Add a MutGlobal utility

Can be used for specifying dynamic components for tests, such as the signer.

lightning/src/util/mod.rs

@@ -15,7 +15,11 @@ pub(crate) mod fuzz_wrappers;
 #[macro_use]
 pub mod ser_macros;
 
+#[cfg(any(test, feature = "_test_utils"))]
+pub mod mut_global;
+
 pub mod anchor_channel_reserves;
+
 #[cfg(fuzzing)]
 pub mod base32;
 #[cfg(not(fuzzing))]

lightning/src/util/mut_global.rs

@@ -0,0 +1,64 @@
+//! A settable global variable.
+//!
+//! Used for testing purposes only.
+
+use std::sync::Mutex;
+
+/// A global variable that can be set exactly once.
+pub struct MutGlobal<T> {
+	value: Mutex<Option<T>>,
+	default_fn: fn() -> T,
+}
+
+impl<T: Clone> MutGlobal<T> {
+	/// Create a new `MutGlobal` with no value set.
+	///
+	/// default_fn will be called to get the default value if the value is unset
+	/// at the time the first call to `get` is made.
+	pub const fn new(default_fn: fn() -> T) -> Self {
+		Self { value: Mutex::new(None), default_fn }
+	}
+
+	/// Set the value of the global variable.
+	pub fn set(&self, value: T) {
+		let mut lock = self.value.lock().unwrap();
+		*lock = Some(value);
+	}
+
+	/// Get the value of the global variable, or the default if unset.
+	pub fn get(&self) -> T {
+		let mut lock = self.value.lock().unwrap();
+		if let Some(value) = &*lock {
+			value.clone()
+		} else {
+			let value = (self.default_fn)();
+			*lock = Some(value.clone());
+			value
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn test() {
+		let v = MutGlobal::<u8>::new(|| 0);
+		assert_eq!(v.get(), 0);
+		v.set(42);
+		assert_eq!(v.get(), 42);
+		v.set(43);
+		assert_eq!(v.get(), 43);
+	}
+
+	static G: MutGlobal<u8> = MutGlobal::new(|| 0);
+
+	#[test]
+	fn test_global() {
+		G.set(42);
+		assert_eq!(G.get(), 42);
+		G.set(43);
+		assert_eq!(G.get(), 43);
+	}
+}