Merge #17

17: Aeron tests r=bspeice a=bspeice

Port over tests for `ManyToOneRingBuffer` from Aeron C++

Co-authored-by: Bradlee Speice <bradlee@speice.io>

aeron-rs/src/client/concurrent/mod.rs

@@ -62,6 +62,20 @@ pub trait AtomicBuffer: Deref<Target = [u8]> + DerefMut<Target = [u8]> {
             })
     }
 
+    /// Overlay a mutable value on the buffer.
+    ///
+    /// NOTE: Has the potential to cause undefined behavior if alignment is incorrect
+    fn overlay_mut<T>(&mut self, offset: IndexT) -> Result<&mut T>
+    where
+        T: Sized,
+    {
+        self.bounds_check(offset, size_of::<T>() as IndexT)
+            .map(|_| {
+                let offset_ptr = unsafe { self.as_mut_ptr().offset(offset as isize) };
+                unsafe { &mut *(offset_ptr as *mut T) }
+            })
+    }
+
     /// Overlay a struct on a buffer, and perform a volatile read
     ///
     /// ```rust
@@ -155,6 +169,11 @@ pub trait AtomicBuffer: Deref<Target = [u8]> + DerefMut<Target = [u8]> {
         self.overlay_volatile::<i64>(offset)
     }
 
+    /// Read an `i64` value from the buffer without performing any synchronization
+    fn get_i64(&self, offset: IndexT) -> Result<i64> {
+        self.overlay::<i64>(offset).map(|i| *i)
+    }
+
     /// Perform a volatile write of an `i64` value
     ///
     /// ```rust
@@ -167,6 +186,18 @@ pub trait AtomicBuffer: Deref<Target = [u8]> + DerefMut<Target = [u8]> {
         self.write_volatile::<i64>(offset, value)
     }
 
+    /// Write an `i64` value into the buffer without performing any synchronization
+    ///
+    /// ```rust
+    /// # use aeron_rs::client::concurrent::AtomicBuffer;
+    /// let mut buffer = vec![0u8; 8];
+    /// buffer.put_i64(0, 12);
+    /// assert_eq!(buffer.get_i64(0), Ok(12));
+    /// ```
+    fn put_i64(&mut self, offset: IndexT, value: i64) -> Result<()> {
+        self.overlay_mut::<i64>(offset).map(|i| *i = value)
+    }
+
     /// Write the contents of one buffer to another. Does not perform any synchronization
     fn put_bytes<B>(
         &mut self,
@@ -209,6 +240,11 @@ pub trait AtomicBuffer: Deref<Target = [u8]> + DerefMut<Target = [u8]> {
         self.overlay_volatile::<i32>(offset)
     }
 
+    /// Read an `i32` value from the buffer without performing any synchronization
+    fn get_i32(&self, offset: IndexT) -> Result<i32> {
+        self.overlay::<i32>(offset).map(|i| *i)
+    }
+
     /// Perform a volatile write of an `i32` into the buffer
     ///
     /// ```rust
@@ -220,6 +256,23 @@ pub trait AtomicBuffer: Deref<Target = [u8]> + DerefMut<Target = [u8]> {
     fn put_i32_ordered(&mut self, offset: IndexT, value: i32) -> Result<()> {
         self.write_volatile::<i32>(offset, value)
     }
+
+    /// Write an `i32` value into the buffer without performing any synchronization
+    ///
+    /// ```rust
+    /// # use aeron_rs::client::concurrent::AtomicBuffer;
+    /// let mut buffer = vec![0u8; 5];
+    /// buffer.put_i32(0, 255 + 1);
+    /// assert_eq!(buffer.get_i32(1), Ok(1));
+    /// ```
+    fn put_i32(&mut self, offset: IndexT, value: i32) -> Result<()> {
+        self.overlay_mut::<i32>(offset).map(|i| *i = value)
+    }
+
+    /// Return the total number of bytes in this buffer
+    fn capacity(&self) -> IndexT {
+        self.len() as IndexT
+    }
 }
 
 impl AtomicBuffer for Vec<u8> {}

aeron-rs/src/client/concurrent/ringbuffer.rs

@@ -2,7 +2,7 @@
 use crate::client::concurrent::AtomicBuffer;
 use crate::util::bit::align;
 use crate::util::{bit, AeronError, IndexT, Result};
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 
 /// Description of the Ring Buffer schema.
 pub mod buffer_descriptor {
@@ -97,10 +97,16 @@ pub mod record_descriptor {
         record_offset + HEADER_LENGTH
     }
 
-    pub(super) fn length_offset(record_offset: IndexT) -> IndexT {
+    /// Return the position of the record length field given a record's starting position
+    pub fn length_offset(record_offset: IndexT) -> IndexT {
         record_offset
     }
 
+    /// Return the position of the record message type field given a record's starting position
+    pub fn type_offset(record_offset: IndexT) -> IndexT {
+        record_offset + size_of::<i32>() as IndexT
+    }
+
     pub(super) fn record_length(header: i64) -> i32 {
         header as i32
     }
@@ -110,6 +116,8 @@ pub mod record_descriptor {
     }
 }
 
+const INSUFFICIENT_CAPACITY: IndexT = -2;
+
 /// Multi-producer, single-consumer ring buffer implementation.
 pub struct ManyToOneRingBuffer<A>
 where
@@ -150,6 +158,11 @@ where
             .unwrap()
     }
 
+    /// Return the total number of bytes in this buffer
+    pub fn capacity(&self) -> IndexT {
+        self.capacity
+    }
+
     /// Write a message into the ring buffer
     pub fn write<B>(
         &mut self,
@@ -157,7 +170,7 @@ where
         source: &B,
         source_index: IndexT,
         length: IndexT,
-    ) -> Result<()>
+    ) -> Result<bool>
     where
         B: AtomicBuffer,
     {
@@ -168,6 +181,10 @@ where
         let required = bit::align(record_len as usize, record_descriptor::ALIGNMENT as usize);
         let record_index = self.claim_capacity(required as IndexT)?;
 
+        if record_index == INSUFFICIENT_CAPACITY {
+            return Ok(false);
+        }
+
         // UNWRAP: `claim_capacity` performed bounds checking
         self.buffer
             .put_i64_ordered(
@@ -189,20 +206,22 @@ where
             .put_i32_ordered(record_descriptor::length_offset(record_index), record_len)
             .unwrap();
 
-        Ok(())
+        Ok(true)
     }
 
     /// Read messages from the ring buffer and dispatch to `handler`, up to `message_count_limit`
-    pub fn read<F>(&mut self, mut handler: F, message_count_limit: usize) -> Result<usize>
+    ///
+    /// NOTE: The C++ API will stop reading and clean up if an exception is thrown in the handler
+    /// function; by contrast, the Rust API makes no attempt to catch panics and currently
+    /// has no way of stopping reading once started.
+    // QUESTION: Is there a better way to handle dispatching the handler function?
+    // We can't give it a `&dyn AtomicBuffer` because of the monomorphized generic functions,
+    // don't know if having a separate handler trait would be helpful.
+    pub fn read_n<F>(&mut self, mut handler: F, message_count_limit: usize) -> Result<usize>
     where
         F: FnMut(i32, &A, IndexT, IndexT) -> (),
     {
-        // QUESTION: Should I implement the `get_i64` method that C++ uses?
+        let head = self.buffer.get_i64(self.head_position_index)?;
-        // UNWRAP: Bounds check performed during buffer creation
-        let head = self
-            .buffer
-            .get_i64_volatile(self.head_position_index)
-            .unwrap();
         let head_index = (head & i64::from(self.capacity - 1)) as i32;
         let contiguous_block_length = self.capacity - head_index;
         let mut messages_read = 0;
@@ -245,6 +264,9 @@ where
         // in Rust (since the main operation also needs mutable access to self).
         let mut cleanup = || {
             if bytes_read != 0 {
+                // UNWRAP: Need to justify this one.
+                // Should be safe because we've already done length checks, but I want
+                // to spend some more time thinking about it.
                 self.buffer
                     .set_memory(head_index, bytes_read as usize, 0)
                     .unwrap();
@@ -261,12 +283,24 @@ where
         Ok(messages_read)
     }
 
