From 98d140664639cffc1b48167adb2dff3d24c12624 Mon Sep 17 00:00:00 2001
From: Alexander Medvedev <lilalexmed@proton.me>
Date: Fri, 27 Dec 2024 16:04:40 +0100
Subject: [PATCH] Implement session.lock

Lock session.lock so other processes know that we currently read and write to the level. This is vanilla behavior.

Test:
Tested to start the a new server while on was locking the file, and got `Failed to lock level: AlreadyLocked("session.lock")` as expected
---
 pumpkin-world/Cargo.toml        |  2 ++
 pumpkin-world/src/level.rs      |  9 +++++++++
 pumpkin-world/src/lib.rs        |  1 +
 pumpkin-world/src/lock/anvil.rs | 32 ++++++++++++++++++++++++++++++++
 pumpkin-world/src/lock/mod.rs   | 18 ++++++++++++++++++
 5 files changed, 62 insertions(+)
 create mode 100644 pumpkin-world/src/lock/anvil.rs
 create mode 100644 pumpkin-world/src/lock/mod.rs

diff --git a/pumpkin-world/Cargo.toml b/pumpkin-world/Cargo.toml
index 495ee3ff..1c7eacd8 100644
--- a/pumpkin-world/Cargo.toml
+++ b/pumpkin-world/Cargo.toml
@@ -25,6 +25,8 @@ dashmap = "6.1.0"
 flate2 = "1.0"
 lz4 = "1.28.0"
 
+file-guard = "0.2.0"
+
 enum_dispatch = "0.3.13"
 
 fastnbt = { git = "https://github.com/owengage/fastnbt.git" }
diff --git a/pumpkin-world/src/level.rs b/pumpkin-world/src/level.rs
index 930363ec..ede2d616 100644
--- a/pumpkin-world/src/level.rs
+++ b/pumpkin-world/src/level.rs
@@ -14,6 +14,7 @@ use crate::{
         anvil::AnvilChunkReader, ChunkData, ChunkParsingError, ChunkReader, ChunkReadingError,
     },
     generation::{get_world_gen, Seed, WorldGenerator},
+    lock::{anvil::AnvilLevelLocker, LevelLocker},
     world_info::{anvil::AnvilLevelInfo, LevelData, WorldInfoReader, WorldInfoWriter},
 };
 
@@ -35,6 +36,9 @@ pub struct Level {
     chunk_watchers: Arc<DashMap<Vector2<i32>, usize>>,
     chunk_reader: Arc<dyn ChunkReader>,
     world_gen: Arc<dyn WorldGenerator>,
+    // Gets unlocked when dropped
+    // TODO: Make this a trait
+    _locker: Arc<AnvilLevelLocker>,
 }
 
 #[derive(Clone)]
@@ -55,6 +59,10 @@ impl Level {
             region_folder,
         };
 
+        // if we fail to lock, lets crash ???. maybe not the best soultion when we have a large server with many worlds and one is locked.
+        // So TODO
+        let locker = AnvilLevelLocker::look(&level_folder).expect("Failed to lock level");
+
         // TODO: Load info correctly based on world format type
         let level_info = AnvilLevelInfo
             .read_world_info(&level_folder)
@@ -71,6 +79,7 @@ impl Level {
             loaded_chunks: Arc::new(DashMap::new()),
             chunk_watchers: Arc::new(DashMap::new()),
             level_info,
+            _locker: Arc::new(locker),
         }
     }
 
diff --git a/pumpkin-world/src/lib.rs b/pumpkin-world/src/lib.rs
index 8e26b106..ec23b151 100644
--- a/pumpkin-world/src/lib.rs
+++ b/pumpkin-world/src/lib.rs
@@ -16,6 +16,7 @@ pub mod dimension;
 mod generation;
 pub mod item;
 pub mod level;
+mod lock;
 pub mod world_info;
 pub const WORLD_HEIGHT: usize = 384;
 pub const WORLD_LOWEST_Y: i16 = -64;
diff --git a/pumpkin-world/src/lock/anvil.rs b/pumpkin-world/src/lock/anvil.rs
new file mode 100644
index 00000000..7864cf71
--- /dev/null
+++ b/pumpkin-world/src/lock/anvil.rs
@@ -0,0 +1,32 @@
+use file_guard::{FileGuard, Lock};
+
+use super::{LevelLocker, LockError};
+
+use std::{fs::File, io::Write, sync::Arc};
+
+pub struct AnvilLevelLocker {
+    _lock: Option<FileGuard<Arc<File>>>,
+}
+
+const SESSION_LOCK_FILE_NAME: &str = "session.lock";
+
+const SNOWMAN: &[u8] = "☃".as_bytes();
+
+impl LevelLocker<Self> for AnvilLevelLocker {
+    fn look(folder: &crate::level::LevelFolder) -> Result<Self, LockError> {
+        let file_path = folder.root_folder.join(SESSION_LOCK_FILE_NAME);
+        let mut file = File::options()
+            .create(true)
+            .truncate(false)
+            .write(true)
+            .open(file_path)
+            .unwrap();
+        // im not joking, mojang writes a snowman into the lock file
+        file.write_all(SNOWMAN)
+            .map_err(|_| LockError::FailedWrite)?;
+        let file_arc = Arc::new(file);
+        let lock = file_guard::try_lock(file_arc, Lock::Exclusive, 0, 1)
+            .map_err(|_| LockError::AlreadyLocked(SESSION_LOCK_FILE_NAME.to_string()))?;
+        Ok(Self { _lock: Some(lock) })
+    }
+}
diff --git a/pumpkin-world/src/lock/mod.rs b/pumpkin-world/src/lock/mod.rs
new file mode 100644
index 00000000..bf4b262f
--- /dev/null
+++ b/pumpkin-world/src/lock/mod.rs
@@ -0,0 +1,18 @@
+use thiserror::Error;
+
+use crate::level::LevelFolder;
+
+pub mod anvil;
+
+// Gets unlocked when dropped
+pub trait LevelLocker<T>: Send + Sync {
+    fn look(folder: &LevelFolder) -> Result<T, LockError>;
+}
+
+#[derive(Error, Debug)]
+pub enum LockError {
+    #[error("Oh no, Level is already locked by {0}")]
+    AlreadyLocked(String),
+    #[error("Failed to write into lock file")]
+    FailedWrite,
+}