RPC: Add alias blockhash to getblockheader for backward compatibi…

…lity

Summary
----

It was found that recent changes to our RPC API introduced here: https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node/-/merge_requests/443
caused the named parameter "blockhash" to be renamed to
"hash_or_height".  It turns out some extant software in the wild relied
on this named parameter to not change.  See: trezor/blockbook#498 (comment)

Fortunately, this codebase supports named parameter aliasing by using
the '|' character inside the parameter names in the RPC method table.
So, we leverage this facility and make `getblockheader` also accept
`blockhash` interchangeably with the `hash_or_height` named parameter.
This ensures continued 100% RPC compatibility with ABC and older BCHN.

The rpc_blockchain unit test for `getblockheader` was also
updated/modified to test that this alias continues to work.

Closes Bitcoin-ABC#173 .

Test Plan
---

This should cover it:

- `ninja && test/functional/test_runner rpc_blockchain`

Also manual testing to verify in case you are paranoid, do the below
with and without this commit (testnet3 assumed below):

`curl --user USER_NAME_HERE --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "getblockheader", "params": {"blockhash":"000000008a91ac6ac787eda6a2f5a7ddc94311322c3f80ddf0f144cb7c2ba7bc"} }' -H 'content-type: text/plain;' http://127.0.0.1:18332/`

- Without this commit: should complain about unknown named arg `blockhash`
- With this commit: should accept arg and return a header for block

Note that you may need to adjust the above username and port for your
setup. The above example assumes testnet3.

src/rpc/blockchain.cpp

@@ -2563,7 +2563,7 @@ static const ContextFreeRPCCommand commands[] = {
     { "blockchain",         "getblockchaininfo",      getblockchaininfo,      {} },
     { "blockchain",         "getblockcount",          getblockcount,          {} },
     { "blockchain",         "getblockhash",           getblockhash,           {"height"} },
-    { "blockchain",         "getblockheader",         getblockheader,         {"hash_or_height","verbose"} },
+    { "blockchain",         "getblockheader",         getblockheader,         {"blockhash|hash_or_height","verbose"} },
     { "blockchain",         "getblockstats",          getblockstats,          {"hash_or_height","stats"} },
     { "blockchain",         "getchaintips",           getchaintips,           {} },
     { "blockchain",         "getchaintxstats",        getchaintxstats,        {"nblocks", "blockhash"} },

test/functional/rpc_blockchain.py

@@ -284,6 +284,22 @@ def _test_getblockheader(self):
         header_by_height = node.getblockheader(hash_or_height=200)
         assert_equal(header, header_by_height)
 
+        # Next, check that the old alias 'blockhash' still works
+        # and is interchangeable with hash_or_height
+        # First, make sure errors work as expected for unknown named params
+        self.log.info("Testing that getblockheader(blockhashhh=\"HEX\") produces the proper error")
+        assert_raises_rpc_error(
+            -8,
+            "Unknown named parameter blockhashhh",
+            node.getblockheader,
+            blockhashhh=header['hash'])
+        # Next, actually try the old legacy blockhash="xx" style arg
+        self.log.info("Testing that legacy getblockheader(blockhash=\"HEX\") still works ok")
+        header_by_hash2 = node.getblockheader(blockhash=header['hash'])
+        assert_equal(header, header_by_hash2)
+        header_by_height2 = node.getblockheader(blockhash=200)
+        assert_equal(header, header_by_height2)
+
         # check that we actually get a hex string back from getblockheader
         # if verbose is set to false.
         header_verbose_false = node.getblockheader(200, False)