simplify code that locks log buffer for reading

signal-gateway/src/circular_buffer.rs

@@ -39,6 +39,7 @@ impl<T> CircularBuffer<T> {
     }
 
     /// Returns the number of elements in the buffer.
+    #[allow(unused)]
     pub fn len(&self) -> usize {
         self.buf.len()
     }
@@ -56,7 +57,7 @@ impl<T> CircularBuffer<T> {
     }
 
     /// Returns an iterator over the elements, from oldest to newest.
-    pub fn iter(&self) -> impl DoubleEndedIterator<Item = &T> {
+    pub fn iter(&self) -> impl DoubleEndedIterator<Item = &T> + ExactSizeIterator {
         self.buf.iter()
     }
 

signal-gateway/src/gateway/log_buffer.rs

@@ -41,18 +41,19 @@ impl LogBuffer {
         buf.clear();
     }
 
-    /// Iterate over all messages without modifying the buffer.
+    /// Access the buffer contents via an iterator.
     ///
-    /// Messages are passed to `f` in reverse order (newest first).
-    pub fn for_each(&self, mut f: impl FnMut(&LogMessage)) {
+    /// The iterator yields messages in reverse order (newest first) and
+    /// implements `ExactSizeIterator`, so `iter.len()` returns the count.
+    // NOTE: We use `&mut dyn ExactSizeIterator` rather than `impl FnOnce(impl ExactSizeIterator)`
+    // because Rust doesn't allow nested `impl Trait` in that position. A generic parameter
+    // `F: FnOnce(I) where I: ExactSizeIterator` doesn't work either because `I` would be
+    // caller-determined, but we need to pass our concrete iterator type. HRTB with the
+    // concrete type (`F: for<'a> FnOnce(Rev<vec_deque::Iter<'a, T>>)`) works but leaks
+    // implementation details.
+    pub fn with_iter<R>(&self, f: impl FnOnce(&mut dyn ExactSizeIterator<Item = &LogMessage>) -> R) -> R {
         let buf = self.buf.lock().unwrap();
-        for log_msg in buf.iter().rev() {
-            f(log_msg);
-        }
-    }
-
-    /// Returns the number of messages currently in the buffer.
-    pub fn len(&self) -> usize {
-        self.buf.lock().unwrap().len()
+        let mut iter = buf.iter().rev();
+        f(&mut iter)
     }
 }

signal-gateway/src/gateway/log_handler.rs

@@ -129,10 +129,11 @@ impl LogHandler {
 
                     use std::fmt::Write;
                     writeln!(&mut text, "=== [{origin}] ===").unwrap();
-                    writeln!(&mut text, "{} log messages (newest first):", buffer.len()).unwrap();
-
-                    buffer.for_each(|log_msg| {
-                        self.write_log_msg(&mut text, log_msg, now);
+                    buffer.with_iter(|iter| {
+                        writeln!(&mut text, "{} log messages (newest first):", iter.len()).unwrap();
+                        for log_msg in iter {
+                            self.write_log_msg(&mut text, log_msg, now);
+                        }
                     });
                     text.push('\n');
                 }