From ef74fefe846ed07bd513b1c5d13edf7d84fad0df Mon Sep 17 00:00:00 2001
From: Simon Warta <simon@warta.it>
Date: Tue, 18 Aug 2020 13:05:34 +0200
Subject: [PATCH 1/2] Fall back on query when header from subscription is too
 high

---
 packages/stargate/src/queries/queryclient.ts | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/packages/stargate/src/queries/queryclient.ts b/packages/stargate/src/queries/queryclient.ts
index d5052478..10b7723b 100644
--- a/packages/stargate/src/queries/queryclient.ts
+++ b/packages/stargate/src/queries/queryclient.ts
@@ -230,7 +230,7 @@ export class QueryClient {
     }
 
     const searchHeight = height + 1;
-    let nextHeader: Header;
+    let nextHeader: Header | undefined;
     let headersSubscription: Stream<NewBlockHeaderEvent> | undefined;
     try {
       headersSubscription = this.tmClient.subscribeNewBlockHeader();
@@ -239,9 +239,14 @@ export class QueryClient {
     }
 
     if (headersSubscription) {
-      // get the header for height+1
-      nextHeader = await firstEvent(headersSubscription); // TODO: fall back on polling if this returns a too high header
-    } else {
+      const firstHeader = await firstEvent(headersSubscription);
+      // The first header we get might not be n+1 but n+2 or even higher. In such cases we fall back on a query.
+      if (firstHeader.height === searchHeight) {
+        nextHeader = firstHeader;
+      }
+    }
+
+    if (!nextHeader) {
       // start from current height to avoid backend error for minHeight in the future
       let header = (await this.tmClient.blockchain(height, searchHeight)).blockMetas
         .map((meta) => meta.header)

From ac7cd44c0fc8e9d35eb287e390315e7adce470d5 Mon Sep 17 00:00:00 2001
From: Simon Warta <simon@warta.it>
Date: Tue, 18 Aug 2020 13:11:34 +0200
Subject: [PATCH 2/2] Refactor code to avoid repeating query

---
 packages/stargate/src/queries/queryclient.ts | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/packages/stargate/src/queries/queryclient.ts b/packages/stargate/src/queries/queryclient.ts
index 10b7723b..c835a816 100644
--- a/packages/stargate/src/queries/queryclient.ts
+++ b/packages/stargate/src/queries/queryclient.ts
@@ -246,26 +246,19 @@ export class QueryClient {
       }
     }
 
-    if (!nextHeader) {
+    while (!nextHeader) {
       // start from current height to avoid backend error for minHeight in the future
-      let header = (await this.tmClient.blockchain(height, searchHeight)).blockMetas
+      const correctHeader = (await this.tmClient.blockchain(height, searchHeight)).blockMetas
         .map((meta) => meta.header)
         .find((h) => h.height === searchHeight);
-      while (!header) {
+      if (correctHeader) {
+        nextHeader = correctHeader;
+      } else {
         await sleep(1000);
-        header = (await this.tmClient.blockchain(height, searchHeight)).blockMetas
-          .map((meta) => meta.header)
-          .find((h) => h.height === searchHeight);
       }
-      nextHeader = header;
-    }
-
-    if (nextHeader.height !== searchHeight) {
-      throw new Error(
-        `Query requires header at height ${searchHeight} for proof verification, but next header was ${nextHeader.height}`,
-      );
     }
 
+    assert(nextHeader.height === searchHeight, "Got wrong header. This is a bug in the logic above.");
     return nextHeader;
   }
 }