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cpio: add FromLayers for multi-layer CPIO archives #2216

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Merged
a-crate merged 1 commit into from
May 12, 2026
+19 −3
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cpio: add FromLayers for multi-layer CPIO archives #2216

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22 changes: 19 additions & 3 deletions pkg/cpio/layer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,24 @@ import (

// FromLayer converts a container layer to CPIO format.
func FromLayer(layer v1.Layer, dest io.Writer) error {
return FromLayers([]v1.Layer{layer}, dest)
}

// FromLayers converts multiple container layers to a single CPIO archive.
func FromLayers(layers []v1.Layer, dest io.Writer) error {
w := cpio.NewDedupWriter(cpio.Newc.Writer(dest))

for _, layer := range layers {
if err := recordsFromLayer(layer, w); err != nil {
return err
}
}

return w.WriteRecord(cpio.TrailerRecord)
}

// recordsFromLayer reads tar entries from a layer and writes them as CPIO records.
func recordsFromLayer(layer v1.Layer, w cpio.RecordWriter) error {
// Open the filesystem layer to walk through the file.
u, err := layer.Uncompressed()
if err != nil {
Expand All @@ -35,8 +53,6 @@ func FromLayer(layer v1.Layer, dest io.Writer) error {

tarReader := tar.NewReader(u)

w := cpio.NewDedupWriter(cpio.Newc.Writer(dest))

// Iterate through the tar archive entries
for {
header, err := tarReader.Next()
Expand Down Expand Up @@ -95,5 +111,5 @@ func FromLayer(layer v1.Layer, dest io.Writer) error {
}
}

return w.WriteRecord(cpio.TrailerRecord)
return nil
}