[mauriciopoppe]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

In kube-apiserver, pkg/handler3 caches the marshaled JSON bytes of each accessed GroupVersion's OpenAPI V3 spec in openAPIV3Group.jsonCache for the lifetime of the server (6.85 MB of retained heap for just 3 GroupVersions, and ~18+ MB across all built-in GroupVersions).

This PR stores gzip-compressed JSON bytes in jsonCache without changing ETag hashes, discovery URLs, or response payloads:

1. Compress cached JSON in openAPIV3Group.jsonCache (pkg/handler3/handler.go and newOpenAPIV3Group): Computes computeETag(jsonBytes) on the uncompressed JSON first, then stores the gzip-compressed slice (bytes.Clone(buf.Bytes())) in timedSpec.spec using sync.Pool (gzipWriterPool and gzipReaderPool) to reuse gzip.Writer and gzip.Reader state machines without lock contention. Only readers that complete io.ReadAll and Close without error are returned to gzipReaderPool.
2. Serve pre-compressed bytes on Accept-Encoding: gzip and lazy-decompress via lazyGzipReadSeeker (pkg/handler3/handler.go and HandleGroupVersion): HandleGroupVersion sets Vary: Accept-Encoding on JSON responses so HTTP caches and reverse proxies distinguish compressed and uncompressed representations. When acceptsGzip(r) is true (iterating all r.Header.Values("Accept-Encoding") header lines per RFC 9110 and verifying no Range header is present), HandleGroupVersion sets Content-Encoding: gzip and serves timedSpec.spec directly. Plain JSON requests pass &lazyGzipReadSeeker{compressed: data} to http.ServeContent, delaying decompression until Seek or Read is invoked after http.ServeContent evaluates If-None-Match and If-Modified-Since preconditions (304 Not Modified responses perform zero gzip decompression).

Special notes for your reviewer:

1. pkg/handler3 Benchmarks (go test -bench=. -benchmem -count=5 ./pkg/handler3/...)

Benchmark	Metric	Before (e2e80c32a35f)	After (This PR)	Delta
BenchmarkHandler3CachedSpecMemory	Retained cached-bytes (len)	604,218 B	16,386 B	-97.3% (36.9x smaller)
BenchmarkHandler3CachedSpecMemory	Retained cached-cap-bytes (cap)	622,592 B	18,432 B	-97.0% (33.8x smaller)
BenchmarkServeGroupVersionGzip	Serve Latency (ns/op)	263,455 ns/op	17,590 ns/op	-93.3% (15.0x faster)
BenchmarkServeGroupVersionGzip	Serve Bytes (B/op)	526,292 B/op	11,784 B/op	-97.8% (-514.5 KB/op)

2. kube-apiserver Heap Impact (pprof inuse_space via TestAPIServerMemoryIntegration, 3 GroupVersions Queried)

Metric / pprof Symbol (inuse_space)	Before	After	Delta
bytes.Clone (from handler3 json.Marshal)	6,853.25 kB	< 218 kB	-6.67 MB (-96.8%)
Net kube-apiserver Live Heap Impact	6,853.25 kB	~1,481 kB	-5.51 MB net live heap

Does this PR introduce a user-facing change?

NONE

[jpbetz]

/assign @seans3 @Jefftree

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mauriciopoppe
Once this PR has been reviewed and has the lgtm label, please ask for approval from jefftree. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment