allow manually dequeueing buffers

src/io/mmap/stream.rs

@@ -196,6 +196,10 @@ impl<'a, 'b> CaptureStream<'b> for Stream<'a> {
         Ok(self.arena_index)
     }
 
+    fn get(&self, index: usize) -> io::Result<(&Self::Item, &Metadata)> {
+        Ok((&self.arena.bufs[index], &self.buf_meta[index]))
+    }
+
     fn next(&'b mut self) -> io::Result<(&Self::Item, &Metadata)> {
         if !self.active {
             // Enqueue all buffers once on stream start
@@ -208,13 +212,8 @@ impl<'a, 'b> CaptureStream<'b> for Stream<'a> {
             CaptureStream::queue(self, self.arena_index)?;
         }
 
-        self.arena_index = CaptureStream::dequeue(self)?;
-
-        // The index used to access the buffer elements is given to us by v4l2, so we assume it
-        // will always be valid.
-        let bytes = &self.arena.bufs[self.arena_index];
-        let meta = &self.buf_meta[self.arena_index];
-        Ok((bytes, meta))
+        let index = CaptureStream::dequeue(self)?;
+        CaptureStream::get(self, index)
     }
 }
 

src/io/traits.rs

@@ -20,6 +20,9 @@ pub trait CaptureStream<'a>: Stream {
     /// Remove a buffer from the drivers' outgoing queue
     fn dequeue(&mut self) -> io::Result<usize>;
 
+    /// Access the buffer at the specified index.
+    fn get(&self, index: usize) -> io::Result<(&Self::Item, &Metadata)>;
+
     /// Fetch a new frame by first queueing and then dequeueing.
     /// First time initialization is performed if necessary.
     fn next(&'a mut self) -> io::Result<(&Self::Item, &Metadata)>;

src/io/userptr/stream.rs

@@ -191,6 +191,10 @@ impl<'a> CaptureStream<'a> for Stream {
         Ok(self.arena_index)
     }
 
+    fn get(&self, index: usize) -> io::Result<(&Self::Item, &Metadata)> {
+        Ok((&self.arena.bufs[index], &self.buf_meta[index]))
+    }
+
     fn next(&'a mut self) -> io::Result<(&Self::Item, &Metadata)> {
         if !self.active {
             // Enqueue all buffers once on stream start
@@ -203,12 +207,7 @@ impl<'a> CaptureStream<'a> for Stream {
             self.queue(self.arena_index)?;
         }
 
-        self.arena_index = self.dequeue()?;
-
-        // The index used to access the buffer elements is given to us by v4l2, so we assume it
-        // will always be valid.
-        let bytes = &mut self.arena.bufs[self.arena_index];
-        let meta = &self.buf_meta[self.arena_index];
-        Ok((bytes, meta))
+        let index = self.dequeue()?;
+        self.get(index)
     }
 }