- Closes#23328
- Related to #19914
- Related to #22319
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [x] Errors have helpful link attached, see `contributing.md`
This PR changes the implementation of `placeholder=blur` when using `next dev` so that it lazy loads on-demand.
This will improve the developer experience for web apps with many blurred images.
This pull request adds "Vary: Accept" header to responses from the image optimizer (i.e. the /_next/image endpoint).
The image optimizer prefers re-encoding JPG files to WebP, but some browsers (such as Safari 14 on Catalina) do not yet support WebP. In such cases the optimizer uses the Accept header sent by the browser to send out a JPG response. Thus the optimizer's response may depend on the Accept header.
Potential caching proxies can be informed of this fact by adding "Vary: Accept" to the response headers. Otherwise WebP data may be served to browsers that do not support it, for example in the following scenario:
* A browser that supports WebP requests the JPG. The optimizer re-encodes it to WebP. The proxy caches the WebP data.
* After this another browser that doesn't support WebP requests the JPG. The proxy sends the WebP data to the browser.
- [x] Integration tests added
- [x] Make sure the linting passes
This solves the main use case from Issue #19914.
Previously, we would set the `Cache-Control` header to a constant and rely on the server cache. This would mean the browser would always request the image and the server could response with 304 Not Modified to omit the response body.
This PR changes the behavior such that the `max-age` will propagate from the upstream server to the Next.js Image Optimization Server and allow browser caching. ("upstream" meaning external server or just an internal route to an image)
This PR does not change the `max-age` for static imports which will remain `public, max-age=315360000, immutable`.
#### Pros:
- Fewer HTTP requests after initial browser visit
- User configurable `max-age` via the upstream image `Cache-Control` header
#### Cons:
- ~~Might be annoying for `next dev` when modifying a source image~~ (solved: use `max-age=0` for dev)
- Might cause browser to cache longer than expected (up to 2x longer than the server cache if requested in the last second before expiration)
## Bug
- [x] Related issues linked using `fixes #number`