+    /// Read messages from the ring buffer and dispatch to `handler`, up to `message_count_limit`
+    ///
+    /// NOTE: The C++ API will stop reading and clean up if an exception is thrown in the handler
+    /// function; by contrast, the Rust API makes no attempt to catch panics and currently
+    /// has no way of stopping reading once started.
+    pub fn read<F>(&mut self, handler: F) -> Result<usize>
+    where
+        F: FnMut(i32, &A, IndexT, IndexT) -> (),
+    {
+        self.read_n(handler, usize::max_value())
+    }
+
     /// Claim capacity for a specific message size in the ring buffer. Returns the offset/index
     /// at which to start writing the next record.
     fn claim_capacity(&mut self, required: IndexT) -> Result<IndexT> {
         // QUESTION: Is this mask how we handle the "ring" in ring buffer?
         // Would explain why we assert buffer capacity is a power of two during initialization
-        let mask = self.capacity - 1;
+        let mask: IndexT = self.capacity - 1;
 
         // UNWRAP: Known-valid offset calculated during initialization
         let mut head = self
@@ -279,28 +313,18 @@ where
         let mut padding: IndexT;
         // Note the braces, making this a do-while loop
         while {
-            // UNWRAP: Known-valid offset calculated during initialization
+            tail = self.buffer.get_i64_volatile(self.tail_position_index)?;
-            tail = self
-                .buffer
-                .get_i64_volatile(self.tail_position_index)
-                .unwrap();
             let available_capacity = self.capacity - (tail - head) as IndexT;
 
             if required > available_capacity {
-                // UNWRAP: Known-valid offset calculated during initialization
+                head = self.buffer.get_i64_volatile(self.head_position_index)?;
-                head = self
-                    .buffer
-                    .get_i64_volatile(self.head_position_index)
-                    .unwrap();
 
                 if required > (self.capacity - (tail - head) as IndexT) {
-                    return Err(AeronError::InsufficientCapacity);
+                    return Ok(INSUFFICIENT_CAPACITY);
                 }
 
-                // UNWRAP: Known-valid offset calculated during initialization
                 self.buffer
-                    .put_i64_ordered(self.head_cache_position_index, head)
+                    .put_i64_ordered(self.head_cache_position_index, head)?;
-                    .unwrap();
             }
 
             padding = 0;
@@ -315,45 +339,32 @@ where
                 let mut head_index = (head & i64::from(mask)) as IndexT;
 
                 if required > head_index {
-                    // UNWRAP: Known-valid offset calculated during initialization
+                    head = self.buffer.get_i64_volatile(self.head_position_index)?;
-                    head = self
-                        .buffer
-                        .get_i64_volatile(self.head_position_index)
-                        .unwrap();
                     head_index = (head & i64::from(mask)) as IndexT;
 
                     if required > head_index {
-                        return Err(AeronError::InsufficientCapacity);
+                        return Ok(INSUFFICIENT_CAPACITY);
                     }
 
-                    // UNWRAP: Known-valid offset calculated during initialization
                     self.buffer
-                        .put_i64_ordered(self.head_cache_position_index, head)
+                        .put_i64_ordered(self.head_cache_position_index, head)?;
-                        .unwrap();
                 }
 
                 padding = to_buffer_end_length;
             }
 
-            // UNWRAP: Known-valid offset calculated during initialization
+            !self.buffer.compare_and_set_i64(
-            !self
-                .buffer
-                .compare_and_set_i64(
                 self.tail_position_index,
                 tail,
                 tail + i64::from(required) + i64::from(padding),
-                )
+            )?
-                .unwrap()
         } {}
 
         if padding != 0 {
-            // UNWRAP: Known-valid offset calculated during initialization
+            self.buffer.put_i64_ordered(
-            self.buffer
-                .put_i64_ordered(
                 tail_index,
                 record_descriptor::make_header(padding, record_descriptor::PADDING_MSG_TYPE_ID),
-                )
+            )?;
-                .unwrap();
             tail_index = 0;
         }
 
