Implement Happy Eyeballs for C4Network2HTTPClient

This should make masterserver requests more reliable for users with a
bad IPv6 connection.

See RFC6555

src/network/C4Network2Reference.cpp

@@ -407,7 +407,7 @@ bool C4Network2HTTPClient::Decompress(StdBuf *pData)
 bool C4Network2HTTPClient::OnConn(const C4NetIO::addr_t &AddrPeer, const C4NetIO::addr_t &AddrConnect, const C4NetIO::addr_t *pOwnAddr, C4NetIO *pNetIO)
 {
 	// Make sure we're actually waiting for this connection
-	if (AddrConnect != ServerAddr)
+	if (fConnected || (AddrConnect != ServerAddr && AddrConnect != ServerAddrFallback))
 		return false;
 	// Save pack peer address
 	PeerAddr = AddrPeer;
@@ -445,10 +445,19 @@ void C4Network2HTTPClient::OnPacket(const class C4NetIOPacket &rPacket, C4NetIO
 bool C4Network2HTTPClient::Execute(int iMaxTime)
 {
 	// Check timeout
-	if (fBusy && time(nullptr) > iRequestTimeout)
+	if (fBusy)
 	{
-		Cancel("Request timeout");
-		return true;
+		if (C4TimeMilliseconds::Now() > HappyEyeballsTimeout)
+		{
+			HappyEyeballsTimeout = C4TimeMilliseconds::PositiveInfinity;
+			Application.InteractiveThread.ThreadLogS("HTTP: Starting fallback connection to %s (%s)", Server.getData(), ServerAddrFallback.ToString().getData());
+			Connect(ServerAddrFallback);
+		}
+		if (time(nullptr) > iRequestTimeout)
+		{
+			Cancel("Request timeout");
+			return true;
+		}
 	}
 	// Execute normally
 	return C4NetIOTCP::Execute(iMaxTime);
@@ -462,7 +471,9 @@ C4TimeMilliseconds C4Network2HTTPClient::GetNextTick(C4TimeMilliseconds tNow)
 
 	C4TimeMilliseconds tHTTPClientTick = tNow + 1000 * std::max<time_t>(iRequestTimeout - time(nullptr), 0);
 
-	return std::max(tNetIOTCPTick, tHTTPClientTick);
+	C4TimeMilliseconds HappyEyeballsTick = tNow + std::max(HappyEyeballsTimeout - C4TimeMilliseconds::Now(), 0);
+
+	return std::min({tNetIOTCPTick, tHTTPClientTick, HappyEyeballsTick});
 }
 
 bool C4Network2HTTPClient::Query(const StdBuf &Data, bool fBinary)
@@ -512,6 +523,11 @@ bool C4Network2HTTPClient::Query(const StdBuf &Data, bool fBinary)
 	// Start connecting
 	if (!Connect(ServerAddr))
 		return false;
+	// Also try the fallback address after some time (if there is one)
+	if (!ServerAddrFallback.IsNull())
+		HappyEyeballsTimeout = C4TimeMilliseconds::Now() + C4Network2HTTPHappyEyeballsTimeout;
+	else
+		HappyEyeballsTimeout = C4TimeMilliseconds::PositiveInfinity;
 	// Okay, request will be performed when connection is complete
 	fBusy = true;
 	iDataOffset = 0;
@@ -529,7 +545,7 @@ void C4Network2HTTPClient::ResetRequestTimeout()
 void C4Network2HTTPClient::Cancel(const char *szReason)
 {
 	// Close connection - and connection attempt
-	Close(ServerAddr); Close(PeerAddr);
+	Close(ServerAddr); Close(ServerAddrFallback); Close(PeerAddr);
 	// Reset flags
 	fBusy = fSuccess = fConnected = fBinary = false;
 	iDownloadedSize = iTotalSize = iDataOffset = 0;
@@ -567,6 +583,14 @@ bool C4Network2HTTPClient::SetServer(const char *szServerAddress)
 		return false;
 	}
 	ServerAddr.SetDefaultPort(GetDefaultPort());
+	if (ServerAddr.GetFamily() == C4NetIO::HostAddress::IPv6)
+	{
+		// Try to find a fallback IPv4 address for Happy Eyeballs.
+		ServerAddrFallback.SetAddress(Server, C4NetIO::HostAddress::IPv4);
+		ServerAddrFallback.SetDefaultPort(GetDefaultPort());
+	}
+	else
+		ServerAddrFallback.Clear();
 	// Remove port
 	const char *firstColon = strchr(Server.getData(), ':');
 	const char *lastColon = strrchr(Server.getData(), ':');

src/network/C4Network2Reference.h

@@ -24,6 +24,7 @@
 #include "lib/C4InputValidation.h"
 
 const int C4Network2HTTPQueryTimeout = 10; // (s)
+const uint32_t C4Network2HTTPHappyEyeballsTimeout = 300; // (ms)
 
 // Session data
 class C4Network2Reference
@@ -129,7 +130,7 @@ public:
 private:
 
 	// Address information
-	C4NetIO::addr_t ServerAddr, PeerAddr;
+	C4NetIO::addr_t ServerAddr, ServerAddrFallback, PeerAddr;
 	StdCopyStrBuf Server, RequestPath;
 
 	bool fBinary;
@@ -137,6 +138,7 @@ private:
 	size_t iDataOffset;
 	StdCopyBuf Request;
 	time_t iRequestTimeout;
+	C4TimeMilliseconds HappyEyeballsTimeout;
 
 	// Response header data
 	size_t iDownloadedSize, iTotalSize;