Merge #14353: REST: add blockhash call, fetch blockhash by height

42ff30ec6 [Docs] add short documentation for /rest/blockhashbyheight (Jonas Schnelli)
579d418f7 [QA] add rest tests for /rest/blockhashbyheight/<HEIGHT>.<FORMAT> (Jonas Schnelli)
eb9ef04c4 REST: add "blockhashbyheight" call, fetch blockhash by height (Jonas Schnelli)

Pull request description:

  Completes the REST interface for trivial block exploring by adding a call that allows to fetch the blockhash in the main chain by a given height.

Tree-SHA512: 94be9e56718f857279b11cc16dfa8d04f3b5a762e87ae54281b4d87247c71c844895f4944d5a47f09056bf851f4c4761ac4fbdbaaee957265d14de5c1c73e8d2

3 files changed, 70 insertions, 1 deletions

diff --git a/doc/REST-interface.md b/doc/REST-interface.md
index ff7ef6ce1c..d21df36130 100644
--- a/doc/REST-interface.md
+++ b/doc/REST-interface.md
@@ -39,6 +39,11 @@ With the /notxdetails/ option JSON response will only contain the transaction ha
 Given a block hash: returns <COUNT> amount of blockheaders in upward direction.
 Returns empty if the block doesn't exist or it isn't in the active chain.
 
+#### Blockhash by height
+`GET /rest/blockhashbyheight/<HEIGHT>.<bin|hex|json>`
+
+Given a height: returns hash of block in best-block-chain at height provided.
+
 #### Chaininfos
 `GET /rest/chaininfo.json`
 
diff --git a/src/rest.cpp b/src/rest.cpp
index 4f26e3afb5..c7a627d14e 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -575,6 +575,52 @@ static bool rest_getutxos(HTTPRequest* req, const std::string& strURIPart)
     }
 }
 
+static bool rest_blockhash_by_height(HTTPRequest* req,
+                       const std::string& str_uri_part)
+{
+    if (!CheckWarmup(req)) return false;
+    std::string height_str;
+    const RetFormat rf = ParseDataFormat(height_str, str_uri_part);
+
+    int32_t blockheight;
+    if (!ParseInt32(height_str, &blockheight) || blockheight < 0) {
+        return RESTERR(req, HTTP_BAD_REQUEST, "Invalid height: " + SanitizeString(height_str));
+    }
+
+    CBlockIndex* pblockindex = nullptr;
+    {
+        LOCK(cs_main);
+        if (blockheight > chainActive.Height()) {
+            return RESTERR(req, HTTP_NOT_FOUND, "Block height out of range");
+        }
+        pblockindex = chainActive[blockheight];
+    }
+    switch (rf) {
+    case RetFormat::BINARY: {
+        CDataStream ss_blockhash(SER_NETWORK, PROTOCOL_VERSION);
+        ss_blockhash << pblockindex->GetBlockHash();
+        req->WriteHeader("Content-Type", "application/octet-stream");
+        req->WriteReply(HTTP_OK, ss_blockhash.str());
+        return true;
+    }
+    case RetFormat::HEX: {
+        req->WriteHeader("Content-Type", "text/plain");
+        req->WriteReply(HTTP_OK, pblockindex->GetBlockHash().GetHex() + "\n");
+        return true;
+    }
+    case RetFormat::JSON: {
+        req->WriteHeader("Content-Type", "application/json");
+        UniValue resp = UniValue(UniValue::VOBJ);
+        resp.pushKV("blockhash", pblockindex->GetBlockHash().GetHex());
+        req->WriteReply(HTTP_OK, resp.write() + "\n");
+        return true;
+    }
+    default: {
+        return RESTERR(req, HTTP_NOT_FOUND, "output format not found (available: " + AvailableDataFormatsString() + ")");
+    }
+    }
+}
+
 static const struct {
     const char* prefix;
     bool (*handler)(HTTPRequest* req, const std::string& strReq);
@@ -587,6 +633,7 @@ static const struct {
       {"/rest/mempool/contents", rest_mempool_contents},
       {"/rest/headers/", rest_headers},
       {"/rest/getutxos", rest_getutxos},
+      {"/rest/blockhashbyheight/", rest_blockhash_by_height},
 };
 
 void StartREST()
diff --git a/test/functional/interface_rest.py b/test/functional/interface_rest.py
index 23b13fc4f1..d5a1b53408 100755
--- a/test/functional/interface_rest.py
+++ b/test/functional/interface_rest.py
@@ -198,7 +198,7 @@ class RESTTest (BitcoinTestFramework):
         self.nodes[0].generate(1)  # generate block to not affect upcoming tests
         self.sync_all()
 
-        self.log.info("Test the /block and /headers URIs")
+        self.log.info("Test the /block, /blockhashbyheight and /headers URIs")
         bb_hash = self.nodes[0].getbestblockhash()
 
         # Check result if block does not exists
@@ -237,6 +237,23 @@ class RESTTest (BitcoinTestFramework):
         # Check json format
         block_json_obj = self.test_rest_request("/block/{}".format(bb_hash))
         assert_equal(block_json_obj['hash'], bb_hash)
+        assert_equal(self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']))['blockhash'], bb_hash)
+
+        # Check hex/bin format
+        resp_hex = self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']), req_type=ReqType.HEX, ret_type=RetType.OBJ)
+        assert_equal(resp_hex.read().decode('utf-8').rstrip(), bb_hash)
+        resp_bytes = self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']), req_type=ReqType.BIN, ret_type=RetType.BYTES)
+        blockhash = binascii.hexlify(resp_bytes[::-1]).decode('utf-8')
+        assert_equal(blockhash, bb_hash)
+
+        # Check invalid blockhashbyheight requests
+        resp = self.test_rest_request("/blockhashbyheight/abc", ret_type=RetType.OBJ, status=400)
+        assert_equal(resp.read().decode('utf-8').rstrip(), "Invalid height: abc")
+        resp = self.test_rest_request("/blockhashbyheight/1000000", ret_type=RetType.OBJ, status=404)
+        assert_equal(resp.read().decode('utf-8').rstrip(), "Block height out of range")
+        resp = self.test_rest_request("/blockhashbyheight/-1", ret_type=RetType.OBJ, status=400)
+        assert_equal(resp.read().decode('utf-8').rstrip(), "Invalid height: -1")
+        self.test_rest_request("/blockhashbyheight/", ret_type=RetType.OBJ, status=400)
 
         # Compare with json block header
         json_obj = self.test_rest_request("/headers/1/{}".format(bb_hash))