@@ -367,6 +378,11 @@ where
             Ok(())
         }
     }
+
+    /// Return the largest possible message size for this buffer
+    pub fn max_msg_length(&self) -> IndexT {
+        self.max_msg_length
+    }
 }
 
 impl<A> Deref for ManyToOneRingBuffer<A>
@@ -380,12 +396,19 @@ where
     }
 }
 
+impl<A> DerefMut for ManyToOneRingBuffer<A>
+where
+    A: AtomicBuffer,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.buffer
+    }
+}
+
 #[cfg(test)]
 mod tests {
-    use crate::client::concurrent::ringbuffer::{record_descriptor, ManyToOneRingBuffer};
+    use crate::client::concurrent::ringbuffer::ManyToOneRingBuffer;
     use crate::client::concurrent::AtomicBuffer;
-    use crate::util::IndexT;
-    use std::mem::size_of;
 
     const BUFFER_SIZE: usize = 512 + super::buffer_descriptor::TRAILER_LENGTH as usize;
 
@@ -421,81 +444,4 @@ mod tests {
         let write_start = ring_buf.claim_capacity(16).unwrap();
         assert_eq!(write_start, 16);
     }
-
-    #[test]
-    fn write_basic() {
-        let mut ring_buffer =
-            ManyToOneRingBuffer::new(vec![0u8; BUFFER_SIZE]).expect("Invalid buffer size");
-
-        let source_bytes = &mut [12u8, 0, 0, 0][..];
-        let source_len = source_bytes.len() as IndexT;
-        let type_id = 1;
-        ring_buffer
-            .write(type_id, &source_bytes, 0, source_len)
-            .unwrap();
-
-        let record_len = source_len + record_descriptor::HEADER_LENGTH;
-        assert_eq!(
-            ring_buffer.get_i64_volatile(0).unwrap(),
-            record_descriptor::make_header(record_len, type_id)
-        );
-        assert_eq!(
-            ring_buffer
-                .get_i64_volatile(size_of::<i64>() as IndexT)
-                .unwrap(),
-            12
-        );
-    }
-
-    #[test]
-    fn read_basic() {
-        // Similar to write basic, put something into the buffer
-        let mut ring_buffer =
-            ManyToOneRingBuffer::new(vec![0u8; BUFFER_SIZE]).expect("Invalid buffer size");
-
-        let mut source_buffer = &mut [12u8, 0, 0, 0, 0, 0, 0, 0][..];
-        let source_len = source_buffer.len() as IndexT;
-        let type_id = 1;
-        ring_buffer
-            .write(type_id, &source_buffer, 0, source_len)
-            .unwrap();
-
-        // Now we can start the actual read process
-        let c = |_, buf: &Vec<u8>, offset, _| assert_eq!(buf.get_i64_volatile(offset).unwrap(), 12);
-        ring_buffer.read(c, 1).unwrap();
-
-        // Make sure that the buffer was zeroed on finish
-        for i in (0..record_descriptor::ALIGNMENT * 1).step_by(4) {
-            assert_eq!(ring_buffer.get_i32_volatile(i).unwrap(), 0);
-        }
-    }
-
-    #[test]
-    fn read_multiple() {
-        let mut ring_buffer =
-            ManyToOneRingBuffer::new(vec![0u8; BUFFER_SIZE]).expect("Invalid buffer size");
-
-        let mut source_buffer = &mut [12u8, 0, 0, 0, 0, 0, 0, 0][..];
-        let source_len = source_buffer.len() as IndexT;
-        let type_id = 1;
-        ring_buffer
-            .write(type_id, &source_buffer, 0, source_len)
-            .unwrap();
-        ring_buffer
-            .write(type_id, &source_buffer, 0, source_len)
-            .unwrap();
-
-        let mut msg_count = 0;
-        let c = |_, buf: &Vec<u8>, offset, _| {
-            msg_count += 1;
-            assert_eq!(buf.get_i64_volatile(offset).unwrap(), 12);
-        };
-        ring_buffer.read(c, 2).unwrap();
-        assert_eq!(msg_count, 2);
-
-        // Make sure that the buffer was zeroed on finish
-        for i in (0..record_descriptor::ALIGNMENT * 2).step_by(4) {
-            assert_eq!(ring_buffer.get_i32_volatile(i).unwrap(), 0);
-        }
-    }
 }

