fuse_loop: handle unmap on crashed instance

If a fuse_loop instance dies suddenly (e.g. SIGKILL), the FUSE mount and
loop device assignment are left behind. We can determine this scenario
on specific unmap, when the PID file is either missing or contains a PID
of a non-running process, but the backing file and potentially loop
device are still there.

If that's the case, do an "emergency cleanup", by unassigning the
loopdev, calling 'fusermount -u' and then cleaning any leftover files
manually.

With this in place, pretty much any situation is now recoverable via
only the 'proxmox-backup-client' binary, by either calling 'unmap' with
or without parameters.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/tools/fuse_loop.rs

@@ -236,7 +236,7 @@ pub fn cleanup_unused_run_files(filter_name: Option<String>) {
 
                 // clean leftover FUSE instances (e.g. user called 'losetup -d' or similar)
                 // does nothing if files are already stagnant (e.g. instance crashed etc...)
-                if let Ok(_) = unmap_from_backing(&path) {
+                if let Ok(_) = unmap_from_backing(&path, None) {
                     // we have reaped some leftover instance, tell the user
                     eprintln!(
                         "Cleaned up dangling mapping '{}': no loop device assigned",
@@ -280,19 +280,53 @@ fn get_backing_file(loopdev: &str) -> Result<String, Error> {
     Ok(backing_file.to_owned())
 }
 
-fn unmap_from_backing(backing_file: &Path) -> Result<(), Error> {
+// call in broken state: we found the mapping, but the client is already dead,
+// only thing to do is clean up what we can
+fn emerg_cleanup (loopdev: Option<&str>, mut backing_file: PathBuf) {
+    eprintln!(
+        "warning: found mapping with dead process ({:?}), attempting cleanup",
+        &backing_file
+    );
+
+    if let Some(loopdev) = loopdev {
+        let _ = loopdev::unassign(loopdev);
+    }
+
+    // killing the backing process does not cancel the FUSE mount automatically
+    let mut command = std::process::Command::new("fusermount");
+    command.arg("-u");
+    command.arg(&backing_file);
+    let _ = crate::tools::run_command(command, None);
+
+    let _ = remove_file(&backing_file);
+    backing_file.set_extension("pid");
+    let _ = remove_file(&backing_file);
+}
+
+fn unmap_from_backing(backing_file: &Path, loopdev: Option<&str>) -> Result<(), Error> {
     let mut pid_path = PathBuf::from(backing_file);
     pid_path.set_extension("pid");
 
-    let pid_str = read_to_string(&pid_path).map_err(|err|
+    let pid_str = read_to_string(&pid_path).map_err(|err| {
-        format_err!("error reading pidfile {:?}: {}", &pid_path, err))?;
+        if err.kind() == std::io::ErrorKind::NotFound {
+            emerg_cleanup(loopdev, backing_file.to_owned());
+        }
+        format_err!("error reading pidfile {:?}: {}", &pid_path, err)
+    })?;
     let pid = pid_str.parse::<i32>().map_err(|err|
         format_err!("malformed PID ({}) in pidfile - {}", pid_str, err))?;
 
     let pid = Pid::from_raw(pid);
 
     // send SIGINT to trigger cleanup and exit in target process
-    signal::kill(pid, Signal::SIGINT)?;
+    match signal::kill(pid, Signal::SIGINT) {
+        Ok(()) => {},
+        Err(nix::Error::Sys(nix::errno::Errno::ESRCH)) => {
+            emerg_cleanup(loopdev, backing_file.to_owned());
+            return Ok(());
+        },
+        Err(e) => return Err(e.into()),
+    }
 
     // block until unmap is complete or timeout
     let start = time::epoch_i64();
@@ -364,16 +398,16 @@ pub fn unmap_loopdev<S: AsRef<str>>(loopdev: S) -> Result<(), Error> {
     }
 
     let backing_file = get_backing_file(loopdev)?;
-    unmap_from_backing(Path::new(&backing_file))
+    unmap_from_backing(Path::new(&backing_file), Some(loopdev))
 }
 
 /// Try and unmap a running proxmox-backup-client instance from the given name
 pub fn unmap_name<S: AsRef<str>>(name: S) -> Result<(), Error> {
-    for (mapping, _) in find_all_mappings()? {
+    for (mapping, loopdev) in find_all_mappings()? {
         if mapping.ends_with(name.as_ref()) {
             let mut path = PathBuf::from(RUN_DIR);
             path.push(&mapping);
-            return unmap_from_backing(&path);
+            return unmap_from_backing(&path, loopdev.as_deref());
         }
     }
     Err(format_err!("no mapping for name '{}' found", name.as_ref()))