Extracting text from a Blob within a transaction panics

[ctm]

I've been using indexed-db for a while to support client-side uploads of assets. It worked great for image files. However, when I tried to support uploading of CSS files, I tried to get the text from the Blob using the Blob::text method and it panicked when I callted text() within the transaction where I was calling get_all. Moving the text() call to outside the transaction works fine, so my code no longer panics, but I assume you want to know about the panic.

I've put together a trunk using small example. The README.md explains a bit more and includes these instructions for running it:

Once it's running, click on the Choose File button to "upload" a file to your Browser (i.e., this is client-side only upload; you're not sending anything to a server). Then—after uploading a file—restart the app by refreshing the page. In the JavaScript console, you should see a Transaction blocked without any request under way panic followed by a back-trace (at least until the indexed-db bug has been fixed).

[Ekleog]

Unfortunately this is not a bug: IndexedDB will auto-commit your transaction if you use .await on anything but IndexedDB futures inside the transaction.

And unfortunately it looks like your code is using it:

JsFuture::from(blob.text()).await

In particular, as the code will return to the event loop, if I were to remove the panic your transaction would already be committed by the time your await completes.

Does that make sense? Unfortunately I don't know of any good solution to that, because this is a limitation of the IndexedDB standard. Panicking seems to be the least bad solution to me, because it's a programming error 🥲

[ctm]

That makes perfect sense, and I had even read about the auto-commit semantics months ago when I first started using IndexedDB functionality, but I'd totally forgotten about it yesterday.

It didn't take me long to figure out that I had to move the text call out of the transaction, but perhaps a wording other than "Transaction blocked without any request under way" might help someone else. Maybe a sentence or two on the indexed-db summary page and then a link to that paragraph in the panic? OTOH, if I'm the only person to have hit this and been surprised, it's unclear that it makes sense to spend any more time on it.

Thanks!

[Ekleog]

I just expanded on the panic message, which I think is the place that makes most sense to definitely fix. You can see the outcome at 92dabd8

For the readme (which gets automatically included in the summary page), do you want to submit a PR to clarify the wording? 😄

[Ekleog]

As for the panic message, it'll be released when I publish the next version, which you can follow here: #17 — I don't think it's worth doing a new release just for it, so it might be released only with 0.5.0 :)