Add get_header_by_hash client method

src/client.rs

@@ -247,6 +247,30 @@ impl Requester {
         rx.await.map_err(|_| ClientError::RecvError)
     }
 
+    /// Look up a header by block hash in the locally synced header chain. Unlike
+    /// [`Self::get_header`], this method returns the header even when the block
+    /// has been demoted from the chain of most work by a reorg, so callers that
+    /// receive a [`BlockHash`] from an [`Event`](crate::Event) (e.g. an
+    /// [`IndexedFilter`](crate::IndexedFilter)) can always resolve the exact
+    /// header the event referred to.
+    ///
+    /// Returns `None` if the hash is not known to the local header chain.
+    ///
+    /// # Errors
+    ///
+    /// If the node has stopped running.
+    pub async fn get_header_by_hash(
+        &self,
+        hash: BlockHash,
+    ) -> Result<Option<IndexedHeader>, ClientError> {
+        let (tx, rx) = tokio::sync::oneshot::channel::<Option<IndexedHeader>>();
+        let request = ClientRequest::new(hash, tx);
+        self.ntx
+            .send(ClientMessage::GetHeaderByHash(request))
+            .map_err(|_| ClientError::SendError)?;
+        rx.await.map_err(|_| ClientError::RecvError)
+    }
+
     /// Look up the height of a block hash in the locally synced chain of most work.
     /// Returns `None` if the hash is not in the chain of most work.
     ///

src/messages.rs

@@ -153,6 +153,9 @@ pub(crate) enum ClientMessage {
     GetPeerInfo(ClientRequest<(), Vec<(AddrV2, ServiceFlags)>>),
     /// Look up a header at a specific height in the chain of most work.
     GetHeader(ClientRequest<u32, Option<IndexedHeader>>),
+    /// Look up a header by its block hash. Returns headers for stale/reorganized
+    /// branches as well as the canonical chain.
+    GetHeaderByHash(ClientRequest<BlockHash, Option<IndexedHeader>>),
     /// Look up the height of a block hash in the chain of most work.
     HeightOfHash(ClientRequest<BlockHash, Option<u32>>),
     /// Send an empty message to see if the node is running.

src/node.rs

@@ -272,6 +272,22 @@ impl Node {
                                     self.dialog.send_warning(Warning::ChannelDropped);
                                 };
                             }
+                            ClientMessage::GetHeaderByHash(request) => {
+                                let (hash, oneshot) = request.into_values();
+                                let indexed = self
+                                    .chain
+                                    .header_chain
+                                    .header_at_hash(hash)
+                                    .and_then(|header| {
+                                        self.chain
+                                            .header_chain
+                                            .height_of_hash(hash)
+                                            .map(|height| IndexedHeader::new(height, header))
+                                    });
+                                if oneshot.send(indexed).is_err() {
+                                    self.dialog.send_warning(Warning::ChannelDropped);
+                                };
+                            }
                             ClientMessage::NoOp => (),
                         }
                     }