Refactor and simplify go fetcher code.

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

Benjy Weinberger
2018, 3427, 3439

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, 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,,
and 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.


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.


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 and, and obsoletes, 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.


CI passes after all merges:

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.


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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:


Benjy Weinberger

Submitted! Thanks for the reviews.