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

8dd5946c0b add functional test (Larry Ruane)
b5a80fa7e4 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 8dd5946c0b
  luke-jr:
    utACK 8dd5946c0b (no changes since previous utACK)
  hebasto:
    re-ACK 8dd5946c0b, only suggested changes since my [previous](https://github.com/bitcoin/bitcoin/pull/18335#pullrequestreview-460621350) review.
  darosior:
    ACK 8dd5946c0b

Tree-SHA512: 33e25f6ff05d9b56fae2bdb68b132557bb8e995f5438ac4fbbc53c304c5152a98aa43c43600c31d8a6a2830cbd48bf8ec7d89dce50190b29ec00a43830126913

test/functional/interface_rpc.py

@@ -8,6 +8,9 @@ import os
 from test_framework.authproxy import JSONRPCException
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_greater_than_or_equal
+from threading import Thread
+import subprocess
+
 
 def expect_http_status(expected_http_status, expected_rpc_code,
                        fcn, *args):
@@ -18,6 +21,16 @@ def expect_http_status(expected_http_status, expected_rpc_code,
         assert_equal(exc.error["code"], expected_rpc_code)
         assert_equal(exc.http_status, expected_http_status)
 
+
+def test_work_queue_getblock(node, got_exceeded_error):
+    while not got_exceeded_error:
+        try:
+            node.cli('getrpcinfo').send_cli()
+        except subprocess.CalledProcessError as e:
+            assert_equal(e.output, 'error: Server response: Work queue depth exceeded\n')
+            got_exceeded_error.append(True)
+
+
 class RPCInterfaceTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
@@ -67,10 +80,23 @@ class RPCInterfaceTest(BitcoinTestFramework):
         expect_http_status(404, -32601, self.nodes[0].invalidmethod)
         expect_http_status(500, -8, self.nodes[0].getblockhash, 42)
 
+    def test_work_queue_exceeded(self):
+        self.log.info("Testing work queue exceeded...")
+        self.restart_node(0, ['-rpcworkqueue=1', '-rpcthreads=1'])
+        got_exceeded_error = []
+        threads = []
+        for _ in range(3):
+            t = Thread(target=test_work_queue_getblock, args=(self.nodes[0], got_exceeded_error))
+            t.start()
+            threads.append(t)
+        for t in threads:
+            t.join()
+
     def run_test(self):
         self.test_getrpcinfo()
         self.test_batch_request()
         self.test_http_status_codes()
+        self.test_work_queue_exceeded()
 
 
 if __name__ == '__main__':