fetch CurseForge Last-Modified header

Author: Pathoschild

src/SMAPI.Toolkit/Framework/Clients/CurseForgeExport/CurseForgeExportApiClient.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Threading.Tasks;
 using Pathoschild.Http.Client;
 using StardewModdingAPI.Toolkit.Framework.Clients.CurseForgeExport.ResponseModels;
@@ -28,9 +29,12 @@ public CurseForgeExportApiClient(string userAgent, string baseUrl)
         /// <inheritdoc />
         public async Task<CurseForgeFullExport> FetchExportAsync()
         {
-            return await this.Client
-                .GetAsync("")
-                .As<CurseForgeFullExport>();
+            IResponse response = await this.Client.GetAsync("");
+
+            CurseForgeFullExport export = await response.As<CurseForgeFullExport>();
+            export.LastModified = response.Message.Content.Headers.LastModified ?? throw new InvalidOperationException("Can't fetch from CurseForge export API: expected Last-Modified header wasn't set.");
+
+            return export;
         }
 
         /// <inheritdoc />

src/SMAPI.Toolkit/Framework/Clients/CurseForgeExport/ResponseModels/CurseForgeFullExport.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 
 namespace StardewModdingAPI.Toolkit.Framework.Clients.CurseForgeExport.ResponseModels
@@ -7,5 +8,8 @@ public class CurseForgeFullExport
     {
         /// <summary>The mod data indexed by public mod ID.</summary>
         public Dictionary<uint, CurseForgeModExport> Mods { get; set; } = new();
+
+        /// <summary>When the data was last updated.</summary>
+        public DateTimeOffset LastModified { get; set; }
     }
 }

src/SMAPI.Web/Framework/Caching/CurseForgeExport/CurseForgeExportCacheMemoryRepository.cs

@@ -13,9 +13,6 @@ internal class CurseForgeExportCacheMemoryRepository : BaseCacheRepository, ICur
         /// <summary>The cached mod data from the CurseForge export API.</summary>
         private CurseForgeFullExport? Data;
 
-        /// <summary>When the data was last updated.</summary>
-        private DateTimeOffset LastRefreshed;
-
 
         /*********
         ** Public methods
@@ -29,7 +26,7 @@ public bool IsLoaded()
         /// <inheritdoc />
         public DateTimeOffset? GetLastRefreshed()
         {
-            return this.LastRefreshed;
+            return this.Data?.LastModified;
         }
 
         /// <inheritdoc />
@@ -50,13 +47,14 @@ public bool TryGetMod(uint id, [NotNullWhen(true)] out CurseForgeModExport? mod)
         public void SetData(CurseForgeFullExport? export)
         {
             this.Data = export;
-            this.LastRefreshed = DateTimeOffset.UtcNow;
         }
 
         /// <inheritdoc />
         public bool IsStale(int staleMinutes)
         {
-            return this.IsStale(this.LastRefreshed, staleMinutes);
+            return
+                this.Data is null
+                || this.IsStale(this.Data.LastModified, staleMinutes);
         }
     }
 }