aeron-rs/tests/many_to_one_ring_buffer.rs

@@ -0,0 +1,381 @@
+/// Tests based on the C++ tests included with Aeron
+use aeron_rs::client::concurrent::ringbuffer::{
+    buffer_descriptor, record_descriptor, ManyToOneRingBuffer,
+};
+use aeron_rs::client::concurrent::AtomicBuffer;
+use aeron_rs::util::bit::align;
+use aeron_rs::util::IndexT;
+use std::ops::Deref;
+
+const CAPACITY: usize = 1024;
+const BUFFER_SZ: usize = CAPACITY + buffer_descriptor::TRAILER_LENGTH as usize;
+const ODD_BUFFER_SZ: usize = (CAPACITY - 1) + buffer_descriptor::TRAILER_LENGTH as usize;
+
+const MSG_TYPE_ID: i32 = 101;
+const HEAD_COUNTER_INDEX: IndexT = 1024 as IndexT + buffer_descriptor::HEAD_POSITION_OFFSET;
+const TAIL_COUNTER_INDEX: IndexT = 1024 as IndexT + buffer_descriptor::TAIL_POSITION_OFFSET;
+
+#[test]
+fn should_calculate_capacity_for_buffer() {
+    let buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+
+    assert_eq!(AtomicBuffer::capacity(buffer.deref()), BUFFER_SZ as IndexT);
+    assert_eq!(buffer.capacity(), CAPACITY as IndexT);
+}
+
+#[test]
+fn should_throw_for_capacity_not_power_of_two() {
+    let buffer = ManyToOneRingBuffer::new(vec![0u8; ODD_BUFFER_SZ]);
+
+    assert!(buffer.is_err());
+}
+
+#[test]
+fn should_throw_when_max_message_size_exceeded() {
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+
+    let bytes = vec![0u8; buffer.max_msg_length() as usize + 1];
+    let write_res = buffer.write(MSG_TYPE_ID, &bytes, 0, bytes.len() as IndexT);
+
+    assert!(write_res.is_err());
+}
+
+#[test]
+fn should_write_to_empty_buffer() {
+    let tail: IndexT = 0;
+    let tail_index: IndexT = 0;
+    let length: IndexT = 8;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let src_index: IndexT = 0;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    let src_bytes = vec![0u8; BUFFER_SZ];
+
+    assert!(buffer
+        .write(MSG_TYPE_ID, &src_bytes, src_index, length)
+        .unwrap());
+
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(tail_index)),
+        Ok(record_length)
+    );
+    assert_eq!(
+        buffer.get_i32(record_descriptor::type_offset(tail_index)),
+        Ok(MSG_TYPE_ID)
+    );
+    assert_eq!(
+        buffer.get_i64(TAIL_COUNTER_INDEX),
+        Ok((tail + aligned_record_length) as i64)
+    );
+}
+
+#[test]
+fn should_reject_write_when_insufficient_space() {
+    let length: IndexT = 100;
+    let head: IndexT = 0;
+    let tail: IndexT = head
+        + (CAPACITY
+            - align(
+                (length - record_descriptor::ALIGNMENT) as usize,
+                record_descriptor::ALIGNMENT as usize,
+            )) as IndexT;
+    let src_index: IndexT = 0;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    let src_bytes = vec![0u8; BUFFER_SZ];
+    let write_res = buffer.write(MSG_TYPE_ID, &src_bytes, src_index, length);
+
+    assert_eq!(write_res, Ok(false));
+    assert_eq!(buffer.get_i64(TAIL_COUNTER_INDEX), Ok(tail as i64));
+}
+
+#[test]
+fn should_reject_write_when_buffer_full() {
+    let length: IndexT = 8;
+    let head: IndexT = 0;
+    let tail: IndexT = head + CAPACITY as IndexT;
+    let src_index: IndexT = 0;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    let src_bytes = vec![0u8; BUFFER_SZ];
+    let write_res = buffer.write(MSG_TYPE_ID, &src_bytes, src_index, length);
+    assert_eq!(write_res, Ok(false));
+    assert_eq!(buffer.get_i64(TAIL_COUNTER_INDEX), Ok(tail as i64));
+}
+
+#[test]
+fn should_insert_padding_record_plus_message_on_buffer_wrap() {
