Merge bitcoin/bitcoin#30065: init: fixes file descriptor accounting

d4c7c40 init: error out if -maxconnections is negative (Sergi Delgado Segura)
c773649 init: improves file descriptors accounting and docs (Sergi Delgado Segura)
29008a7 init: fixes fd accounting regarding poll/select (Sergi Delgado Segura)

Pull request description:

  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 the first commit of this PR).

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

  Fixes #18911

ACKs for top commit:
  sr-gi:
    > ACK [d4c7c40](bitcoin/bitcoin@d4c7c40)
  naumenkogs:
    ACK d4c7c40
  vasild:
    ACK d4c7c40
  TheCharlatan:
    ACK d4c7c40

Tree-SHA512: 1524d10c8ad8f354f6ab9c244699adbcdae2dd7aba37de5b8f9e177c629e8a2cce0f6e8117e076dde3a592f5283bd30a4201db96a3c011e335c02d1fde7414bc

Author: ryanofsky

src/init.cpp

@@ -148,11 +148,12 @@ 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";
 
 /**
@@ -838,8 +839,7 @@ 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;
@@ -991,27 +991,33 @@ 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);
-
-    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
+    // 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
 
-#ifdef USE_POLL
-    int fd_max = nFD;
-#else
-    int fd_max = FD_SETSIZE;
+    // 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);
+    if (user_max_connection < 0) {
+        return InitError(Untranslated("-maxconnections must be greater or equal than zero"));
+    }
+    // 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(user_max_connection + min_required_fds);
+    // If we are using select instead of poll, our actual limit may be even smaller
+#ifndef USE_POLL
+    available_fds = std::min(FD_SETSIZE, available_fds);
 #endif
+    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, 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);
+    nMaxConnections = std::min(available_fds - min_required_fds, user_max_connection);
 
-    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));
@@ -1159,7 +1165,7 @@ 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()) {