init: fixes fd accounting regarding poll/select

We are computing our file descriptors limits based on whether we use
poll or select. However, we are taking that into account only partially
(subtracting from fd_max in one case, but from nFD later on). Moreover,
nBind is also only accounted for partially.

Simplify and fix this logic

Author: sr-gi

src/init.cpp

@@ -993,18 +993,16 @@ bool AppInitParameterInteraction(const ArgsManager& args)
     nMaxConnections = std::max(nUserMaxConnections, 0);
 
     nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
-
-#ifdef USE_POLL
-    int fd_max = nFD;
-#else
-    int fd_max = FD_SETSIZE;
+    // If we are using select instead of poll, our actual limit may be even smaller
+#ifndef USE_POLL
+    nFD = std::min(FD_SETSIZE, nFD);
 #endif
-    // Trim requested connection counts, to fit into system limitations
-    // <int> in std::min<int>(...) to work around FreeBSD compilation issue described in #2695
-    nMaxConnections = std::max(std::min<int>(nMaxConnections, fd_max - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE), 0);
     if (nFD < MIN_CORE_FILEDESCRIPTORS)
         return InitError(_("Not enough file descriptors available."));
-    nMaxConnections = std::min(nFD - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE, nMaxConnections);
+
+    // Trim requested connection counts, to fit into system limitations
+    // <int> in std::min<int>(...) to work around FreeBSD compilation issue described in #2695
+    nMaxConnections = std::max(std::min<int>(nMaxConnections, nFD - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE), 0);
 
     if (nMaxConnections < nUserMaxConnections)
         InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));