Update signal-hook to version 0.3.0

The dependency on version v0.1.16 of signal-hook makes this library
impossible to use in a project alongside signal-hook version v0.3.0.
This is caused by an arguably incorrect dependency from signal-hook
v0.1.16 on `~1.2` of signal-hook-registry (I think it should just depend
on `^1.2` instead), however it seemed more beneficial overall to upgrade
this crate's dependency instead.

Cargo.toml

@@ -20,7 +20,11 @@ once_cell = "1.4.1"
 
 [target.'cfg(unix)'.dependencies]
 async-io = "1.0.0"
-signal-hook = { version = "0.1.16", default-features = false }
+
+[target.'cfg(unix)'.dependencies.signal-hook]
+version = "0.3.0"
+features = ["iterator"]
+default-features = false
 
 [target.'cfg(windows)'.dependencies]
 blocking = "1.0.0"

src/lib.rs

@@ -186,9 +186,11 @@ impl Child {
                 }
 
             } else if #[cfg(unix)] {
-                static SIGNALS: Lazy<signal_hook::iterator::Signals> = Lazy::new(|| {
-                    signal_hook::iterator::Signals::new(&[signal_hook::SIGCHLD])
-                        .expect("cannot set signal handler for SIGCHLD")
+                static SIGNALS: Lazy<Mutex<signal_hook::iterator::Signals>> = Lazy::new(|| {
+                    Mutex::new(
+                        signal_hook::iterator::Signals::new(&[signal_hook::consts::SIGCHLD])
+                            .expect("cannot set signal handler for SIGCHLD"),
+                    )
                 });
 
                 // Make sure the signal handler is registered before interacting with the process.
@@ -196,7 +198,7 @@ impl Child {
 
                 // Waits for the next SIGCHLD signal.
                 fn wait_sigchld() {
-                    SIGNALS.forever().next();
+                    SIGNALS.lock().unwrap().forever().next();
                 }
 
                 // Wraps a sync I/O type into an async I/O type.