Don't recreate cached layers

[eyusupov]

This caching optimization reuses existing layer from a cache image for a RUN command instead of unpacking it and recreating.
It also helps with multi-stage build caching issues like this #575. The issue is that when recreating cached layers, some attributes like timestamps are not recreated and the digest for the new first stage image was different, breaking caching for subsequent stages.

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[eyusupov]

There is an integration test failing for CMD command due to ArgsEscaped flag in the metadata having incorrect value. It doesn't seem related to my changes and it seems to be failing on master locally for me too. Am I missing something or is it failing for others too?
Here's the output I'm getting on master (I commented out all tests other than Dockerfile_test_cmd fwiw)

--- FAIL: TestRun (0.00s)
    --- FAIL: TestRun/test_Dockerfile_test_cmd (0.92s)
        integration_test.go:224: diff = [
              {
                "Image1": "172.17.0.2:5000/docker-dockerfile_test_cmd",
                "Image2": "172.17.0.2:5000/kaniko-dockerfile_test_cmd",
                "DiffType": "File",
                "Diff": {
                  "Adds": null,
                  "Adds": null,
                  "Dels": null,
                  "Mods": null
                }
              },
              {
                "Image1": "172.17.0.2:5000/docker-dockerfile_test_cmd",
                "Image2": "172.17.0.2:5000/kaniko-dockerfile_test_cmd",
                "DiffType": "Metadata",
                "Diff": {
                  "Adds": [
                    "ArgsEscaped: true"
                  ],
                  "Dels": [
                    "ArgsEscaped: false"
                  ]
                }
              }
            ]
        integration_test.go:227: json: cannot unmarshal string into Go struct field .Adds of type integration.fileDiff
        integration_test.go:227: []integration.diffOutput differ (-got, +want): {[]integration.diffOutput}[1].Diff.Adds[0->?]:
                -: integration.fileDiff{}
                +: <non-existent>
            {[]integration.diffOutput}[1].Diff.Dels[0->?]:
                -: integration.fileDiff{}
                +: <non-existent>

[tejal29]

@eyusupov Thanks for the PR. Can you please add some tests?

[eyusupov]

Will do soon!

[tejal29]

@eyusupov This looks great! Please add tests and we will try to get this in!

[eyusupov]

Sorry for not coming back to it sooner, will try to do it as soon as I can

…

On Fri, 27 Sep 2019, 23:17 Tejal Desai, ***@***.***> wrote: @eyusupov <https://github.com/eyusupov> This looks great! Please add tests and we will try to get this in! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#715?email_source=notifications&email_token=AABKKDV3CKPIPHLVD77K2K3QLZS4HA5CNFSM4IECYDFKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD7Z7PQY#issuecomment-536082371>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AABKKDQ4AD4WAVAIK46UMMDQLZS4HANCNFSM4IECYDFA> .

[strax]

@eyusupov Is there anything I could do to help with this? We too are affected by #575 and it would be great to get these changes shipped.

[raijinsetsu]

This is huge for my team. The step of creating the new archive from exactly the same content as the prior archive is causing a significant slow-down in our build pipeline due to the number of files (this is a LARGE NodeJS project).

[cvgw]

I'm going to close this. #882 will be merged in favor