Split domain to which LIO requests are directed to envvar

On several occasions, I've tried to test something involving LIO
full-stack and forgot that the LIO helper hardcodes production.
Thankfully I never did damage due to the interop secret check, but it's
a bit bone-chilling every time. Additionally I believe this situation
can't be correct as far as staging is involved, either.

Same as the shared interop secret, this envvar is optional in debug (and
it missing causes LIO to become a no-op), and required with no default
in release.

README.md

@@ -103,10 +103,20 @@ Defaults to `localhost`.
 
 Enables DataDog origin detection when running in a container. See [DataDog documentation](https://docs.datadoghq.com/developers/dogstatsd/?tab=kubernetes&code-lang=dotnet#origin-detection-over-udp).
 
+### LEGACY_IO_DOMAIN
+
+Root domain to which LegacyIO requests should be directed.
+
+- In Debug configuration, this parameter is not required; when missing, all LIO requests are no-ops.
+- In Release configuration, this parameter is required; all LIO requests will hard-fail with an exception if missing.
+
 ### SHARED_INTEROP_SECRET
 
 Secret key used to sign LegacyIO requests to osu-web. Required to award medals.
 
+- In Debug configuration, this parameter is not required; when missing, all LIO requests are no-ops.
+- In Release configuration, this parameter is required; all LIO requests will hard-fail with an exception if missing.
+
 ### PROCESS_USER_MEDALS
 
 Whether to process user medals. Set to `0` to disable processing.

osu.Server.Queues.ScoreStatisticsProcessor/Helpers/LegacyDatabaseHelper.cs

@@ -75,12 +75,21 @@ public RulesetDatabaseInfo(int rulesetId, string rulesetIdentifier, bool legacyS
             }
         }
 
-        private static readonly string shared_interop_secret = Environment.GetEnvironmentVariable("SHARED_INTEROP_SECRET") ?? string.Empty;
+        private static readonly string? legacy_io_domain = Environment.GetEnvironmentVariable("LEGACY_IO_DOMAIN");
+        private static readonly string? shared_interop_secret = Environment.GetEnvironmentVariable("SHARED_INTEROP_SECRET");
 
         private static readonly HttpClient http = new HttpClient();
 
         public static HttpResponseMessage RunLegacyIO(string command, string method = "GET", dynamic? postObject = null)
         {
+            if (string.IsNullOrEmpty(legacy_io_domain))
+            {
+#if !DEBUG
+                throw new InvalidOperationException($"Attempted legacy IO call without target domain specified ({command})");
+#endif
+                return null!;
+            }
+
             if (string.IsNullOrEmpty(shared_interop_secret))
             {
 #if !DEBUG
@@ -97,7 +106,7 @@ public static HttpResponseMessage RunLegacyIO(string command, string method = "G
             {
                 long time = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
 
-                string url = $"https://osu.ppy.sh/_lio/{command}{(command.Contains('?') ? "&" : "?")}timestamp={time}";
+                string url = $"{legacy_io_domain}/_lio/{command}{(command.Contains('?') ? "&" : "?")}timestamp={time}";
                 string signature = hmacEncode(url, Encoding.UTF8.GetBytes(shared_interop_secret));
 
 #pragma warning disable SYSLIB0014