init: improves file descriptors accounting and docs

The current logic for file descriptor accounting is pretty convoluted and hard
to follow. This is partially caused by the lack of documentation plus non-intuitive
variable naming (which was more intuitive when fewer things were accounted for, but
hasn't aged well). This has led to this accounting being error-prone and hard to maintain
(as shown in he previous commit).

Redefine some of the constants, plus document what are we accounting for so this can be
extended more easily

src/init.cpp

@@ -142,11 +142,12 @@ static constexpr bool DEFAULT_STOPAFTERBLOCKIMPORT{false};
 // Win32 LevelDB doesn't use filedescriptors, and the ones used for
 // accessing block files don't count towards the fd_set size limit
 // anyway.
-#define MIN_CORE_FILEDESCRIPTORS 0
+#define MIN_LEVELDB_FDS 0
 #else
-#define MIN_CORE_FILEDESCRIPTORS 150
+#define MIN_LEVELDB_FDS 150
 #endif
 
+static const int MIN_CORE_FDS = MIN_LEVELDB_FDS + NUM_FDS_MESSAGE_CAPTURE;
 static const char* DEFAULT_ASMAP_FILENAME="ip_asn.map";
 
 /**
@@ -967,22 +968,29 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         return InitError(Untranslated("Cannot set -listen=0 together with -listenonion=1"));
     }
 
-    // Make sure enough file descriptors are available
+    // Make sure enough file descriptors are available. We need to reserve enough FDs to account for the bare minimum,
+    // plus all manual connections and all bound interfaces. Any remainder will be available for connection sockets
+
+    // Number of bound interfaces (we have at least one)
     int nBind = std::max(nUserBind, size_t(1));
+    // Maximum number of connetions with other nodes, this accounts for all types of outbound and inbounds expect for manual
     nUserMaxConnections = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
     nMaxConnections = std::max(nUserMaxConnections, 0);
+    // Reserve enough FDs to account for the bare minimum, plus any manual connections, plus the bound interfaces
+    int nMinRequiredFds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + nBind;
 
-    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
+    // Try raising the FD limit to what we need (nFD may be smaller than the requested amount if this fails)
+    nFD = RaiseFileDescriptorLimit(nMaxConnections + nMinRequiredFds);
     // If we are using select instead of poll, our actuall limit may be even smaller
 #ifndef USE_POLL
     nFD = std::min(FD_SETSIZE, nFD);
 #endif
-    if (nFD < MIN_CORE_FILEDESCRIPTORS)
-        return InitError(_("Not enough file descriptors available."));
+    if (nFD < MIN_CORE_FDS)
+        return InitError(strprintf(_("Not enough file descriptors available. %d available, %d required."), nFD, MIN_CORE_FDS));
 
     // 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);
+    nMaxConnections = std::max(std::min<int>(nMaxConnections, nFD - nMinRequiredFds), 0);
 
     if (nMaxConnections < nUserMaxConnections)
         InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));