6 files changed, 64 insertions, 7 deletions
diff --git a/src/network/core/game.h b/src/network/core/game.h
index c82c5aefe..a9da29118 100644
--- a/src/network/core/game.h
+++ b/src/network/core/game.h
@@ -56,6 +56,8 @@ struct NetworkGameInfo : NetworkServerGameInfo {
 	byte map_set;                                   ///< Graphical set
 };
 
+const char * GetNetworkRevisionString();
+
 #endif /* ENABLE_NETWORK */
 
 #endif /* NETWORK_CORE_GAME_H */
diff --git a/src/network/core/tcp_http.cpp b/src/network/core/tcp_http.cpp
index a8bb64080..aec198bf5 100644
--- a/src/network/core/tcp_http.cpp
+++ b/src/network/core/tcp_http.cpp
@@ -45,14 +45,14 @@ NetworkHTTPSocketHandler::NetworkHTTPSocketHandler(SOCKET s,
 	redirect_depth(depth),
 	sock(s)
 {
-	size_t bufferSize = strlen(url) + strlen(host) + strlen(_openttd_revision) + (data == NULL ? 0 : strlen(data)) + 128;
+	size_t bufferSize = strlen(url) + strlen(host) + strlen(GetNetworkRevisionString()) + (data == NULL ? 0 : strlen(data)) + 128;
 	char *buffer = AllocaM(char, bufferSize);
 
 	DEBUG(net, 7, "[tcp/http] requesting %s%s", host, url);
 	if (data != NULL) {
-		seprintf(buffer, buffer + bufferSize - 1, "POST %s HTTP/1.0\r\nHost: %s\r\nUser-Agent: OpenTTD/%s\r\nContent-Type: text/plain\r\nContent-Length: %d\r\n\r\n%s\r\n", url, host, _openttd_revision, (int)strlen(data), data);
+		seprintf(buffer, buffer + bufferSize - 1, "POST %s HTTP/1.0\r\nHost: %s\r\nUser-Agent: OpenTTD/%s\r\nContent-Type: text/plain\r\nContent-Length: %d\r\n\r\n%s\r\n", url, host, GetNetworkRevisionString(), (int)strlen(data), data);
 	} else {
-		seprintf(buffer, buffer + bufferSize - 1, "GET %s HTTP/1.0\r\nHost: %s\r\nUser-Agent: OpenTTD/%s\r\n\r\n", url, host, _openttd_revision);
+		seprintf(buffer, buffer + bufferSize - 1, "GET %s HTTP/1.0\r\nHost: %s\r\nUser-Agent: OpenTTD/%s\r\n\r\n", url, host, GetNetworkRevisionString());
 	}
 
 	ssize_t size = strlen(buffer);
diff --git a/src/network/network.cpp b/src/network/network.cpp
index 0bbdd0d06..d70d800f9 100644
--- a/src/network/network.cpp
+++ b/src/network/network.cpp
@@ -1101,12 +1101,67 @@ void NetworkShutDown()
 }
 
