mutate.Time should be lazy

[imjasonh]

pkg/v1/mutate.Time sets all timestamps in an image to the given time, typically to make the image more reproducible. For timestamps in layers, it calls layerTime which reads the layer contents and writes them to a buffered tar.Writer. This means that if the layer is huge, we may run out of memory buffering its contents.

We should consider making this a lazy transformation, so that layer mutations aren't made until they're read. We did a similar thing in pkg/v1/cache, where layer contents aren't cached until they're read via Compressed or Uncompressed.

[jonjohnsonjr]

At one point I experimented with this. The API was gross due to streaming layers, but that was so long ago there might be a less naive way to do it: jonjohnsonjr@542d74b

[imjasonh]

Yeah I like this Mutation concept. It doesn't help that much with mutate.Time though since that wants to deal with files in the tar archive, and that's still a huge pain to extract from the v1.Image, especially streamily.

Maybe we could expose some adapter that takes a func(tar.Header) (tar.Header, error)?

[jonjohnsonjr]

For the layerTime, I ended up having a ReadCloser -> ReadCloser: jonjohnsonjr@542d74b#diff-05a233d3122c90ba0cf547c623e3a34194cf56c4ff05cdf414d5a265300b31e6R39

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