Add libfuse 3 support, default on Linux

configure prefers fuse3 (>= 3.4.0) on Linux and falls back to fuse2
when the fuse3 development files are absent, so existing build
environments keep working; --with-fuse2 forces the legacy API and
remains the default on macOS, FreeBSD, and NetBSD. FUSE_USE_VERSION
becomes 31 for fuse3 builds. README documents the new prerequisites
and mount options.

API changes for FUSE 3:
- getattr/truncate absorb fgetattr/ftruncate via the fuse_file_info
  argument; chmod, chown, and utimens gain the argument and use the
  file handle when the path is NULL (possible under nullpath_ok).
- rename handles flags: RENAME_NOREPLACE returns EEXIST when the
  target exists, RENAME_EXCHANGE is rejected with EINVAL.
- readdir and the directory filler gain flag arguments.
- init receives struct fuse_config; use_ino, hard_remove, and
  nullpath_ok move there from mount options.
- FUSE_CAP_ASYNC_READ is cleared in init to keep tape reads ordered;
  FUSE 3 removed the -o sync_read option and enables asynchronous
  reads by default.
- big_writes is gone (always enabled); fuse_parse_cmdline uses
  struct fuse_cmdline_opts.

The fuse2 code paths are unchanged and selected with --with-fuse2.

Author: matejk

README.md

@@ -38,7 +38,7 @@ These instructions will get you a copy of the project up and running on your loc
   * automake 1.13.4 or later
   * autoconf 2.69 or later
   * libtool 2.4.2 or later
-  * fuse 2.6.0 or later
+  * fuse3 3.4.0 or later (default), or fuse 2.6.0 or later with `--with-fuse2`
   * uuid 1.36 or later (Linux)
   * libxml-2.0 2.6.16 or later
   * net-snmp 5.3 or later
@@ -198,8 +198,26 @@ make install
 
 `./configure --help` shows various options for build and install.
 
+On Linux the build uses libfuse 3 (package `libfuse3-dev` on Debian/Ubuntu,
+`fuse3-devel` on Fedora/RHEL). Pass `--with-fuse2` to build against the
+legacy libfuse 2 API instead; macOS, FreeBSD, and NetBSD currently use the
+libfuse 2 API by default.
+
+FUSE 3 builds negotiate request sizes up to 1 MiB (tunable with
+`-o max_write=<bytes>`), serve directory listings through readdirplus, and
+support `-o direct_io` to bypass the kernel page cache entirely so large
+archive jobs do not fill it (mmap does not work on files opened this way).
+
 In some systems, you might need `sudo ldconfig -v` after `make install` to load the shared libraries correctly.
 
+## Running the test suite
+
+`make check` runs integration tests against a tape emulated in a local
+directory by the file backend; no tape hardware is required. The tests need
+a Linux host with `/dev/fuse` and are skipped elsewhere. On macOS,
+`tests/run-in-docker.sh [configure-options...]` builds and runs the suite
+inside an Ubuntu container.
+
 #### Parameter settings of the sg driver
 
 LTFS uses the sg driver by default. You can improve reliability to change parameters of the sg driver below.

configure.ac

@@ -289,7 +289,39 @@ fi
 dnl
 dnl Check for FUSE, libuuid, and libxml2
 dnl
-PKG_CHECK_MODULES([FUSE_MODULE], [fuse >= 2.6.0])
+dnl libfuse 3 is preferred on Linux, falling back to libfuse 2 when the
+dnl fuse3 development files are not installed. --with-fuse2 forces the
+dnl legacy libfuse 2 API; it is also the default on the other platforms
+dnl until their fuse3 stacks are verified.
+dnl
+AC_ARG_WITH([fuse2],
+    [AS_HELP_STRING([--with-fuse2],
+        [build against libfuse 2 instead of libfuse 3])],
+    [with_fuse2=$withval],
+    [with_fuse2=default])
+if test "x${with_fuse2}" = "xdefault" && test "x${host_linux}" != "xyes"
+then
+    with_fuse2=yes
+fi
+
+if test "x${with_fuse2}" = "xyes"
+then
+    PKG_CHECK_MODULES([FUSE_MODULE], [fuse >= 2.6.0])
+elif test "x${with_fuse2}" = "xno"
+then
+    PKG_CHECK_MODULES([FUSE_MODULE], [fuse3 >= 3.4.0])
+else
+    PKG_CHECK_MODULES([FUSE_MODULE], [fuse3 >= 3.4.0],
+        [with_fuse2=no],
+        [PKG_CHECK_MODULES([FUSE_MODULE], [fuse >= 2.6.0], [with_fuse2=yes])])
+fi
+AC_MSG_CHECKING([for the FUSE API version to use])
+if test "x${with_fuse2}" = "xyes"
+then
+    AC_MSG_RESULT([2])
+else
+    AC_MSG_RESULT([3])
+fi
 PKG_CHECK_MODULES([LIBXML2_MODULE], [libxml-2.0 >= 2.6.16])
 
 if test "x${host_mac}" = "xyes"
@@ -461,6 +493,10 @@ dnl Update flags
 dnl Sets CFLAGS to force optimization and debugging options, which isn't quite kosher
 dnl
 AM_CPPFLAGS="-D_GNU_SOURCE -I\$(top_srcdir)/src -DLTFS_CONFIG_FILE='\"${sysconfdir}/ltfs.conf\"' -DLTFS_BASE_DIR='\"${prefix}\"'"
+if test "x${with_fuse2}" != "xyes"
+then
+    AM_CPPFLAGS="${AM_CPPFLAGS} -DHAVE_FUSE3"
+fi
 AM_CFLAGS="-Wall -Wsign-compare -fsigned-char ${FUSE_MODULE_CFLAGS} ${UUID_MODULE_CFLAGS} ${LIBXML2_MODULE_CFLAGS} ${ICU_MODULE_CFLAGS} ${SNMP_ENABLE} ${SNMP_MODULE_CFLAGS}"
 
 if test "x$use_fast" = "xyes"

src/libltfs/ltfs_fuse_version.h

@@ -50,6 +50,12 @@
 #ifndef __ltfs_fuse_version_h__
 #define __ltfs_fuse_version_h__
 
+/* HAVE_FUSE3 is set on the compiler command line by configure
+ * (default on Linux; --with-fuse2 selects the libfuse 2 API). */
+#ifdef HAVE_FUSE3
+#define FUSE_USE_VERSION 31
+#else
 #define FUSE_USE_VERSION 26
+#endif
 
 #endif /* __ltfs_fuse_version_h__ */