Resolve thread mentions on demand (#1480)

* Initial plan

* Fix unresolved thread mentions in HTML export (#1261)

- Add TryGetChannelAsync to DiscordClient for on-demand channel/thread lookup
- Add PopulateChannelAsync to ExportContext with negative caching
- Update HtmlMarkdownVisitor to resolve thread mentions on demand

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Refactor GetChannelAsync to use TryGetChannelAsync for parent resolution

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Add test for thread mention resolution in HTML export

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Apply PopulateChannelAsync to PlainTextMarkdownVisitor; add JSON thread mention test

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Cli.Tests/Specs/HtmlMentionSpecs.cs

@@ -61,4 +61,17 @@ public class HtmlMentionSpecs
         // Assert
         message.Text().Should().Contain("Role mention: @Role 1");
     }
+
+    [Fact]
+    public async Task I_can_export_a_channel_that_contains_a_message_with_a_thread_mention()
+    {
+        // Act
+        var message = await ExportWrapper.GetMessageAsHtmlAsync(
+            ChannelIds.MentionTestCases,
+            Snowflake.Parse("1474874276828938290")
+        );
+
+        // Assert
+        message.Text().Should().Contain("Thread mention: #Thread starting message");
+    }
 }

DiscordChatExporter.Cli.Tests/Specs/JsonMentionSpecs.cs

@@ -75,4 +75,21 @@ public class JsonMentionSpecs
         // Assert
         message.GetProperty("content").GetString().Should().Be("Role mention: @Role 1");
     }
+
+    [Fact]
+    public async Task I_can_export_a_channel_that_contains_a_message_with_a_thread_mention()
+    {
+        // Act
+        var message = await ExportWrapper.GetMessageAsJsonAsync(
+            ChannelIds.MentionTestCases,
+            Snowflake.Parse("1474874276828938290")
+        );
+
+        // Assert
+        message
+            .GetProperty("content")
+            .GetString()
+            .Should()
+            .Be("Thread mention: #Thread starting message");
+    }
 }

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -371,21 +371,40 @@ public class DiscordClient(
             ?.GetNonWhiteSpaceStringOrNull()
             ?.Pipe(Snowflake.Parse);
 
-        try
-        {
-            var parent = parentId is not null
-                ? await GetChannelAsync(parentId.Value, cancellationToken)
-                : null;
-
-            return Channel.Parse(response, parent);
-        }
         // It's possible for the parent channel to be inaccessible, despite the
         // child channel being accessible.
         // https://github.com/Tyrrrz/DiscordChatExporter/issues/1108
-        catch (DiscordChatExporterException)
+        var parent = parentId is not null
+            ? await TryGetChannelAsync(parentId.Value, cancellationToken)
+            : null;
+
+        return Channel.Parse(response, parent);
+    }
+
+    public async ValueTask<Channel?> TryGetChannelAsync(
+        Snowflake channelId,
+        CancellationToken cancellationToken = default
+    )
+    {
+        var response = await TryGetJsonResponseAsync($"channels/{channelId}", cancellationToken);
+        if (response is null)
+            return null;
+
+        var parentId = response
+            .Value.GetPropertyOrNull("parent_id")
+            ?.GetNonWhiteSpaceStringOrNull()
+            ?.Pipe(Snowflake.Parse);
+
+        Channel? parent = null;
+        if (parentId is not null)
         {
-            return Channel.Parse(response);
+            // It's possible for the parent channel to be inaccessible, despite the
+            // child channel being accessible.
+            // https://github.com/Tyrrrz/DiscordChatExporter/issues/1108
+            parent = await TryGetChannelAsync(parentId.Value, cancellationToken);
         }
+
+        return Channel.Parse(response.Value, parent);
     }
 
     public async IAsyncEnumerable<Channel> GetChannelThreadsAsync(

DiscordChatExporter.Core/Exporting/ExportContext.cs

@@ -16,7 +16,7 @@ namespace DiscordChatExporter.Core.Exporting;
 internal class ExportContext(DiscordClient discord, ExportRequest request)
 {
     private readonly Dictionary<Snowflake, Member?> _membersById = new();
-    private readonly Dictionary<Snowflake, Channel> _channelsById = new();
+    private readonly Dictionary<Snowflake, Channel?> _channelsById = new();
     private readonly Dictionary<Snowflake, Role> _rolesById = new();
 
     private readonly ExportAssetDownloader _assetDownloader = new(
@@ -51,6 +51,21 @@ internal class ExportContext(DiscordClient discord, ExportRequest request)
         }
     }
 
+    // Threads are not preloaded, so we resolve them on demand
+    public async ValueTask PopulateChannelAsync(
+        Snowflake id,
+        CancellationToken cancellationToken = default
+    )
+    {
+        if (_channelsById.ContainsKey(id))
+            return;
+
+        var channel = await Discord.TryGetChannelAsync(id, cancellationToken);
+
+        // Store the result even if it's null, to avoid re-fetching non-existing channels
+        _channelsById[id] = channel;
+    }
+
     // Because members cannot be pulled in bulk, we need to populate them on demand
     private async ValueTask PopulateMemberAsync(
         Snowflake id,

DiscordChatExporter.Core/Exporting/HtmlMarkdownVisitor.cs

@@ -270,6 +270,12 @@ internal partial class HtmlMarkdownVisitor(
         }
         else if (mention.Kind == MentionKind.Channel)
         {
+            // Channel/thread mentions may reference threads that are not preloaded,
+            // so we resolve them on demand.
+            // https://github.com/Tyrrrz/DiscordChatExporter/issues/1261
+            if (mention.TargetId is not null)
+                await context.PopulateChannelAsync(mention.TargetId.Value, cancellationToken);
+
             var channel = mention.TargetId?.Pipe(context.TryGetChannel);
             var symbol = channel?.IsVoice == true ? "🔊" : "#";
             var name = channel?.Name ?? "deleted-channel";

DiscordChatExporter.Core/Exporting/PlainTextMarkdownVisitor.cs

@@ -57,6 +57,12 @@ internal partial class PlainTextMarkdownVisitor(ExportContext context, StringBui
         }
         else if (mention.Kind == MentionKind.Channel)
         {
+            // Channel/thread mentions may reference threads that are not preloaded,
+            // so we resolve them on demand.
+            // https://github.com/Tyrrrz/DiscordChatExporter/issues/1261
+            if (mention.TargetId is not null)
+                await context.PopulateChannelAsync(mention.TargetId.Value, cancellationToken);
+
             var channel = mention.TargetId?.Pipe(context.TryGetChannel);
             var name = channel?.Name ?? "deleted-channel";