messageFlagsAdd deadlocks/freezes if fetch is used

[PRR24]

Describe the bug
messageFlagsAdd does not return if fetch was used on the client earlier

To Reproduce

await client.connect();
let lock = await client.getMailboxLock("INBOX");
try {
    let processed: number[] = [];
    for await (let msg of client.fetch({ seen: false }, { source: { maxLength: 2 * 1024 * 1024 } })) {
        console.log(msg.seq);
        processed.push(msg.seq);
    }
    if (processed.length) {
        await client.messageFlagsAdd(processed, ["\\Seen"]); // Freezes without any error/log
    }
// ...

If I take the seq logged to the console and use the solution without fetch:

await client.connect();
let lock = await client.getMailboxLock("INBOX");
try {
    let processed: number[] = [4517];
    if (processed.length) {
        await client.messageFlagsAdd(processed, ["\\Seen"]); // Works perfectly
    }

everything works as expected.

Ubuntu 22.04, imapflow 1.0.119

[PRR24]

Made a mistake while oversimplifying the repro case.
The problem occurs when fetch() is not fully processed, eg either with:

await client.connect();
let lock = await client.getMailboxLock("INBOX");
try {
    let processed: number[] = [];
    for await (let msg of client.fetch({ seen: false }, { source: { maxLength: 2 * 1024 * 1024 } })) {
        console.log(msg.seq);
        processed.push(msg.seq);
        break; // Important for repro
    }
    if (processed.length) {
        await client.messageFlagsAdd(processed, ["\\Seen"]); // Freezes without any error/log
    }
// ...

Or with (the original code I started to track the problem down from):

await client.connect();
let lock = await client.getMailboxLock("INBOX");
try {
    for await (let msg of client.fetch({ seen: false }, { source: { maxLength: 2 * 1024 * 1024 } })) {
        console.log(msg.seq);
        client.messageFlagsAdd({ seq: msg.seq }, ["\\Seen"]); // Freezes without any error/log
    }
// ...

Just for future record, what is the recommended way to safely cancel the fetch loop?

[andris9]

Oh, I guess I haven't documented it well enough (if at all 🤔). You can not run any IMAP command before the previous one has finished. So you can't run messageFlagsAdd inside the fetch loop. This is because you can only process a single IMAP command at a time (can't do anything in parallel in the same session), and the fetch command has not finished yet. For other commands, it's not a big issue, as ImapFlow just pipelines these commands, but the fetch loop is special – it allows you to run your code while the command is still being executed.