vm-virtio: Introduce DescriptorChain iterator

In order to iterate over a chain of descriptor chains, this code has
been ported over from crosvm, based on the commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.

The main modification compared to the original code is the way the
sorting between readable and writable descriptors happens.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/queue.rs

@@ -29,6 +29,37 @@ pub(super) const VIRTQ_DESC_F_WRITE: u16 = 0x2;
 // The Virtio Spec 1.0 defines the alignment of VirtIO descriptor is 16 bytes,
 // which fulfills the explicit constraint of GuestMemoryMmap::read_obj().
 
+/// An iterator over a single descriptor chain.  Not to be confused with AvailIter,
+/// which iterates over the descriptor chain heads in a queue.
+pub struct DescIter<'a> {
+    next: Option<DescriptorChain<'a>>,
+}
+
+impl<'a> DescIter<'a> {
+    /// Returns an iterator that only yields the readable descriptors in the chain.
+    pub fn readable(self) -> impl Iterator<Item = DescriptorChain<'a>> {
+        self.filter(|d| !d.is_write_only())
+    }
+
+    /// Returns an iterator that only yields the writable descriptors in the chain.
+    pub fn writable(self) -> impl Iterator<Item = DescriptorChain<'a>> {
+        self.filter(DescriptorChain::is_write_only)
+    }
+}
+
+impl<'a> Iterator for DescIter<'a> {
+    type Item = DescriptorChain<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(current) = self.next.take() {
+            self.next = current.next_descriptor();
+            Some(current)
+        } else {
+            None
+        }
+    }
+}
+
 /// A virtio descriptor constraints with C representive.
 #[repr(C)]
 #[derive(Default, Clone, Copy)]
@@ -42,6 +73,7 @@ struct Descriptor {
 unsafe impl ByteValued for Descriptor {}
 
 /// A virtio descriptor chain.
+#[derive(Clone)]
 pub struct DescriptorChain<'a> {
     desc_table: GuestAddress,
     queue_size: u16,
@@ -69,7 +101,7 @@ pub struct DescriptorChain<'a> {
 }
 
 impl<'a> DescriptorChain<'a> {
-    fn checked_new(
+    pub fn checked_new(
         mem: &GuestMemoryMmap,
         desc_table: GuestAddress,
         queue_size: u16,
@@ -167,6 +199,15 @@ impl<'a> DescriptorChain<'a> {
     }
 }
 
+impl<'a> IntoIterator for DescriptorChain<'a> {
+    type Item = DescriptorChain<'a>;
+    type IntoIter = DescIter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        DescIter { next: Some(self) }
+    }
+}
+
 /// Consuming iterator over all available descriptor chain heads in the queue.
 pub struct AvailIter<'a, 'b> {
     mem: &'a GuestMemoryMmap,