Introduce FileHandle (#1063)

### What does this PR do?

This commit introduces a FileHandle concept, allowing us to rig the filesystem
up so that we can keep track of bytes lost when files are unlinked and then
the last fp is closed. Unfortunately as of this commit the property tests
are still failing but it's a start.

Author: blt

lading/proptest-regressions/generator/file_gen/model.txt

@@ -12,6 +12,7 @@ use tracing_subscriber::{fmt::format::FmtSpan, util::SubscriberInitExt};
 use nix::libc::{self, ENOENT};
 use serde::Deserialize;
 use std::{
+    collections::HashMap,
     ffi::OsStr,
     num::NonZeroU32,
     path::PathBuf,
@@ -83,6 +84,8 @@ const TTL: Duration = Duration::from_secs(1); // Attribute cache timeout
 #[derive(Debug)]
 struct LogrotateFS {
     state: Arc<Mutex<model::State>>,
+    open_files: Arc<Mutex<HashMap<u64, model::FileHandle>>>,
+
     start_time: std::time::Instant,
     start_time_system: std::time::SystemTime,
 }
@@ -189,7 +192,7 @@ impl Filesystem for LogrotateFS {
         &mut self,
         _req: &Request,
         ino: u64,
-        _fh: u64,
+        fh: u64,
         offset: i64,
         size: u32,
         _flags: i32,
@@ -199,13 +202,51 @@ impl Filesystem for LogrotateFS {
         let tick = self.get_current_tick();
         let mut state = self.state.lock().expect("lock poisoned");
 
-        // NOTE this call to State::read is almost surely allocating. It'd be
-        // pretty slick if we could get the buffer directly from the OS to pass
-        // down for writing but we can't. I suppose we could send up the raw
-        // blocks and then chain them together as needed but absent a compelling
-        // reason to do that the simplicity of this API is nice.
-        if let Some(data) = state.read(ino as usize, offset as usize, size as usize, tick) {
-            reply.data(&data);
+        // Get the FileHandle from fh
+        let file_handle = {
+            let open_files = self.open_files.lock().expect("lock poisoned");
+            open_files.get(&fh).cloned()
+        };
+
+        if let Some(file_handle) = file_handle {
+            assert!(
+                file_handle.inode() as u64 == ino,
+                "file handle inode and passed ino do not match"
+            );
+            if let Some(data) = state.read(file_handle, offset as usize, size as usize, tick) {
+                reply.data(&data);
+            } else {
+                reply.error(ENOENT);
+            }
+        } else {
+            reply.error(ENOENT);
+        }
+    }
+
+    #[tracing::instrument(skip(self, _req, reply))]
+    fn release(
+        &mut self,
+        _req: &Request,
+        _ino: u64,
+        fh: u64,
+        _flags: i32,
+        _lock_owner: Option<u64>,
+        _flush: bool,
+        reply: fuser::ReplyEmpty,
+    ) {
+        let tick = self.get_current_tick();
+        let mut state = self.state.lock().expect("lock poisoned");
+
+        // Remove the FileHandle from the mapping
+        let file_handle = {
+            let mut open_files = self.open_files.lock().expect("lock poisoned");
+            open_files.remove(&fh)
+        };
+
+        if let Some(file_handle) = file_handle {
+            // Close the file in the state
+            state.close_file(tick, file_handle);
+            reply.ok();
         } else {
             reply.error(ENOENT);
         }
@@ -276,7 +317,19 @@ impl Filesystem for LogrotateFS {
 
     #[tracing::instrument(skip(self, _req, reply))]
     fn open(&mut self, _req: &Request, ino: u64, flags: i32, reply: fuser::ReplyOpen) {
-        reply.opened(ino, flags as u32);
+        let tick = self.get_current_tick();
+        let mut state = self.state.lock().expect("lock poisoned");
+
+        if let Some(file_handle) = state.open_file(tick, ino as usize) {
+            let fh = file_handle.id();
+            {
+                let mut open_files = self.open_files.lock().expect("lock poisoned");
+                open_files.insert(fh, file_handle);
+            }
+            reply.opened(fh, flags as u32);
+        } else {
+            reply.error(ENOENT);
+        }
     }
 }
 
@@ -321,6 +374,7 @@ fn main() -> Result<(), Error> {
     // Initialize the FUSE filesystem
     let fs = LogrotateFS {
         state: Arc::new(Mutex::new(state)),
+        open_files: Arc::new(Mutex::new(HashMap::new())),
         start_time: std::time::Instant::now(),
         start_time_system: std::time::SystemTime::now(),
     };