using Pages Router with basePath set in next.config.js results in 404 error when making <basePath>/_next/data/<BUILD_ID>/<path>.json requests

[chiakaiming]

Hello!

I'm consistently facing a 404 error on <basePath>/_next/data/<BUILD_ID>/<path>.json requests.

The issue arises when providing a value for basePath in next.config.js (tested on a project using pages router).

Looking at the matchRoute function in packages/open-next/src/core/routing/routeMatcher.ts, the regex array used to match a path is derived from optionalBasepathPrefixRegex which prepends the basePath to the path regex. This results is a regex pattern that looks something this:

/^\/basePath\/?(?:)?path(?:\/)?$/

In the function fixDataPage in packages/open-next/src/core/routing/matcher.ts, we are removing the dataPattern (<basePath>/_next/data/<BUILD_ID>) from rawPath which includes the basePath resulting in a transformed rawPath of just /path.

When matchRoute is eventually called, it will try to match the above regex pattern with /path which will fail, resulting in a 404 response return for the data route.

When newPath is initialised in fixDataPage, can we prepend it with ${NextConfig.basePath ?? ""} so that the transformed rawPath would be basePath/path?

Thank you!

PS: Currently, when handling a route, fixDataPage outputs /path for both .../basePath/.../path and .../path requests which I don't think is the intended behaviour?