Simplify and fix AtomicCounter

`AtomicCounter` was slightly race-y on 32-bit platforms because it
increments the high `AtomicUsize` independently from the low
`AtomicUsize`, leading to a potential race where another thread
could observe the low increment but not the high increment and see
a value of 0 twice.

This isn't a big deal because (a) most platforms are 64-bit these
days, (b) 32-bit platforms aren't super likely to have their
counter overflow 32 bits anyway, and (c) the two writes are
back-to-back so having another thread read during that window is
very unlikely.

However, we can also optimize the counter somewhat by using the
`target_has_atomic = "64"` cfg flag, which we do here, allowing us
to use `AtomicU64` even on 32-bit platforms where 64-bit atomics
are available.

This changes some test behavior slightly, which requires
adaptation.

Fixes #3000

lightning/src/ln/functional_tests.rs

@@ -7668,8 +7668,8 @@ fn test_bump_penalty_txn_on_revoked_htlcs() {
 		assert_ne!(node_txn[0].input[0].previous_output, node_txn[2].input[0].previous_output);
 		assert_ne!(node_txn[1].input[0].previous_output, node_txn[2].input[0].previous_output);
 
-		assert_eq!(node_txn[0].input[0].previous_output, revoked_htlc_txn[1].input[0].previous_output);
-		assert_eq!(node_txn[1].input[0].previous_output, revoked_htlc_txn[0].input[0].previous_output);
+		assert_eq!(node_txn[1].input[0].previous_output, revoked_htlc_txn[1].input[0].previous_output);
+		assert_eq!(node_txn[0].input[0].previous_output, revoked_htlc_txn[0].input[0].previous_output);
 
 		// node_txn[3] spends the revoked outputs from the revoked_htlc_txn (which only have one
 		// output, checked above).

lightning/src/ln/monitor_tests.rs

@@ -2251,6 +2251,10 @@ fn do_test_restored_packages_retry(check_old_monitor_retries_after_upgrade: bool
 
 	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 
+	// Reset our RNG counters to mirror the RNG output from when this test was written.
+	nodes[0].keys_manager.backing.inner.entropy_source.set_counter(0x1_0000_0004);
+	nodes[1].keys_manager.backing.inner.entropy_source.set_counter(0x1_0000_0004);
+
 	// Open a channel, lock in an HTLC, and immediately broadcast the commitment transaction. This
 	// ensures that the HTLC timeout package is held until we reach its expiration height.
 	let (_, _, chan_id, funding_tx) = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, 50_000_000);

lightning/src/sign/mod.rs

@@ -1853,6 +1853,9 @@ pub struct KeysManager {
 	channel_master_key: Xpriv,
 	channel_child_index: AtomicUsize,
 
+	#[cfg(test)]
+	pub(crate) entropy_source: RandomBytes,
+	#[cfg(not(test))]
 	entropy_source: RandomBytes,
 
 	seed: [u8; 32],
@@ -2309,6 +2312,9 @@ impl SignerProvider for KeysManager {
 /// Switching between this struct and [`KeysManager`] will invalidate any previously issued
 /// invoices and attempts to pay previous invoices will fail.
 pub struct PhantomKeysManager {
+	#[cfg(test)]
+	pub(crate) inner: KeysManager,
+	#[cfg(not(test))]
 	inner: KeysManager,
 	inbound_payment_key: KeyMaterial,
 	phantom_secret: SecretKey,
@@ -2487,6 +2493,13 @@ impl RandomBytes {
 	pub fn new(seed: [u8; 32]) -> Self {
 		Self { seed, index: AtomicCounter::new() }
 	}
+
+	#[cfg(test)]
+	/// Force the counter to a value to produce the same output again. Mostly useful in tests where
+	/// we need to maintain behavior with a previous version which didn't use as much RNG output.
+	pub(crate) fn set_counter(&self, count: u64) {
+		self.index.set_counter(count);
+	}
 }
 
 impl EntropySource for RandomBytes {

lightning/src/util/atomic_counter.rs

@@ -1,32 +1,44 @@
-//! A simple atomic counter that uses AtomicUsize to give a u64 counter.
+//! A simple atomic counter that uses mutexes if the platform doesn't support atomic u64s.
 
-#[cfg(not(any(target_pointer_width = "32", target_pointer_width = "64")))]
-compile_error!("We need at least 32-bit pointers for atomic counter (and to have enough memory to run LDK)");
+#[cfg(target_has_atomic = "64")]
+use core::sync::atomic::{AtomicU64, Ordering};
+#[cfg(not(target_has_atomic = "64"))]
+use crate::sync::Mutex;
 
-use core::sync::atomic::{AtomicUsize, Ordering};
-
-#[derive(Debug)]
 pub(crate) struct AtomicCounter {
-	// Usize needs to be at least 32 bits to avoid overflowing both low and high. If usize is 64
-	// bits we will never realistically count into high:
-	counter_low: AtomicUsize,
-	counter_high: AtomicUsize,
+	#[cfg(target_has_atomic = "64")]
+	counter: AtomicU64,
+	#[cfg(not(target_has_atomic = "64"))]
+	counter: Mutex<u64>,
 }
 
 impl AtomicCounter {
 	pub(crate) fn new() -> Self {
 		Self {
-			counter_low: AtomicUsize::new(0),
-			counter_high: AtomicUsize::new(0),
+			#[cfg(target_has_atomic = "64")]
+			counter: AtomicU64::new(0),
+			#[cfg(not(target_has_atomic = "64"))]
+			counter: Mutex::new(0),
 		}
 	}
 	pub(crate) fn get_increment(&self) -> u64 {
-		let low = self.counter_low.fetch_add(1, Ordering::AcqRel) as u64;
-		let high = if low == 0 {
-			self.counter_high.fetch_add(1, Ordering::AcqRel) as u64
-		} else {
-			self.counter_high.load(Ordering::Acquire) as u64
-		};
-		(high << 32) | low
+		#[cfg(target_has_atomic = "64")] {
+			self.counter.fetch_add(1, Ordering::AcqRel)
+		}
+		#[cfg(not(target_has_atomic = "64"))] {
+			let mut mtx = self.counter.lock().unwrap();
+			*mtx += 1;
+			*mtx - 1
+		}
+	}
+	#[cfg(test)]
+	pub(crate) fn set_counter(&self, count: u64) {
+		#[cfg(target_has_atomic = "64")] {
+			self.counter.store(count, Ordering::Release);
+		}
+		#[cfg(not(target_has_atomic = "64"))] {
+			let mut mtx = self.counter.lock().unwrap();
+			*mtx  = count;
+		}
 	}
 }