pxar: add basic code for FUSE implementation.

This adds the basic code in order to create a fuse session and mount an archive.
It adds libfuse3-3 as runtime dependency and libfuse3-dev as build dependency.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

src/pxar/fuse.rs

@@ -0,0 +1,159 @@
+//! Low level FUSE implementation for pxar.
+//!
+//! Allows to mount the archive as read-only filesystem to inspect its contents.
+
+use std::ffi::{OsStr, CString};
+use std::fs::File;
+use std::os::unix::ffi::OsStrExt;
+use std::path::Path;
+
+use libc;
+use libc::{c_int, c_void, c_char, size_t};
+use failure::*;
+
+#[link(name = "fuse3")]
+extern "C" {
+    fn fuse_session_new(args: *const FuseArgs, oprs: *const Operations, size: size_t, op: *const c_void) -> *mut c_void;
+    fn fuse_set_signal_handlers(session: *const c_void) -> c_int;
+    fn fuse_remove_signal_handlers(session: *const c_void);
+    fn fuse_daemonize(foreground: c_int) -> c_int;
+    fn fuse_session_mount(session: *const c_void, mountpoint: *const c_char) -> c_int;
+    fn fuse_session_unmount(session: *const c_void);
+    fn fuse_session_loop(session: *const c_void) -> c_int;
+    fn fuse_session_destroy(session: *const c_void);
+}
+
+/// Command line arguments passed to fuse.
+#[repr(C)]
+#[derive(Debug)]
+struct FuseArgs {
+    argc: c_int,
+    argv: *const *const c_char,
+    allocated: c_int,
+}
+
+/// `Session` stores a pointer to the session context and is used to mount the
+/// archive to the given mountpoint.
+#[derive(Debug)]
+pub struct Session {
+    ptr: *mut c_void,
+    archive: File,
+    verbose: bool,
+}
+
+/// `Operations` defines the callback function table of supported operations.
+#[repr(C)]
+struct Operations {
+    init: extern fn(userdata: *mut c_void) -> *mut c_void,
+    destroy: extern fn(userdata: *mut c_void) -> *mut c_void,
+}
+
+impl Session {
+    /// Create a new low level fuse session.
+    ///
+    /// `Session` is created using the provided mount options and sets the
+    /// default signal handlers.
+    /// Options have to be provided as comma separated OsStr, e.g.
+    /// ("ro,default_permissions").
+    pub fn new(archive_path: &Path, options: &OsStr, verbose: bool)-> Result<Self, Error> {
+        let file = File::open(archive_path)?;
+        // First argument should be the executable name
+        let arguments = vec![
+            CString::new("pxar-mount").unwrap(),
+            CString::new("-o").unwrap(),
+            CString::new(options.as_bytes())?,
+        ];
+
+        let arg_ptrs: Vec<_> = arguments.iter().map(|opt| opt.as_ptr()).collect();
+        let args = FuseArgs {
+            argc: arg_ptrs.len() as i32,
+            argv: arg_ptrs.as_ptr(),
+            allocated: 0,
+        };
+
+        // Register the callback funcitons for the session
+        let oprs = Operations {
+            init: init,
+            destroy: destroy,
+        };
+
+        let session_ptr = unsafe { fuse_session_new(
+            &args as *const FuseArgs,
+            &oprs as *const Operations,
+            std::mem::size_of::<Operations>(),
+            std::ptr::null()
+        )};
+
+        if session_ptr.is_null() {
+            bail!("error while creating new fuse session");
+        }
+
+        if unsafe { fuse_set_signal_handlers(session_ptr) } != 0 {
+            bail!("error while setting signal handlers");
+        }
+
+        Ok(Self {
+            ptr: session_ptr,
+            archive: file,
+            verbose: verbose,
+        })
+    }
+
+    /// Actually mount the filesystem for this session on the provided mountpoint
+    /// and daemonize process.
+    pub fn mount(&mut self, mountpoint: &Path) -> Result<(), Error> {
+        if self.verbose {
+            println!("Mounting archive to {:#?}", mountpoint);
+        }
+        let mountpoint = mountpoint.canonicalize()?;
+        let path_cstr = CString::new(mountpoint.as_os_str().as_bytes())
+            .map_err(|err| format_err!("invalid mountpoint - {}", err))?;
+        if unsafe { fuse_session_mount(self.ptr, path_cstr.as_ptr()) } != 0 {
+            bail!("mounting on {:#?} failed", mountpoint);
+        }
+
+        // Do not send process to background if verbose flag is set
+        if !self.verbose && unsafe { fuse_daemonize(0) } != 0 {
+            bail!("could not send process to background");
+        }
+
+        Ok(())
+    }
+
+    /// Execute session loop which handles requests from kernel.
+    pub fn run_loop(&mut self) -> Result<(), Error> {
+        if self.verbose {
+            println!("Executing fuse session loop");
+        }
+        let result = unsafe { fuse_session_loop(self.ptr) };
+        if result < 0 {
+            bail!("fuse session loop exited with - {}", result);
+        }
+        if result > 0 {
+            eprintln!("fuse session loop recieved signal - {}", result);
+        }
+
+        Ok(())
+    }
+}
+
+impl Drop for Session {
+    fn drop(&mut self) {
+        unsafe {
+            fuse_session_unmount(self.ptr);
+            fuse_remove_signal_handlers(self.ptr);
+            fuse_session_destroy(self.ptr);
+        }
+    }
+}
+
+/// Callback functions for fuse kernel driver.
+extern "C" fn init(_userdata: *mut c_void) -> *mut c_void {
+    println!("Init callback");
+    return std::ptr::null_mut();
+}
+
+extern "C" fn destroy(_userdata: *mut c_void) -> *mut c_void {
+    println!("Destroy callback");
+    return std::ptr::null_mut();
+}