### Issue
When the og module is a shared module being imported in both page and metadata image routes, it will be shared in the module graph. Especially in the edge runtime, since the `default` export is being used in the metadata image routes, then it can't be easily tree-shaked out.
### Solution
Separate the image route to a separate layer which won't share modules with the page, so that image route is always bundling separately and the `@vercel/og` module only stays inside in that layer, when import image metadata named exports (size / alt / etc..) it can be still tree shaked.
Co-authored-by: Jiachi Liu <4800338+huozhi@users.noreply.github.com>
This PR fixes tree-shaking for the metadata image generation module
(e.g. `opengraph-image.js` and other conventions) when the page has
`runtime = 'edge'`.
## Details
The first step of this fix is to change this from the loader:
```js
import * as exported from "./opengraph-image.js"
```
to be necessary fields only (so the `default` export can potentially be
removed):
```js
import { alt, size } from "./opengraph-image.js"
```
To know which fields are exported, we need to load the module first via
Webpack loader's `loadModule` API and check its
`HarmonyExportSpecifierDependency` dependencies.
This is the first step to make it tree-shakable. Since we have
`./opengraph-image.js` used in another entry, the actual image API route
`opengraph-image/route.js`:
```js
import * as image from "./opengraph-image.js"
```
Webpack still treats both as the same module and generates one chunk for
it. We want to "fork" it into two modules. The technique here is to add
a noop resource query and make it:
```js
import { alt, size } from "./opengraph-image.js?__next_metadata_image_meta__"
```
So it won't be shared in the chunk (as it's a different request), and
can be concatenated inline.
However that's not enough, the inlined result will still have all
imports from our `opengraph-image.js`, including `import { ImageResponse
} from 'next/server'`. Because we can't simply add `"sideEffects":
false` in Next.js' package.json, we need a way to mark this import as
side-effect free. I went through
https://github.com/webpack/webpack/blob/main/lib/optimize/SideEffectsFlagPlugin.js
and used the same method to mark that module with
`module.factoryMeta.sideEffectFree = true`.
With all these added, the page bundle will no longer contain the
`ImageResponse` instance.
## Result
The difference is quite amazing, for the new added test (an empty Edge
runtime page with an opengrah image file) here're the before/after
metrics for the `page.js` server bundle:
Edge bundle size: 892kB → 500kB.
Build time: 26.792s → 8.830s.
