Add lockfile for build state directory

Use flock() to ensure that there are no concurrent instances using the
same build state. More fine-gained locking may be implemented in the
future.

2 files changed, 21 insertions, 4 deletions

diff --git a/crates/runner/src/lib.rs b/crates/runner/src/lib.rs
index 658a833..45366f3 100644
--- a/crates/runner/src/lib.rs
+++ b/crates/runner/src/lib.rs
@@ -6,9 +6,12 @@ mod tar;
 mod task;
 mod util;
 
+use std::{fs::File, os::unix::prelude::AsRawFd, path::Path};
+
 use capctl::prctl;
 use ipc_channel::ipc;
 use nix::{
+	fcntl,
 	sched::CloneFlags,
 	sys::{signal, stat, wait},
 	unistd::{self, Gid, Uid},
@@ -16,6 +19,7 @@ use nix::{
 use serde::{Deserialize, Serialize};
 
 use common::{error::*, types::*};
+use util::fs;
 
 use self::{
 	jobserver::Jobserver,
@@ -72,7 +76,7 @@ fn runner_loop(mut channel: ipc::IpcReceiver<Message>) {
 	}
 }
 
-fn runner(uid: Uid, gid: Gid, channel: ipc::IpcReceiver<Message>) {
+fn runner(uid: Uid, gid: Gid, channel: ipc::IpcReceiver<Message>, _lockfile: File) {
 	ns::mount_proc();
 	ns::setup_userns(Uid::from_raw(0), Gid::from_raw(0), uid, gid);
 
@@ -106,6 +110,8 @@ impl Runner {
 	///
 	/// Unsafe: Do not call in multithreaded processes
 	pub unsafe fn new() -> Result<Self> {
+		let lockfile = Runner::lock()?;
+
 		let uid = unistd::geteuid();
 		let gid = unistd::getegid();
 
@@ -116,7 +122,7 @@ impl Runner {
 			(tx, rx),
 			|(tx, rx)| {
 				drop(tx);
-				runner(uid, gid, rx);
+				runner(uid, gid, rx, lockfile);
 			},
 		)
 		.expect("clone()")
@@ -124,9 +130,18 @@ impl Runner {
 
 		Ok(Runner { channel: tx })
 	}
-}
 
-impl Runner {
+	fn lock() -> Result<File> {
+		let path = Path::new(paths::LOCKFILE);
+		fs::mkdir(path.parent().unwrap())?;
+		let file = fs::create(path)?;
+
+		fcntl::flock(file.as_raw_fd(), fcntl::FlockArg::LockExclusiveNonblock)
+			.context("Failed to get lock on build directory, is another instance running?")?;
+
+		Ok(file)
+	}
+
 	pub fn spawn(&self, task: &Task) -> ipc::IpcReceiver<Result<TaskOutput>> {
 		let (reply_tx, reply_rx) = ipc::channel().expect("IPC channel creation failed");
 
diff --git a/crates/runner/src/paths.rs b/crates/runner/src/paths.rs
index 632f030..9539782 100644
--- a/crates/runner/src/paths.rs
+++ b/crates/runner/src/paths.rs
@@ -8,6 +8,7 @@
 //! ├── downloads/
 //! │   └── ...
 //! ├── state/
+//! │   ├── build.lock
 //! │   ├── output/
 //! │   │   ├── <input hash>.tar.tmp  # during packing
 //! │   │   ├── <archive hash>.tar    # files are renamed when packing is finished
@@ -49,6 +50,7 @@ pub const TASK_TMP_DIR: &str = "build/tmp/task";
 pub const TASK_TMP_ROOTFS_SUBDIR: &str = "rootfs";
 pub const TASK_TMP_DEPENDS_SUBDIR: &str = "depends";
 
+pub const LOCKFILE: &str = "build/state/build.lock";
 pub const OUTPUT_STATE_DIR: &str = "build/state/output";
 pub const TASK_STATE_DIR: &str = "build/state/task";
 pub const LAYER_STATE_DIR: &str = "build/state/layer";