Merge bitcoin/bitcoin#25471: rpc: Disallow gettxoutsetinfo queries for a specific block with use_index=false

27c8056885 rpc: Disallow gettxoutsetinfo queries for a specific block with use_index=false (Martin Zumsande)

Pull request description:

  In the `gettxoutsetinfo` RPC, if we set `use_index` to false but specify `hash_or_height`, we currently hit a nonfatal error, e.g. `gettxoutsetinfo "muhash" "1" "false"` results in:
  ```
  Internal bug detected: "!pindex || pindex->GetBlockHash() == view->GetBestBlock()"
  rpc/blockchain.cpp:836 (GetUTXOStats)
  ```
  The failing check was added in [#24410](664a14ba7c), but the previous behavior, returning the specified height together with data corresponding to the tip's height, was very confusing too in my opinion.
  Fix this by disallowing the interaction  of `use_index=false` and `hash_or_height` and add a RPC help example with `-named` because users might ask themselves how to use the `use_index` flag witout hitting an error.

  An alternative way would be to allow the interaction if the specified `hash_or_height` happens to correspond to the tip (which should then also be applied to the `HASH_SERIALIZED` check before). If reviewers would prefer that, please say so.

ACKs for top commit:
  fjahr:
    utACK 27c8056885
  shaavan:
    ACK 27c8056885

Tree-SHA512: 1d81c34eaa48c86134a2cf7193246d5de6bfd819d413c3b3fae9cb9290e0297a336111eeaecede2f0f020b0f9a181d240de0da4493e1b387fe63b8189154442b

src/rpc/blockchain.cpp

@@ -845,7 +845,7 @@ static RPCHelpMan gettxoutsetinfo()
                 "Note this call may take some time if you are not using coinstatsindex.\n",
                 {
                     {"hash_type", RPCArg::Type::STR, RPCArg::Default{"hash_serialized_2"}, "Which UTXO set hash should be calculated. Options: 'hash_serialized_2' (the legacy algorithm), 'muhash', 'none'."},
-                    {"hash_or_height", RPCArg::Type::NUM, RPCArg::Optional::OMITTED_NAMED_ARG, "The block hash or height of the target height (only available with coinstatsindex).", "", {"", "string or numeric"}},
+                    {"hash_or_height", RPCArg::Type::NUM, RPCArg::DefaultHint{"the current best block"}, "The block hash or height of the target height (only available with coinstatsindex).", "", {"", "string or numeric"}},
                     {"use_index", RPCArg::Type::BOOL, RPCArg::Default{true}, "Use coinstatsindex, if available."},
                 },
                 RPCResult{
@@ -881,6 +881,7 @@ static RPCHelpMan gettxoutsetinfo()
                     HelpExampleCli("gettxoutsetinfo", R"("none")") +
                     HelpExampleCli("gettxoutsetinfo", R"("none" 1000)") +
                     HelpExampleCli("gettxoutsetinfo", R"("none" '"00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09"')") +
+                    HelpExampleCli("-named gettxoutsetinfo", R"(hash_type='muhash' use_index='false')") +
                     HelpExampleRpc("gettxoutsetinfo", "") +
                     HelpExampleRpc("gettxoutsetinfo", R"("none")") +
                     HelpExampleRpc("gettxoutsetinfo", R"("none", 1000)") +
@@ -917,6 +918,9 @@ static RPCHelpMan gettxoutsetinfo()
             throw JSONRPCError(RPC_INVALID_PARAMETER, "hash_serialized_2 hash type cannot be queried for a specific block");
         }
 
+        if (!index_requested) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot set use_index to false when querying for a specific block");
+        }
         pindex = ParseHashOrHeight(request.params[1], chainman);
     }