+    let length: IndexT = 100;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let tail: IndexT = CAPACITY as IndexT - record_descriptor::ALIGNMENT;
+    let head: IndexT = tail - (record_descriptor::ALIGNMENT * 4);
+    let src_index: IndexT = 0;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    let src_bytes = vec![0u8; BUFFER_SZ];
+    let write_res = buffer.write(MSG_TYPE_ID, &src_bytes, src_index, length);
+    assert_eq!(write_res, Ok(true));
+
+    assert_eq!(
+        buffer.get_i32(record_descriptor::type_offset(tail)),
+        Ok(record_descriptor::PADDING_MSG_TYPE_ID)
+    );
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(tail)),
+        Ok(record_descriptor::ALIGNMENT)
+    );
+
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(0)),
+        Ok(record_length)
+    );
+    assert_eq!(
+        buffer.get_i32(record_descriptor::type_offset(0)),
+        Ok(MSG_TYPE_ID)
+    );
+    assert_eq!(
+        buffer.get_i64(TAIL_COUNTER_INDEX),
+        Ok((tail + aligned_record_length + record_descriptor::ALIGNMENT) as i64)
+    );
+}
+
+#[test]
+fn should_insert_padding_record_plus_message_on_buffer_wrap_with_head_equal_to_tail() {
+    let length: IndexT = 100;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let tail: IndexT = CAPACITY as IndexT - record_descriptor::ALIGNMENT;
+    let head: IndexT = tail;
+    let src_index: IndexT = 0;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    let src_bytes = vec![0u8; BUFFER_SZ];
+    let write_res = buffer.write(MSG_TYPE_ID, &src_bytes, src_index, length);
+    assert_eq!(write_res, Ok(true));
+
+    assert_eq!(
+        buffer.get_i32(record_descriptor::type_offset(tail)),
+        Ok(record_descriptor::PADDING_MSG_TYPE_ID)
+    );
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(tail)),
+        Ok(record_descriptor::ALIGNMENT)
+    );
+
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(0)),
+        Ok(record_length)
+    );
+    assert_eq!(
+        buffer.get_i32(record_descriptor::type_offset(0)),
+        Ok(MSG_TYPE_ID)
+    );
+    assert_eq!(
+        buffer.get_i64(TAIL_COUNTER_INDEX),
+        Ok((tail + aligned_record_length + record_descriptor::ALIGNMENT) as i64)
+    );
+}
+
+#[test]
+fn should_read_single_message() {
+    let length: IndexT = 8;
+    let head: IndexT = 0;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let tail: IndexT = aligned_record_length;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    buffer
+        .put_i32(record_descriptor::type_offset(0), MSG_TYPE_ID)
+        .unwrap();
+    buffer
+        .put_i32(record_descriptor::length_offset(0), record_length)
+        .unwrap();
+
+    let mut times_called = 0;
+    let closure = |_, _buf: &Vec<u8>, _, _| {
+        times_called += 1;
+    };
+    let messages_read = buffer.read(closure);
+
+    assert_eq!(messages_read, Ok(1));
+    assert_eq!(times_called, 1);
+    assert_eq!(
+        buffer.get_i64(HEAD_COUNTER_INDEX),
+        Ok((head + aligned_record_length) as i64)
+    );
+
+    for i in (0..record_descriptor::ALIGNMENT).step_by(4) {
+        assert_eq!(buffer.get_i32(i), Ok(0));
+    }
+}
+
+#[test]
+fn should_not_read_single_message_part_way_through_writing() {
+    let length: IndexT = 8;
+    let head: IndexT = 0;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let end_tail: IndexT = aligned_record_length;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, end_tail as i64).unwrap();
+    buffer
