Broken message in cache after ModifyAsync with added embed

Summary

GetMessageAsync returns a message with no Content, if it was previously edited with ModifyAsync to add an embed. (it doesn’t reproduce if no embed added on edit)

Details

D#+ 3.2.3 (no custom sockets) (got couple of confirmations it also happens on 4.0 nightly) .Net Core 2.2

Steps to reproduce

First, add the following commands:

private ulong testMsgId = 0;

[Command("test1")]
public async Task Test1Async(CommandContext ctx)
{
    var  msg1 = await ctx.RespondAsync("test message");
    testMsgId = msg1.Id;

    msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"A: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");
}

[Command("test2")]
public async Task Test2Async(CommandContext ctx)
{
    if (testMsgId == 0) { return; }

    var msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"A: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");

    if (msg1.Content != null)
    {
        await msg1.ModifyAsync(
            msg1.Content,
            new DiscordEmbedBuilder { Description = "test", Timestamp = DateTimeOffset.Now }
            );
    }

    msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"B: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");
}

Then, run them in the following order:

!test1
!test2
!test2

Every command should output “test message” in place of {msg1.Content}. But after first run of .test2 has modified it, on the second run it is empty.

Notes

I hope to have a fix or workaround for 3.2.x, unless you releasing 4.0 like tomorrow.

[Emzi0767]

It’s a classic Apple goto fail; bug.

[Naamloos]

Fix coming up, will backport to 3.x as well