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feat(rpc): stop flooding a busy node's RPC queue; fix the reconnect storm

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dragonxd serves RPC with 8 workers and a 16-deep queue. While a witness
rebuild holds cs_main, every request needing it waits, and a request the
client gave up on keeps its slot. The GUI kept polling and re-probing
until all 24 slots were its own abandoned requests (even stop was then
refused), treated the busy node as lost (tearing down the wallet state),
and retried connect at frame rate on some pages.

Busy gate (src/rpc/rpc_busy_gate.h), enforced in RPCClient::performCall
for the shared rpc_ / fast_rpc_:
- the first reply timeout or queue-full refusal marks the node busy;
  from then on nothing needing cs_main is sent (calls fail at once with
  the new RpcFailureKind::Deferred). stop, help and getrescaninfo are
  lock-free in dragonxd and always go through
- one canary getinfo with no client timeout waits on its own client and
  thread; its answer clears the gate and triggers an immediate refresh
  (queue-full: retried after 2..30 s; no answer: back to normal handling)
- the timer dispatch skips Core/Tx/Addresses/Peers/chat/mining/opid and
  connect attempts while busy
- the 3-strike teardown counts only failures that put the connection in
  doubt, never busy ones; no balance scans follow a busy chain-info call

Reconnect storm: retries were armed with setTimer(Core, kCoreDefault - 1),
i.e. "4 s elapsed", which against Overview's 2 s interval meant every
frame. RefreshScheduler::deferFor() is an absolute hold, and
ConnectRetryBackoff picks the delay by failure kind (not listening
1/2/5 s, queue full 2..30 s, busy 30 s, else 4 s).

Measured against the fake daemon: abandoned requests during a rebuild
went from ~24 (every slot) to at most 2, with no teardown and recovery
when the rebuild ends; a saturated queue sees about one canary per
back-off step instead of ~190 requests/s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
DanS
2026-10-02 09:49:55 -05:00
co-authored by Claude Opus 5.5
parent 0c821333c8
commit 0f25c67600
12 changed files with 501 additions and 24 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/rpc/rpc_client.cpp
+11 -2
View File
@@ -360,6 +360,8 @@ json RPCClient::makePayload(const std::string& method, const json& params)
std::string RPCClient::performCall(const std::string& method, const json& params, long& httpCodeOut)
{
if (busy_gate_ && !busy_gate_->admit(method))
throw RpcTransportError(RpcFailureKind::Deferred, "RPC deferred: the node is busy", CURLE_OK, 0, false);
emitRpcTrace(method);
json payload = makePayload(method, params);
@@ -380,13 +382,20 @@ std::string RPCClient::performCall(const std::string& method, const json& params
long requestSize = 0;
curl_easy_getinfo(impl_->curl, CURLINFO_REQUEST_SIZE, &requestSize);
const bool sent = requestSize > 0;
throw RpcTransportError(classifyRpcFailure(res, sent, 0, 0, std::string()),
"RPC request failed: " + std::string(curl_easy_strerror(res)), res, 0, sent);
const RpcFailureKind kind = classifyRpcFailure(res, sent, 0, 0, std::string());
if (busy_gate_) busy_gate_->record(method, kind);
throw RpcTransportError(kind, "RPC request failed: " + std::string(curl_easy_strerror(res)), res, 0, sent);
}
// Check HTTP response code
httpCodeOut = 0;
curl_easy_getinfo(impl_->curl, CURLINFO_RESPONSE_CODE, &httpCodeOut);
if (busy_gate_) {
// Any HTTP answer means a daemon worker ran our request, except the queue-full refusal.
const bool queueFull = httpCodeOut == 500 &&
response_data.find("Work queue depth exceeded") != std::string::npos;
busy_gate_->record(method, queueFull ? RpcFailureKind::QueueFull : RpcFailureKind::None);
}
return response_data;
}