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

Remove FreeBSD workaround to #2695

Author: sr-gi

src/init.cpp

@@ -147,11 +147,12 @@ 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 constexpr int MIN_CORE_FDS = MIN_LEVELDB_FDS + NUM_FDS_MESSAGE_CAPTURE;
 static const char* DEFAULT_ASMAP_FILENAME="ip_asn.map";
 
 /**
@@ -834,8 +835,7 @@ void InitLogging(const ArgsManager& args)
 namespace { // Variables internal to initialization process only
 
 int nMaxConnections;
-int nUserMaxConnections;
-int nFD;
+int available_fds;
 ServiceFlags nLocalServices = ServiceFlags(NODE_NETWORK_LIMITED | NODE_WITNESS);
 int64_t peer_connect_timeout;
 std::set<BlockFilterType> g_enabled_filter_types;
@@ -987,25 +987,31 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         return InitError(Untranslated("Cannot set -listen=0 together with -listenonion=1"));
     }
 
-    // Make sure enough file descriptors are available
-    int nBind = std::max(nUserBind, size_t(1));
-    nUserMaxConnections = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
-    nMaxConnections = std::max(nUserMaxConnections, 0);
+    // 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
 
-    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
+    // Number of bound interfaces (we have at least one)
+    int nBind = std::max(nUserBind, size_t(1));
+    // Maximum number of connections with other nodes, this accounts for all types of outbounds and inbounds except for manual
+    int user_max_connection = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
+    nMaxConnections = std::max(user_max_connection, 0);
+    // Reserve enough FDs to account for the bare minimum, plus any manual connections, plus the bound interfaces
+    int min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + nBind;
+
+    // Try raising the FD limit to what we need (available_fds may be smaller than the requested amount if this fails)
+    available_fds = RaiseFileDescriptorLimit(nMaxConnections + min_required_fds);
     // If we are using select instead of poll, our actual limit may be even smaller
 #ifndef USE_POLL
-    nFD = std::min(FD_SETSIZE, nFD);
+    available_fds = std::min(FD_SETSIZE, available_fds);
 #endif
-    if (nFD < MIN_CORE_FILEDESCRIPTORS)
-        return InitError(_("Not enough file descriptors available."));
+    if (available_fds < min_required_fds)
+        return InitError(strprintf(_("Not enough file descriptors available. %d available, %d required."), available_fds, min_required_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::min(available_fds - min_required_fds, nMaxConnections);
 
-    if (nMaxConnections < nUserMaxConnections)
-        InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));
+    if (nMaxConnections < user_max_connection)
+        InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), user_max_connection, nMaxConnections));
 
     // ********************************************************* Step 3: parameter-to-internal-flags
     if (auto result{init::SetLoggingCategories(args)}; !result) return InitError(util::ErrorString(result));
@@ -1153,7 +1159,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         return false;
     }
 
-    LogPrintf("Using at most %i automatic connections (%i file descriptors available)\n", nMaxConnections, nFD);
+    LogPrintf("Using at most %i automatic connections (%i file descriptors available)\n", nMaxConnections, available_fds);
 
     // Warn about relative -datadir path.
     if (args.IsArgSet("-datadir") && !args.GetPathArg("-datadir").is_absolute()) {