Merge #18335: bitcoin-cli: print useful error if bitcoind rpc work queue exceeded

8dd5946c0b7aa8f3976b14a5de4ce84b80a9c32a add functional test (Larry Ruane)
b5a80fa7e487c37b7ac0e3874a2fabade41b9ca8 util: Handle HTTP_SERVICE_UNAVAILABLE in bitcoin-cli (Hennadii Stepanov)

Pull request description:

  If `bitcoind` is processing 16 RPC requests, attempting to submit another request using `bitcoin-cli` produces this less-than-helpful error message: `error: couldn't parse reply from server`. This PR changes the error to: `error: server response: Work queue depth exceeded`.

ACKs for top commit:
  fjahr:
    tACK 8dd5946c0b7aa8f3976b14a5de4ce84b80a9c32a
  luke-jr:
    utACK 8dd5946c0b7aa8f3976b14a5de4ce84b80a9c32a (no changes since previous utACK)
  hebasto:
    re-ACK 8dd5946c0b7aa8f3976b14a5de4ce84b80a9c32a, only suggested changes since my [previous](https://github.com/bitcoin/bitcoin/pull/18335#pullrequestreview-460621350) review.
  darosior:
    ACK 8dd5946c0b7aa8f3976b14a5de4ce84b80a9c32a

Tree-SHA512: 33e25f6ff05d9b56fae2bdb68b132557bb8e995f5438ac4fbbc53c304c5152a98aa43c43600c31d8a6a2830cbd48bf8ec7d89dce50190b29ec00a43830126913

2 files changed, 3 insertions, 1 deletions

diff --git a/src/bitcoin-cli.cpp b/src/bitcoin-cli.cpp
index 49f609be2d..4f3d7a4ffe 100644
--- a/src/bitcoin-cli.cpp
+++ b/src/bitcoin-cli.cpp
@@ -708,6 +708,8 @@ static UniValue CallRPC(BaseRequestHandler* rh, const std::string& strMethod, co
         } else {
             throw std::runtime_error("Authorization failed: Incorrect rpcuser or rpcpassword");
         }
+    } else if (response.status == HTTP_SERVICE_UNAVAILABLE) {
+        throw std::runtime_error(strprintf("Server response: %s", response.body));
     } else if (response.status >= 400 && response.status != HTTP_BAD_REQUEST && response.status != HTTP_NOT_FOUND && response.status != HTTP_INTERNAL_SERVER_ERROR)
         throw std::runtime_error(strprintf("server returned HTTP error %d", response.status));
     else if (response.body.empty())
diff --git a/src/httpserver.cpp b/src/httpserver.cpp
index e1c8ec4ab9..12395f5b24 100644
--- a/src/httpserver.cpp
+++ b/src/httpserver.cpp
@@ -262,7 +262,7 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
             item.release(); /* if true, queue took ownership */
         else {
             LogPrintf("WARNING: request rejected because http work queue depth exceeded, it can be increased with the -rpcworkqueue= setting\n");
-            item->req->WriteReply(HTTP_INTERNAL_SERVER_ERROR, "Work queue depth exceeded");
+            item->req->WriteReply(HTTP_SERVICE_UNAVAILABLE, "Work queue depth exceeded");
         }
     } else {
         hreq->WriteReply(HTTP_NOT_FOUND);