Allow ModifiedUtc to vary between variation and response

Author: rmja

src/HttpClient.Cache/Files/FileCache.cs

@@ -156,7 +156,7 @@ TimeProvider timeProvider
                 return null;
             }
 
-            fileInfo = FindJsonFile(responseKey);
+            fileInfo = FindJsonFile(responseKey, filename.EtagHash);
             if (fileInfo is null)
             {
                 return null;
@@ -397,7 +397,7 @@ DateTimeOffset expiration
                 return;
             }
 
-            fileInfo = FindJsonFile(responseKey);
+            fileInfo = FindJsonFile(responseKey, filename.EtagHash);
             if (fileInfo is null)
             {
                 return;
@@ -422,6 +422,16 @@ DateTimeOffset expiration
         return _rootDirectory.EnumerateFiles($"{hash}_*.json").MaxBy(x => x.Name);
     }
 
+    private FileInfo? FindJsonFile(string key, string? etagHash)
+    {
+        var hash = Hash.ComputeHash(key);
+
+        // Rely on that the file name includes the "modified" timestamp right after the hash
+        return _rootDirectory
+            .EnumerateFiles($"{hash}_*_{etagHash ?? ""}.*.json")
+            .MaxBy(x => x.Name);
+    }
+
     public void Clear()
     {
         foreach (var fileInfo in _rootDirectory.GetFiles("*.json", SearchOption.AllDirectories))

src/HttpClient.Cache/Files/FileName.cs

@@ -110,9 +110,9 @@ public static FileName FromFileInfo(FileInfo fileInfo)
         );
         Debug.Assert(modifiedUtc.Kind == DateTimeKind.Utc);
 
-        var etagHash = basename[(index + 1 + DateTimeFormat.Length + 1)..].ToString();
+        var etagHash = basename[(index + 1 + DateTimeFormat.Length + 1)..];
 
-        return new(hash, modifiedUtc, etagHash, extension);
+        return new(hash, modifiedUtc, etagHash.Length > 0 ? etagHash.ToString() : null, extension);
     }
 
     public static implicit operator string(FileName filename) => filename.ToString();

src/HttpClient.Cache/HttpClient.Cache.csproj

@@ -19,6 +19,7 @@
     <None Include="..\..\README.md" Pack="true" PackagePath="">
       <Link>Properties\README.md</Link>
     </None>
+    <InternalsVisibleTo Include="HttpClient.Cache.Tests" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Http" Version="9.0.6" />

test/HttpClient.Cache.Tests/File/FileNameTests.cs

@@ -0,0 +1,32 @@
+using HttpClientCache.Files;
+
+namespace HttpClientCache.Tests.File;
+
+public class FileNameTests
+{
+    [Fact]
+    public void CanParseFileName_WithEtagHash()
+    {
+        var fileInfo = new FileInfo(
+            "efed661293e37fe7141f28819252c3365aadae65_2025-06-24T102811Z_c6e5a96ed499f364089ebe92d16e13280ef750ce.response.json"
+        );
+        var fileName = FileName.FromFileInfo(fileInfo);
+
+        Assert.Equal("efed661293e37fe7141f28819252c3365aadae65", fileName.KeyHash);
+        Assert.Equal(new DateTime(2025, 6, 24, 10, 28, 11, DateTimeKind.Utc), fileName.ModifiedUtc);
+        Assert.Equal("c6e5a96ed499f364089ebe92d16e13280ef750ce", fileName.EtagHash);
+    }
+
+    [Fact]
+    public void CanParseFileName_WithoutEtag()
+    {
+        var fileInfo = new FileInfo(
+            "efed661293e37fe7141f28819252c3365aadae65_2025-06-24T102811Z_.response.json"
+        );
+        var fileName = FileName.FromFileInfo(fileInfo);
+
+        Assert.Equal("efed661293e37fe7141f28819252c3365aadae65", fileName.KeyHash);
+        Assert.Equal(new DateTime(2025, 6, 24, 10, 28, 11, DateTimeKind.Utc), fileName.ModifiedUtc);
+        Assert.Null(fileName.EtagHash);
+    }
+}