Refactor and simplify go fetcher code.

Review Request #3902 - Created May 17, 2016 and submitted

Information
Benjy Weinberger
pants
2018, 3427, 3439
Reviewers
pants-reviews
jsirois

This is not a small change... It gets rid of a lot of complex logic
that, with hindsight, doesn't appear to be necessary. It replaces
that complexity with a more simple approach.

This mirrors the direction that Go itself is moving in: Go's heuristics
for fetching remote deps (including the meta tag protocol) are becoming
increasingly standard. In fact, there are comments in Go's codebase(1)
to the effect that they encourage code-hosting sites to support the
meta tag protocol so that Go can remove its own hard-coded special cases.

Fetcher Types

Under this change there are two Fetcher types: The existing ArchiveFetcher,
and a new CloningFetcher.

ArchiveFetcher handles special cases where we know how to map an import
path to a tarball path. This is useful for sites like github.com, which
do not currently support the meta tag protocol, and also for diverting
fetches to an internal artifactory, for repos that wish to do so.

CloningFetcher implements (a useful subset of) the standard Go heuristics:
It checks meta tags, and then clones the remote repo and sets its state
to the specified rev (currently this only works for git).

All remote fetches that don't map to an ArchiveFetcher will use the
CloningFetcher. This makes it trivial to use standard git-based remote
repos without any extra config. In particular, gopkg.in, golang.org/x
and google.golang.org now support the meta tag protocol, so there is
no need to special-case them.

The Fetcher class API has also changed a bit. Now a Fetcher encapsulates
the import_path its fetching, so a new Fetcher instance is created for
each fetch operation. The Fetcher classes use Subsystems, but are no longer
themselves subsystems.

Subsystems

For separation of concerns, this commit introduces two utility subsystems
that do what their names imply: GoImportMetaTagReader and ArchiveRetriever.
The fetchers use these. Note that they are not currently unit-tested,
because by the time you mock out the network stuff, there's not that
much left. However they are indirectly tested via several other unit and
integration tests.

FetcherFactory

This change gets rid of the fetcher advertisement+registration mechanism,
which with hindsight seems like overkill, given that we expect that the
current two Fetcher classes are all we're likely to need for the forseeable
future. The Fetchers class is gone. In its stead is much simpler
FetcherFactory subsystem. It now encapsulates the matcher logic, and uses
that to choose a fetcher.

This resolves https://github.com/pantsbuild/pants/issues/3439 and
https://github.com/pantsbuild/pants/issues/3427, and obsoletes
https://github.com/pantsbuild/pants/issues/2018, as we're now
going in a different, simpler, direction.

I can't add dbentley and yujie here because they aren't reviewers on RB, but
I will ask for their review feedback via email.

(1) https://github.com/golang/go/blob/7bc40ffb05d8813bf9b41a331b45d37216f9e747/src/cmd/go/vcs.go#L874

CI passes after all merges: http://jenkins.pantsbuild.org/job/pantsbuild/job/pants/branch/PR-3458/

Added various remote deps to one of the example targets and verified manually that it buildgens and compiles. Also added it to the integration test.

Issues

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Description From Last Updated
Daniel Bentley
Yujie Chen
Daniel Bentley
Yujie Chen
John Sirois
John Sirois
John Sirois
Benjy Weinberger
Benjy Weinberger
Review request changed

Status: Closed (submitted)

Change Summary:

aa9b358e09f1f0157260777c29b3598e0cd9bb70

Benjy Weinberger

Submitted! Thanks for the reviews.

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