`COPY --from` always breaks cache

[apourchet]

[hatemosphere]

@apourchet could you please share more details on this issue?

[yiranwang52]

Currently cache ID calculation is done before the whole build process. The logic is:

• For FROM, the ID is hash(makisu_version + base_image_sha)
• For ADD/COPY without --from, the ID is hash(parent_id + hash(source_content))
• For COPY with --from, the ID is always random
• For other steps, the ID is hash(parent_id + string(current_line))

So we intentionally break the cache chain whenever we see COPY --from, because at the time of cache ID calculation, we don't know the hash of the files to be copied. The fix could be simple - move the calculation at the beginning of each stage.

[bpaquet]

I have the same problem. Any ETA for a fix ?

[yiranwang52]

We will work on this in Q3.