util/compression: use zstd.EncoderLevelFromZstd to map compression levels

[thaJeztah]

• relates to exporter: support for compression-level #2591
• possibly relevant upstream changes;
  • Zstd default compression level klauspost/compress#118
  • zstd: Add better compression mode klauspost/compress#240
  • zstd: use SpeedBestCompression for level >= 10 klauspost/compress#410
  • fix logically dead code klauspost/compress#472

The toZstdEncoderLevel utility was introduced in cab33b1, and has a custom mapping of zstd compressions levels to their go (klauspost/compress) counterparts.

Swap our local implementation for the one in klauspost/compress so that new levels implemented in klauspost/compress will also be mapped if they arrive.

This does change some mappings;

Input level	Before	After
< 0	SpeedDefault	SpeedFastest
6	SpeedDefault	SpeedBetterCompression
9	SpeedBestCompression	SpeedBetterCompression

While updating, also slightly refactor;

• use an options slice to skip setting the option if no config was set, and to allow adding additional options.
• inline zstdWriter where used, as it was only used in a single place.

---

Input level	Before (toZstdEncoderLevel)	After (zstd.EncoderLevelFromZstd)	Change
< 0	SpeedDefault	SpeedFastest	⚠️ Different
0–2	SpeedFastest	SpeedFastest	✅ Same
3–5	SpeedDefault	SpeedDefault	✅ Same
6	SpeedDefault	SpeedBetterCompression	⚠️ Different
7–8	SpeedBetterCompression	SpeedBetterCompression	✅ Same
9	SpeedBestCompression	SpeedBetterCompression	⚠️ Different

= 10 | SpeedBestCompression | SpeedBestCompression | ✅ Same

[thaJeztah]

Test failure output looks a bit wonky; for some reason it does print %, but no value?

buildkit/client/build_test.go

Lines 1175 to 1189 in 64bbec8

	func (p *testPrompt) wait(msg string) string {
	for {
	newOutput := p.output.String()[p.pos:]
	if strings.Contains(newOutput, msg) {
	p.pos += len(newOutput)
	return newOutput
	}
	select {
	case <-p.ctx.Done():
	p.t.Logf("Output at timeout: %s", p.output.String())
	p.t.Fatalf("Timeout waiting for %q", msg)
	case <-time.After(100 * time.Millisecond):
	}
	}
	}

=== FAIL: client TestClientGatewayIntegration/TestClientGatewayContainerCancelExecTty/worker=containerd-1.7 (12.95s)
    build_test.go:1184: Output at timeout: % 
    build_test.go:1185: Timeout waiting for "% "
    build_test.go:1355: 
        	Error Trace:	/src/client/build_test.go:1355
        	            				/src/util/testutil/integration/run.go:105
        	            				/src/util/testutil/integration/run.go:253
        	Error:      	An error is expected but got nil.

[thaJeztah]

From discussion on Slack to see if there was anything to match w.r.t. behavior, but looks like containerd at least does not set any options at all, so always uses the defaults; https://github.com/containerd/containerd/blob/d8f13bf4cce8b8a99ae61457cb574696fcf475e5/pkg/archive/compression/compression.go#L244-L256

buildkit/vendor/github.com/containerd/containerd/v2/pkg/archive/compression/compression.go

Lines 243 to 255 in d5307d8

	// CompressStream compresses the dest with specified compression algorithm.
	func CompressStream(dest io.Writer, compression Compression) (io.WriteCloser, error) {
	switch compression {
	case Uncompressed:
	return &writeCloserWrapper{dest, nil}, nil
	case Gzip:
	return gzip.NewWriter(dest), nil
	case Zstd:
	return zstd.NewWriter(dest)
	default:
	return nil, fmt.Errorf("unsupported compression format %s", (&compression).Extension())
	}
	}