client: Add a lock file to prevent parallel execution.

Cargo.lock

 version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "fslock"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "libc 0.2.71 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "fswatcher"
 version = "0.1.0"
  "criterion 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "dirs 2.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "env_logger 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "fslock 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "fswatcher 0.1.0 (git+https://github.com/mgottschlag/fswatcher-rs.git)",
  "futures 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.8 (registry+https://github.com/rust-lang/crates.io-index)",
 "checksum env_logger 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)" = "44533bbbb3bb3c1fa17d9f2e4e38bbbaf8396ba82193c4cb1b6445d711445d36"
 "checksum fake-simd 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e88a8acf291dafb59c2d96e8f59828f3838bb1a70398823ade51a84de6a6deed"
 "checksum fnv 1.0.7 (registry+https://github.com/rust-lang/crates.io-index)" = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
+"checksum fslock 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)" = "b14c83e47c73f7d62d907ae24a1a98e9132df3c33eb6c54fcf4bce0dbc41d5af"
 "checksum fswatcher 0.1.0 (git+https://github.com/mgottschlag/fswatcher-rs.git)" = "<none>"
 "checksum fuchsia-zircon 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "2e9763c69ebaae630ba35f74888db465e49e259ba1bc0eda7d06f4a067615d82"
 "checksum fuchsia-zircon-sys 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "3dcaa9ae7725d12cdb85b3ad99a434db70b468c09ded17e012d86b5c1010f7a7"

Cargo.toml

 fswatcher = { git = "https://github.com/mgottschlag/fswatcher-rs.git" }
 url = "*"
 range-set = "*"
+fslock = "0.1"
 
 [dev-dependencies]
 criterion = "0.3"

src/bin/client.rs

-use std::fs::{create_dir_all, remove_file};
+use std::fs::{self, create_dir_all, remove_file};
 use std::sync::Arc;
 use std::ffi::{OsStr, OsString};
 
 use tokio::stream::StreamExt;
 use tokio::sync::{mpsc, Mutex};
 use tokio::signal;
+use fslock::LockFile;
 
 use twfss::{Database, FileTree, ClientSideSync, FileSystemWatcher, Error, SynchronizationError, paths};
 
     create_dir_all(data_dir()).unwrap();
 
     // Check whether the client is already running, and exit if it is.
-    // TODO
+    fs::write(&paths::client_lock(), b"").ok();
+    let mut lock = LockFile::open(&paths::client_lock()).expect("cannot open lockfile");
+    if !lock.try_lock().expect("error while locking the lock file") {
+        eprintln!("Client already running, exiting...");
+        return;
+    }
 
     let mut ctrl_c = Box::pin(signal::ctrl_c());
     let (errors_send, mut errors) = mpsc::channel(32);

src/paths.rs

 pub fn client_socket() -> String {
     format!("{}/client.sock", client_data())
 }
+
+pub fn client_lock() -> String {
+    format!("{}/client.lock", client_data())
+}