ~O_APPEND;
   }
 
+  static
+  bool
+  rdonly(const int flags_)
+  {
+    return ((flags_ & O_ACCMODE) == O_RDONLY);
+  }
+
+  static
+  bool
+  calculate_flush(FlushOnClose const flushonclose_,
+                  int const          flags_)
+  {
+    switch(flushonclose_)
+      {
+      case FlushOnCloseEnum::NEVER:
+        return false;
+      case FlushOnCloseEnum::OPENED_FOR_WRITE:
+        return !l::rdonly(flags_);
+      case FlushOnCloseEnum::ALWAYS:
+        return true;
+      }
+
+    return true;
+  }
+
   static
   void
   config_to_ffi_flags(Config::Read     &cfg_,
@@ -200,6 +225,9 @@ namespace FUSE
     if(cfg->writeback_cache)
       l::tweak_flags_writeback_cache(&ffi_->flags);
 
+    ffi_->noflush = !l::calculate_flush(cfg->flushonclose,
+                                        ffi_->flags);
+
     rv = l::create(cfg->func.getattr.policy,
                    cfg->func.create.policy,
                    cfg->branches,

src/fuse_open.cpp

@@ -114,6 +114,24 @@ namespace l
       *flags_ &= ~O_APPEND;
   }
 
+  static
+  bool
+  calculate_flush(FlushOnClose const flushonclose_,
+                  int const          flags_)
+  {
+    switch(flushonclose_)
+      {
+      case FlushOnCloseEnum::NEVER:
+        return false;
+      case FlushOnCloseEnum::OPENED_FOR_WRITE:
+        return !l::rdonly(flags_);
+      case FlushOnCloseEnum::ALWAYS:
+        return true;
+      }
+
+    return true;
+  }
+
   static
   void
   config_to_ffi_flags(Config::Read     &cfg_,
@@ -236,6 +254,9 @@ namespace FUSE
     if(cfg->writeback_cache)
       l::tweak_flags_writeback_cache(&ffi_->flags);
 
+    ffi_->noflush = !l::calculate_flush(cfg->flushonclose,
+                                        ffi_->flags);
+
     rv = l::open(cfg->func.open.policy,
                  cfg->branches,
                  fusepath_,

src/fuse_opendir.cpp

@@ -30,6 +30,8 @@ namespace FUSE
 
     ffi_->fh = reinterpret_cast<uint64_t>(new DirInfo(fusepath_));
 
+    ffi_->noflush = true;
+
     if(cfg->cache_readdir)
       {
         ffi_->keep_cache    = 1;