add getBlobStore method

[mawinter69]

For openstack the right BlobStore depends on the region. So it should be the
task of the provider to get the right BlobStore.
BlobStoreProvider is currently working with BlobStoreContext. But at all
places the context is just used to call the getBlobStore() method.

[jglick]

Is there an OpenStack implementation that would be downstream of this PR?

[mawinter69]

yes, I'm working on an implementation for openstack. Though I would prefer having it in a separate plugin. But that requires #83 first.

[jglick]

Looks harmless. Awaiting downstream PR demonstrating usage. (See JEP-305 instructions.)

[jglick]

Better to revert changes to this file.

[jglick]

Ditto.

[jglick]

#83 is not likely to happen.

Assuming #96 lands, can https://docs.openstack.org/newton/config-reference/object-storage/configure-s3.html be used to access storage via the S3 emulation layer without code changes, or are there unimplemented features that this plugin relies on such as presigned URLs?

[mawinter69]

In order to access swift you have to use signed urls, so I guess it can't be used.

[jglick]

Is there an OSS repo where you are prototyping the downstream implementation?

[jglick]

This could perhaps be made nonabstract (throwing an AbstractMethodError at runtime if neither method is overridden, to avoid a StackOverflowError), since in the other direction there is a BlobStore.getContext() (used in this plugin only to close the context for tests…which is likely a bug).

[mawinter69]

I don't see where BlobStore.getContext() is used. In the test BlobStoreProvider.getContext() is used and on this context the close() is called.
I think it should stay abstract.

[jglick]

I don't see where BlobStore.getContext() is used.

Sorry, to clarify: I think we could write something along the lines of:

public /* final */ BlobStoreContext getContext() throws IOException {
    if (Util.isOverridden(BlobStoreProvider.class, getClass(), "getBlobStore")) {
        return getBlobStore().getContext();
    } else {
        throw new AbstractMethodError("override getBlobStore");
    }
}
public /* abstract */ BlobStore getBlobStore() throws IOException {
    return getContext().getBlobStore();
}

where the modifiers could be uncommented and the code simplified if we did not care to retain backward compatibility for hypothetical external implementations (none are known).

[mawinter69]

ok, this might work as well, when speaking of not retaining backward compatibility one could also consider removing the getContext() method completely.
The getBlobStore should always be abstract I think.

[jglick]

when speaking of not retaining backward compatibility one could also consider removing the getContext() method completely

Yes, you could just inline it in that case.

The getBlobStore should always be abstract

Then calls to this method against a (hypothetical) existing external implementation, which overrode/implemented getContext, would fail with an AbstractMethodError.

The above paired method idiom is common in Jenkins sources, by the way, when we change our minds about what methods an implementation of some interface (or abstract class) should define. Somewhat cleaner is to just deprecate the whole “interface” and introduce, say, a BlobStoreProvider2 while having callers (anything enumerating this ExtensionList) convert from the old implementation.

Again, any of these tricks may just be overkill—the only other implementation of JEP-202 I know about is azure-artifact-manager, which uses the Azure APIs directly rather than via this plugin and jclouds.

[jglick]

For openstack the right BlobStore depends on the region.

Is this something that could be configured more generically? I see that there is a RegionScopedSwiftBlobStore and RegionScopedBlobStoreContext but I am not clear on how they are used.

[mawinter69]

Opened another pull request (#111) that implements openstack swift

[jglick]

Given #111, I do not think this PR is still relevant.