Merge bitcoin/bitcoin#36078: qa: Reduce -maxconnections in the functional test framework

b8a8893 qa: Lower `-rpcmaxconnections` in `interface_http.py` test (Hennadii Stepanov)
6f4109b qa: Reduce `-maxconnections` in the functional test framework (Hennadii Stepanov)

Pull request description:

  This PR follows up on bitcoin/bitcoin#35730 and fixes a [regression](bitcoin/bitcoin#35730 (comment)) on NetBSD.

  Since bitcoin/bitcoin#35730 the HTTP server reserves file descriptors for its listen sockets and for `-rpcmaxconnections` connected clients (16 by default), so `min_required_fds` in `init.cpp` grew.

  On select()-based platforms `available_fds` is capped at FD_SETSIZE, which is 256 on NetBSD. The previous value of 94 no longer fits and every node in the test suite started up with a warning, which the framework treats as unexpected stderr and fails on.

  Recompute the value with the new accounting (256 - 179 = 77) and update the comment to match the current variable names in `init.cpp`.

ACKs for top commit:
  achow101:
    ACK b8a8893
  hodlinator:
    re-ACK b8a8893
  winterrdog:
    re-ACK b8a8893

Tree-SHA512: d6200cc334b98148d71992b1d085ca8f72ba68d330b26d7ba373a0917cca56a444b8d89b0c0827e2a56242893b268e9f581bffc8a631a2ff20cc51f10db3255e

Author: achow101

test/functional/interface_http.py

@@ -612,9 +612,10 @@ def check_connection_limit(self):
 
         # Disable timeout so the initial batch of clients stays connected
         # until the end of the test.
-        for comment,                   extra_args,                                                 limit in [
-            ("default (16)",           ["-rpcservertimeout=0", "-rest"],                           16),
-            ("-rpcmaxconnections=128", ["-rpcservertimeout=0", "-rest", "-rpcmaxconnections=128"], 128)
+        for comment,                  extra_args,                                      limit in [
+            ("default (16)",          ["-rpcservertimeout=0", "-rest"],                16),
+            ("-rpcmaxconnections=64", ["-rpcservertimeout=0", "-rest",
+                                       "-rpcmaxconnections=64", "-maxconnections=16"], 64)
         ]:
             self.log.info(f"Using connection limit: {comment}")
             self.restart_node(0, extra_args=extra_args)

test/functional/test_framework/util.py

@@ -557,15 +557,18 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
             f.write("connect=0\n")
         # Limit max connections to mitigate test failures on some systems caused by the warning:
         # "Warning: Reducing -maxconnections from <...> to <...> due to system limitations".
-        # The value is calculated as follows:
-        #  available_fds = 256          // Same as FD_SETSIZE on NetBSD.
-        #  MIN_CORE_FDS = 151           // Number of file descriptors required for core functionality.
-        #  MAX_ADDNODE_CONNECTIONS = 8  // Maximum number of -addnode outgoing nodes.
-        #  nBind == 3                   // Maximum number of bound interfaces used in a test.
+        # For details, consult the `AppInitParameterInteraction` function in src/init.cpp.
+        #  available_fds = 256                // Same as FD_SETSIZE on NetBSD.
+        #  MIN_CORE_FDS = 151                 // Number of file descriptors required for core functionality.
+        #  MAX_ADDNODE_CONNECTIONS = 8        // Maximum number of -addnode outgoing nodes.
+        #  num_p2p_bind = 3                   // Maximum number of bound P2P interfaces (-bind and -whitebind) used in a test.
+        #  num_rpc_bind = 2                   // Maximum number of HTTP sockets used in a test.
+        #  DEFAULT_MAX_HTTP_CONNECTIONS = 16  // Reserved for connected HTTP clients.
         #
-        #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + nBind = 151 + 8 + 3 = 162;
-        #  nMaxConnections = available_fds - min_required_fds = 256 - 161 = 94;
-        f.write("maxconnections=94\n")
+        #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + num_p2p_bind + num_rpc_bind + DEFAULT_MAX_HTTP_CONNECTIONS =
+        #    = 151 + 8 + 3 + 2 + 16 = 180;
+        #  num_p2p_max_connections = available_fds - min_required_fds = 256 - 180 = 76;
+        f.write("maxconnections=76\n")
         f.write("par=" + str(min(2, os.cpu_count())) + "\n")
         # Use a single prevoutfetch worker thread to keep per-node resource usage low.
         f.write("prevoutfetchthreads=1\n")