 /**
+ * How many hex digits of the git hash to include in network revision string.
+ * Determined as 10 hex digits + 2 characters for -g/-u/-m prefix.
+ */
+static const uint GITHASH_SUFFIX_LEN = 12;
+
+/**
+ * Get the network version string used by this build.
+ * The returned string is guaranteed to be at most NETWORK_REVISON_LENGTH bytes.
+ */
+const char * GetNetworkRevisionString()
+{
+	/* This will be allocated on heap and never free'd, but only once so not a "real" leak. */
+	static char *network_revision = nullptr;
+
+	if (!network_revision) {
+		/* Start by taking a chance on the full revision string. */
+		network_revision = stredup(_openttd_revision);
+		/* Ensure it's not longer than the packet buffer length. */
+		if (strlen(network_revision) >= NETWORK_REVISION_LENGTH) network_revision[NETWORK_REVISION_LENGTH - 1] = '\0';
+
+		/* Release version names are not mangled further. */
+		if (IsReleasedVersion()) return network_revision;
+
+		/* Prepare a prefix of the git hash.
+		* Size is length + 1 for terminator, +2 for -g prefix. */
+		assert(_openttd_revision_modified < 3);
+		char githash_suffix[GITHASH_SUFFIX_LEN + 1] = "-";
+		githash_suffix[1] = "gum"[_openttd_revision_modified];
+		for (uint i = 2; i < GITHASH_SUFFIX_LEN; i++) {
+			githash_suffix[i] = _openttd_revision_hash[i-2];
+		}
+
+		/* Where did the hash start in the original string?
+		 * Overwrite from that position, unless that would go past end of packet buffer length. */
+		ptrdiff_t hashofs = strrchr(_openttd_revision, '-') - _openttd_revision;
+		if (hashofs + strlen(githash_suffix) + 1 > NETWORK_REVISION_LENGTH) hashofs = strlen(network_revision) - strlen(githash_suffix);
+		/* Replace the git hash in revision string. */
+		strecpy(network_revision + hashofs, githash_suffix, network_revision + NETWORK_REVISION_LENGTH);
+		assert(strlen(network_revision) < NETWORK_REVISION_LENGTH); // strlen does not include terminator, constant does, hence strictly less than
+	}
+
+	return network_revision;
+}
+
+static const char *ExtractNetworkRevisionHash(const char *revstr)
+{
+	return strrchr(revstr, '-');
+}
+
+/**
  * Checks whether the given version string is compatible with our version.
+ * First tries to match the full string, if that fails, attempts to compare just git hashes.
  * @param other the version string to compare to
  */
 bool IsNetworkCompatibleVersion(const char *other)
 {
-	return strncmp(_openttd_revision, other, NETWORK_REVISION_LENGTH - 1) == 0;
+	if (strncmp(GetNetworkRevisionString(), other, NETWORK_REVISION_LENGTH - 1) == 0) return true;
+
+	const char *hash1 = ExtractNetworkRevisionHash(GetNetworkRevisionString());
+	const char *hash2 = ExtractNetworkRevisionHash(other);
+	return hash1 && hash2 && (strncmp(hash1, hash2, GITHASH_SUFFIX_LEN) == 0);
 }
 
 #endif /* ENABLE_NETWORK */
diff --git a/src/network/network_admin.cpp b/src/network/network_admin.cpp
index c1ca3d3bb..f70f3d1a5 100644
--- a/src/network/network_admin.cpp
+++ b/src/network/network_admin.cpp
@@ -172,7 +172,7 @@ NetworkRecvStatus ServerNetworkAdminSocketHandler::SendWelcome()
 	Packet *p = new Packet(ADMIN_PACKET_SERVER_WELCOME);
 
 	p->Send_string(_settings_client.network.server_name);
-	p->Send_string(_openttd_revision);
+	p->Send_string(GetNetworkRevisionString());
 	p->Send_bool  (_network_dedicated);
 
 	p->Send_string(_network_game_info.map_name);
diff --git a/src/network/network_client.cpp b/src/network/network_client.cpp
index 4b066681d..9e34ebb09 100644
--- a/src/network/network_client.cpp
+++ b/src/network/network_client.cpp
@@ -343,7 +343,7 @@ NetworkRecvStatus ClientNetworkGameSocketHandler::SendJoin()
 	SetWindowDirty(WC_NETWORK_STATUS_WINDOW, WN_NETWORK_STATUS_WINDOW_JOIN);
 
 	Packet *p = new Packet(PACKET_CLIENT_JOIN);
-	p->Send_string(_openttd_revision);
+	p->Send_string(GetNetworkRevisionString());
 	p->Send_uint32(_openttd_newgrf_version);
 	p->Send_string(_settings_client.network.client_name); // Client name
 	p->Send_uint8 (_network_join_as);     // PlayAs
diff --git a/src/network/network_udp.cpp b/src/network/network_udp.cpp
index 1dc696065..3fd0dd485 100644
--- a/src/network/network_udp.cpp
+++ b/src/network/network_udp.cpp
@@ -191,7 +191,7 @@ void ServerNetworkUDPSocketHandler::Receive_CLIENT_FIND_SERVER(Packet *p, Networ
 
 	strecpy(ngi.map_name, _network_game_info.map_name, lastof(ngi.map_name));
 	strecpy(ngi.server_name, _settings_client.network.server_name, lastof(ngi.server_name));
-	strecpy(ngi.server_revision, _openttd_revision, lastof(ngi.server_revision));
+	strecpy(ngi.server_revision, GetNetworkRevisionString(), lastof(ngi.server_revision));
 
 	Packet packet(PACKET_UDP_SERVER_RESPONSE);
 	this->SendNetworkGameInfo(&packet, &ngi);