Fix nesting depth tracking in incremental parsing

rmp/src/decode/est.rs

@@ -236,6 +236,7 @@ impl MessageLen {
     }
 
     fn read_sequence(&mut self, data: &mut &[u8], mut items_left: u32) -> Option<()> {
+        let parent_depth = self.current_depth;
         self.current_depth += 1;
         if self.current_depth > self.max_depth {
             return self.fail(WIP::LimitExceeded);
@@ -244,17 +245,16 @@ impl MessageLen {
             let position_before_item = self.position;
             self.read_one_item(data).or_else(|| {
                 self.set_max_position(position_before_item + items_left as usize);
-                // -1, because it will increase depth again when resumed
                 self.sequences_wip.push(Seq {
                     items_left: non_zero,
-                    depth: self.current_depth - 1,
+                    depth: parent_depth,
                 });
                 None
             })?;
             items_left -= 1;
         }
         debug_assert!(self.current_depth > 0);
-        self.current_depth -= 1;
+        self.current_depth = parent_depth;
         Some(())
     }
 

rmp/tests/func/est.rs

@@ -88,6 +88,20 @@ fn map() {
     check_estimates(&out, &[1, 7, 7, 7, 9, 9, 11, 11, 11, 11, 11, 14, 14, 21, 21, 21, 21, 21, 21, 21, 21, 22, 26, 26, 28, 28, 28, 28, 29, 31, 31, 35, 35, 35, 35, -35]);
 }
 
+#[test]
+fn incremental_depth() {
+    let msg = [0x92, 0x91, 0xc0, 0x90]; // [[nil], []]
+
+    // Given `msg` is a valid message
+    assert_eq!(msg.len(), MessageLen::with_limits(2, 16).incremental_len(&msg).unwrap());
+
+    // Regression test: when reading the message in chunks, we used to fail due
+    // to incorrect depth tracking
+    let mut incremental = MessageLen::with_limits(2, 16);
+    assert!(matches!(incremental.incremental_len(&msg[..2]), Err(LenError::Truncated(_))));
+    assert_eq!(msg.len(), incremental.incremental_len(&msg[2..]).unwrap());
+}
+
 #[test]
 fn nested() {
     let mut out = Vec::new();