0783f0762d
Prior to this commit, Spring MVC and Spring WebFlux would not support conditional requests with `If-Match` preconditions. As underlined in the RFC9110 Section 13.1, those are related to the `If-None-Match` conditions, but this time only performing requests if the resource matches the given ETag. This feature, and in general the `"*"` request Etag, are generally useful to prevent "lost updates" when performing a POST/PUT request: we want to ensure that we're updating a version with a known version or create a new resource only if it doesn't exist already. This commit adds `If-Match` conditional requests support and ensures that both `If-Match` and `If-None-Match` work well with `"*"` request ETags. We can't rely on `checkNotModified(null)`, as the compiler can't decide between method variants accepting an ETag `String` or a Last Modified `long`. Instead, developers should use empty ETags `""` to signal that no resource is known on the server side. Closes gh-24881
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