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Document the semantics of the metadata created by pkg/compression #2605

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Oct 24, 2024
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Document the semantics of the metadata created by pkg/compression #2605

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14 changes: 12 additions & 2 deletions pkg/compression/compression.go
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Original file line number Diff line number Diff line change
Expand Up @@ -99,8 +99,18 @@ func CompressStream(dest io.Writer, algo Algorithm, level *int) (io.WriteCloser,
return internal.AlgorithmCompressor(algo)(dest, m, level)
}

// CompressStreamWithMetadata returns the compressor by its name. If the compression
// generates any metadata, it is written to the provided metadata map.
// CompressStreamWithMetadata returns the compressor by its name.
//
// Compressing a stream may create integrity data that allows consuming the compressed byte stream
// while only using subsets of the compressed data (if the compressed data is seekable and most
// of the uncompressed data is already present via other means), while still protecting integrity
// of the compressed stream against unwanted modification. (In OCI container images, this metadata
// is usually carried in manifest annotations.)
//
// Such a partial decompression is not implemented by this package; it is consumed e.g. by
// github.com/containers/storage/pkg/chunked .
//
// If the compression generates such metadata, it is written to the provided metadata map.
func CompressStreamWithMetadata(dest io.Writer, metadata map[string]string, algo Algorithm, level *int) (io.WriteCloser, error) {
return internal.AlgorithmCompressor(algo)(dest, metadata, level)
}
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9 changes: 9 additions & 0 deletions pkg/compression/internal/types.go
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Expand Up @@ -3,6 +3,15 @@ package internal
import "io"

// CompressorFunc writes the compressed stream to the given writer using the specified compression level.
//
// Compressing a stream may create integrity data that allows consuming the compressed byte stream
// while only using subsets of the compressed data (if the compressed data is seekable and most
// of the uncompressed data is already present via other means), while still protecting integrity
// of the compressed stream against unwanted modification. (In OCI container images, this metadata
// is usually carried in manifest annotations.)
//
// If the compression generates such metadata, it is written to the provided metadata map.
//
// The caller must call Close() on the stream (even if the input stream does not need closing!).
type CompressorFunc func(io.Writer, map[string]string, *int) (io.WriteCloser, error)

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