+        .put_i32(record_descriptor::type_offset(0), MSG_TYPE_ID)
+        .unwrap();
+    buffer
+        .put_i32(record_descriptor::length_offset(0), -record_length)
+        .unwrap();
+
+    let mut times_called = 0;
+    let closure = |_, _buf: &Vec<u8>, _, _| {
+        times_called += 1;
+    };
+    let messages_read = buffer.read(closure);
+
+    assert_eq!(messages_read, Ok(0));
+    assert_eq!(times_called, 0);
+    assert_eq!(buffer.get_i64(HEAD_COUNTER_INDEX), Ok(head as i64));
+}
+
+#[test]
+fn should_read_two_messages() {
+    let length: IndexT = 8;
+    let head: IndexT = 0;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let tail: IndexT = aligned_record_length * 2;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    buffer
+        .put_i32(record_descriptor::type_offset(0), MSG_TYPE_ID)
+        .unwrap();
+    buffer
+        .put_i32(record_descriptor::length_offset(0), record_length)
+        .unwrap();
+
+    buffer
+        .put_i32(
+            record_descriptor::type_offset(0 + aligned_record_length),
+            MSG_TYPE_ID,
+        )
+        .unwrap();
+    buffer
+        .put_i32(
+            record_descriptor::length_offset(0 + aligned_record_length),
+            record_length,
+        )
+        .unwrap();
+
+    let mut times_called = 0;
+    let closure = |_, _buf: &Vec<u8>, _, _| {
+        times_called += 1;
+    };
+    let messages_read = buffer.read(closure);
+
+    assert_eq!(messages_read, Ok(2));
+    assert_eq!(times_called, 2);
+    assert_eq!(
+        buffer.get_i64(HEAD_COUNTER_INDEX),
+        Ok((head + aligned_record_length * 2) as i64)
+    );
+
+    for i in (0..record_descriptor::ALIGNMENT * 2).step_by(4) {
+        assert_eq!(buffer.get_i32(i), Ok(0));
+    }
+}
+
+#[test]
+fn should_limit_read_of_messages() {
+    let length: IndexT = 8;
+    let head: IndexT = 0;
+    let record_length: IndexT = length + record_descriptor::HEADER_LENGTH;
+    let aligned_record_length: IndexT = align(
+        record_length as usize,
+        record_descriptor::ALIGNMENT as usize,
+    ) as IndexT;
+    let tail: IndexT = aligned_record_length * 2;
+
+    let mut buffer = ManyToOneRingBuffer::new(vec![0u8; BUFFER_SZ]).unwrap();
+    buffer.put_i64(HEAD_COUNTER_INDEX, head as i64).unwrap();
+    buffer.put_i64(TAIL_COUNTER_INDEX, tail as i64).unwrap();
+
+    buffer
+        .put_i32(record_descriptor::type_offset(0), MSG_TYPE_ID)
+        .unwrap();
+    buffer
+        .put_i32(record_descriptor::length_offset(0), record_length)
+        .unwrap();
+
+    buffer
+        .put_i32(
+            record_descriptor::type_offset(0 + aligned_record_length),
+            MSG_TYPE_ID,
+        )
+        .unwrap();
+    buffer
+        .put_i32(
+            record_descriptor::length_offset(0 + aligned_record_length),
+            record_length,
+        )
+        .unwrap();
+
+    let mut times_called = 0;
+    let closure = |_, _buf: &Vec<u8>, _, _| {
+        times_called += 1;
+    };
+    let messages_read = buffer.read_n(closure, 1);
+
+    assert_eq!(messages_read, Ok(1));
+    assert_eq!(times_called, 1);
+    assert_eq!(
+        buffer.get_i64(HEAD_COUNTER_INDEX),
+        Ok((head + aligned_record_length) as i64)
+    );
+
+    for i in (0..record_descriptor::ALIGNMENT).step_by(4) {
+        assert_eq!(buffer.get_i32(i), Ok(0));
+    }
+    assert_eq!(
+        buffer.get_i32(record_descriptor::length_offset(aligned_record_length)),
+        Ok(record_length)
+    );
+}