FUSE compat: add Haiku extension

* Add a way for a FUSE module to supply Haiku-specific extensions.
  This allows it to integrate better with Haiku while only requiring
  minimal changes on the FUSE module itself.

* For now, there is only one extension: another function pointer for
  "get_fs_info", which lets the FUSE module fill in an fs_info struct.
  FUSE provides no good way to otherwise communicate extra information,
  such as the volume flags (e.g. B_FS_IS_SHARED).

* A FUSE module can signal that it supports the Haiku extensions by
    a) defining HAS_HAIKU_FUSE_EXTENSIONS before including the fuse
       headers
    b) setting the global variable gHasHaikuFuseExtensions to 1 in
       its initialization
  Otherwise, the Haiku extensions are completely invisible to the
  FUSE module.

headers/private/userlandfs/fuse/fuse.h

@@ -36,6 +36,11 @@
 extern "C" {
 #endif
 
+#ifdef HAS_FUSE_HAIKU_EXTENSIONS
+struct fs_info;
+extern int gHasHaikuFuseExtensions;
+#endif
+
 /* ----------------------------------------------------------- *
  * Basic FUSE API					       *
  * ----------------------------------------------------------- */
@@ -423,6 +428,10 @@ struct fuse_operations {
 	 * Introduced in version 2.6
 	 */
 	int (*bmap) (const char *, size_t blocksize, uint64_t *idx);
+
+#ifdef HAS_FUSE_HAIKU_EXTENSIONS
+	int (*get_fs_info) (struct fs_info*);
+#endif
 };
 
 /** Extra context that may be needed by some filesystems
@@ -658,6 +667,10 @@ int fuse_fs_bmap(struct fuse_fs *fs, const char *path, size_t blocksize,
 void fuse_fs_init(struct fuse_fs *fs, struct fuse_conn_info *conn);
 void fuse_fs_destroy(struct fuse_fs *fs);
 
+#ifdef HAS_FUSE_HAIKU_EXTENSIONS
+int fuse_fs_get_fs_info(struct fuse_fs* fs, struct fs_info* info);
+#endif
+
 /**
  * Create a new fuse filesystem object
  *

src/add-ons/kernel/file_systems/userlandfs/server/fuse/FUSEVolume.cpp

@@ -783,6 +783,15 @@ FUSEVolume::Sync()
 status_t
 FUSEVolume::ReadFSInfo(fs_info* info)
 {
+	if (gHasHaikuFuseExtensions == 1 && fFS->ops.get_fs_info != NULL) {
+		int fuseError = fuse_fs_get_fs_info(fFS, info);
+		if (fuseError != 0)
+			return fuseError;
+		return B_OK;
+	}
+
+	// No Haiku FUSE extensions, so our knowledge is limited: use some values
+	// from statfs and make reasonable guesses for the rest of them.
 	if (fFS->ops.statfs == NULL)
 		return B_UNSUPPORTED;
 
@@ -791,6 +800,7 @@ FUSEVolume::ReadFSInfo(fs_info* info)
 	if (fuseError != 0)
 		return fuseError;
 
+	memset(info, 0, sizeof(*info));
 	info->flags = B_FS_IS_PERSISTENT;	// assume the FS is persistent
 	info->block_size = st.f_bsize;
 	info->io_size = 64 * 1024;			// some value

src/add-ons/kernel/file_systems/userlandfs/server/fuse/fuse_api.h

@@ -7,6 +7,8 @@
 
 #define FUSE_USE_VERSION FUSE_VERSION
 
+#define HAS_FUSE_HAIKU_EXTENSIONS
+
 #include <fuse.h>
 #include <fuse_lowlevel.h>
 #include <fuse_opt.h>

src/add-ons/kernel/file_systems/userlandfs/server/fuse/fuse_fs.cpp

@@ -380,3 +380,12 @@ fuse_fs_new(const struct fuse_operations* ops, size_t opSize, void* userData)
 
 	return fs;
 }
+
+
+int
+fuse_fs_get_fs_info(struct fuse_fs* fs, struct fs_info* info)
+{
+	if (fs->ops.get_fs_info == NULL)
+		return ENOSYS;
+	return fs->ops.get_fs_info(info);
+}

src/add-ons/kernel/file_systems/userlandfs/server/fuse/fuse_main.cpp

@@ -17,6 +17,11 @@
 #include "../RequestThread.h"
 
 
+int gHasHaikuFuseExtensions = 0;
+	// This global can be set to 1 by a Haiku-aware FUSE add-on to signal
+	// that it implements the Haiku-specific functions in struct
+	// fuse_operations (those which are guarded by HAS_FUSE_HAIKU_EXTENSIONS).
+
 int
 fuse_main_real(int argc, char* argv[], const struct fuse_operations* op,
 	size_t opSize, void* userData)