create pass-through writer and iowriter

[deitch]

As discussed with @SteveLasker , this is the first of 2-3 PRs expanding the capabilities of the store package.

There are two additions here:

• PassthroughWriter that lets you wrap any other underlying store (e.g. file, memory) and have a processor function in the middle. Thus if I wanted to decrypt the content in transit, or uncompress, or any other action, I have a simple framework for doing so.
• IoWriter is very similar to FileStore, except instead of taking a directory and opening files, it takes an actual io.Writer and passes it through. I have used it to, e.g. write directly to block devices, and to write to another stream over a network. It is more fine-grained control as you can pass it any io.Writer you want.

Once these are in, I have two more that take advantage of the PassthroughWriter: GunzipWriter and UntarWriter.

[deitch]

Thanks for rebasing on master @jdolitsky . Any other comments on it?

[jdolitsky]

@deitch code lgtm - can we add a few unit tests for these? Or I suppose they can be added w/ the Gunzip/Untar writers?

[deitch]

Yeah, that is a good idea. At least for the io.Writer one. I will find some time to do it.

[deitch]

A bit odd to do, but I think this works for the test.

[jdolitsky]

@deitch can you check failing CI?

[deitch]

@jdolitsky I ran it multiple times locally, I cannot see the failure. Could be transient. I am repushing to kick off again

[deitch]

Nope that failed too. The failure doesn't appear to have anything to do with this PR, but I am not sure.

--- FAIL: TestORASTestSuite (0.27s)
    --- FAIL: TestORASTestSuite/Test_0_Push (0.09s)
    --- FAIL: TestORASTestSuite/Test_1_Pull (0.06s)
    --- PASS: TestORASTestSuite/Test_2_MediaType (0.05s)
    --- PASS: TestORASTestSuite/Test_3_Conditional_Pull (0.05s)

Any ideas? Here is the output https://github.com/deislabs/oras/pull/182/checks?check_run_id=1210937435

[deitch]

What is wrong with the CI run @jdolitsky ?

[deitch]

@jdolitsky I rebased onto master, but also took advantage to simplify the interface, which ended up simplifying the code and blocksize stuff materially, while hewing much more closely to the classic golang style. Take a look, and let's get this in? It has been open for a good few weeks.

[deitch]

And CI is all clean. Even better.

[deitch]

Shoot. It deleted all my changes! I have to recreate them!

[deitch]

Phew. Managed to recover it all. Back at you @jdolitsky

[deitch]

And CI is green.

[jdolitsky]

lgtm!

[deitch]

Excellent.

I have gunzip and untar writers ready to go based on this.