How should devices which support multiple LANGIDs for string descriptors be handled?

[ArcaneNibble]

USB theoretically supports localization of device string descriptors (although I have never actually seen this implemented in practice). However, in the places where the WebUSB specification refers to string descriptors, it does not specify how a LANGID should be chosen.

This is especially relevant to the serialNumber string, since that string is used in permission checks.

[kevinmehall]

The practical answer is that it should return whichever string the OS has cached so it doesn't have to perform its own IO to retrieve it. On Linux, that's the first supported language ID, with a fallback to 0x0409 (US English), and I'd assume the other platforms are similar, but doesn't seem to be documented.

[ArcaneNibble]

Right, but that's an optimization and not normative behavior. And "try to imitate what the OS does" isn't a very principled way to design a specification. Not to mention that most OSes don't cache all the strings that WebUSB exposes, primarily the strings corresponding to non-active configurations and interface alternate settings.

I also just noticed that the serialNumber attribute is only specified as "SHOULD contain" (with error handling unspecified), which really doesn't feel safe for something that participates in security and permissions concerns.

[reillyeon]

We should probably define an algorithm for fetching a string which boils down to returning the first of:

1. What the OS has cached.
2. The string in the user's configured locale.
3. The string in the en_US locale.

Steps 2 and 3 are designed to mimic what the OS would normally do to populate the cache. The preference for the OS cache is because it'll be faster, less disruptive to the device, and be consistent with any OS UI surfaces which also refer to the device.

Right now what Chromium does for step 2 is actually pick the first language ID returned by the device and only fall back to requesting English if the device doesn't respond to the request for a language list. It would be good to check what Linux and Windows do as well. I suspect Linux's logic is similar to Chromium's since the kernel doesn't (IIRC) have a locale setting while Windows might be cleverer.