make seqwait generic

SeqWait can use any type that is Ord + Debug + Copy. Debug is not
strictly necessary, but allows us to keep the panic message if a caller
wants the sequence number to go backwards.

zenith_utils/src/seqwait.rs

@@ -1,6 +1,7 @@
 #![warn(missing_docs)]
 
 use std::collections::BTreeMap;
+use std::fmt::Debug;
 use std::mem;
 use std::sync::Mutex;
 use std::time::Duration;
@@ -18,9 +19,12 @@ pub enum SeqWaitError {
 }
 
 /// Internal components of a `SeqWait`
-struct SeqWaitInt {
-    waiters: BTreeMap<u64, (Sender<()>, Receiver<()>)>,
-    current: u64,
+struct SeqWaitInt<T>
+where
+    T: Ord,
+{
+    waiters: BTreeMap<T, (Sender<()>, Receiver<()>)>,
+    current: T,
     shutdown: bool,
 }
 
@@ -38,13 +42,19 @@ struct SeqWaitInt {
 /// [`wait_for`]: SeqWait::wait_for
 /// [`advance`]: SeqWait::advance
 ///
-pub struct SeqWait {
-    internal: Mutex<SeqWaitInt>,
+pub struct SeqWait<T>
+where
+    T: Ord,
+{
+    internal: Mutex<SeqWaitInt<T>>,
 }
 
-impl SeqWait {
+impl<T> SeqWait<T>
+where
+    T: Ord + Debug + Copy,
+{
     /// Create a new `SeqWait`, initialized to a particular number
-    pub fn new(starting_num: u64) -> Self {
+    pub fn new(starting_num: T) -> Self {
         let internal = SeqWaitInt {
             waiters: BTreeMap::new(),
             current: starting_num,
@@ -82,7 +92,7 @@ impl SeqWait {
     ///
     /// This call won't complete until someone has called `advance`
     /// with a number greater than or equal to the one we're waiting for.
-    pub async fn wait_for(&self, num: u64) -> Result<(), SeqWaitError> {
+    pub async fn wait_for(&self, num: T) -> Result<(), SeqWaitError> {
         let mut rx = {
             let mut internal = self.internal.lock().unwrap();
             if internal.current >= num {
@@ -116,7 +126,7 @@ impl SeqWait {
     /// [`SeqWaitError::Timeout`] will be returned.
     pub async fn wait_for_timeout(
         &self,
-        num: u64,
+        num: T,
         timeout_duration: Duration,
     ) -> Result<(), SeqWaitError> {
         timeout(timeout_duration, self.wait_for(num))
@@ -130,23 +140,30 @@ impl SeqWait {
     ///
     /// `advance` will panic if you send it a lower number than
     /// a previous call.
-    pub fn advance(&self, num: u64) {
+    pub fn advance(&self, num: T) {
         let wake_these = {
             let mut internal = self.internal.lock().unwrap();
 
             if internal.current > num {
                 panic!(
-                    "tried to advance backwards, from {} to {}",
+                    "tried to advance backwards, from {:?} to {:?}",
                     internal.current, num
                 );
             }
             internal.current = num;
 
             // split_off will give me all the high-numbered waiters,
-            // so split and then swap. Everything at or above (num + 1)
-            // gets to stay.
-            let mut split = internal.waiters.split_off(&(num + 1));
+            // so split and then swap. Everything at or above `num`
+            // stays.
+            let mut split = internal.waiters.split_off(&num);
             std::mem::swap(&mut split, &mut internal.waiters);
+
+            // `split_at` didn't get the value at `num`; if it's
+            // there take that too.
+            if let Some(sleeper) = internal.waiters.remove(&num) {
+                split.insert(num, sleeper);
+            }
+
             split
         };