parallelize retrieval of server version information

Parallelizing the retrieval of the server version information decreases
the program runtime, especially since the impact of slow responses and
timeouts is lowered greatly.

main.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/hashicorp/go-version"
 	"github.com/matrix-org/gomatrixserverlib/fclient"
@@ -15,6 +16,11 @@ import (
 	"maunium.net/go/mautrix/id"
 )
 
+type HomeserverServerVersionInfo struct {
+	Homeserver        string
+	ServerVersionInfo fclient.Version
+}
+
 type ServerPath struct {
 	MaxRoomVersion string
 	Server         string
@@ -36,6 +42,44 @@ var unknownServerVersionInfo = fclient.Version{
 	},
 }
 
+func getServerVersionInfoByHomeserver(homeservers []string, federationClient *fclient.Client) map[string](fclient.Version) {
+	homeserverChannel := make(chan string)
+	go func() {
+		for _, hs := range homeservers {
+			homeserverChannel <- hs
+		}
+		close(homeserverChannel)
+	}()
+
+	homeserverSVInfoChannel := make(chan HomeserverServerVersionInfo)
+	var wg sync.WaitGroup
+	const numHomeserverSVInfoReceivers = 100
+	wg.Add(numHomeserverSVInfoReceivers)
+	for i := 0; i < numHomeserverSVInfoReceivers; i++ {
+		go func() {
+			for hs := range homeserverChannel {
+				serverVersionInfo, err := federationClient.GetVersion(context.Background(), spec.ServerName(hs))
+				if err != nil {
+					serverVersionInfo = unknownServerVersionInfo
+				}
+				homeserverSVInfoChannel <- HomeserverServerVersionInfo{hs, serverVersionInfo}
+			}
+			wg.Done()
+		}()
+	}
+
+	go func() {
+		wg.Wait()
+		close(homeserverSVInfoChannel)
+	}()
+
+	serverVersionInfoByHomeserver := make(map[string](fclient.Version))
+	for homeserverSVInfo := range homeserverSVInfoChannel {
+		serverVersionInfoByHomeserver[homeserverSVInfo.Homeserver] = homeserverSVInfo.ServerVersionInfo
+	}
+	return serverVersionInfoByHomeserver
+}
+
 // Gets the maximum supported room version for the given server-version-pair.
 func getMaxRoomVersion(serverVersionInfo fclient.Version) string {
 	// Use only the first part of the version string as sometimes there are suffixes after a space, like " (<commit>)", which then can't be parsed.
@@ -235,6 +279,13 @@ func main() {
 		membersByHomeservers[key.Homeserver()]++
 	}
 
+	homeservers := make([]string, 0, len(membersByHomeservers))
+	for hs := range membersByHomeservers {
+		homeservers = append(homeservers, hs)
+	}
+
+	serverVersionInfoByHomeserver := getServerVersionInfoByHomeserver(homeservers, federationClient)
+
 	// Info map tree.
 	maxRoomVersionInfo := make(map[string](map[string](map[string](map[string]uint))))
 	// Member counters.
@@ -243,10 +294,7 @@ func main() {
 	membersByVersionPath := make(map[VersionPath]uint)
 
 	for key, value := range membersByHomeservers {
-		serverVersionInfo, err := federationClient.GetVersion(context.Background(), spec.ServerName(key))
-		if err != nil {
-			serverVersionInfo = unknownServerVersionInfo
-		}
+		serverVersionInfo := serverVersionInfoByHomeserver[key]
 
 		maxRoomVersion := getMaxRoomVersion(serverVersionInfo)