This change adds an "Origin" JSON key to the output of
go list -json -m and go mod download -json. The associated value is a
JSON object with metadata about the source control system. For Git,
that metadata is sufficient to evaluate whether the remote server has
changed in any interesting way that might invalidate the cached data.
In most cases, it will not have, and a fetch could then avoid
downloading a full repo from the server.

This origin metadata is also now recorded in the .info file for a
given module@version, for informational and debugging purposes.

This change only adds the metadata. It does not use it to optimize
away unnecessary git fetch operations. (That's the next change.)

For #53644.

Change-Id: I4a1712a2386d1d8ab4e02ffdf0f72ba75d556115
Reviewed-on: https://go-review.googlesource.com/c/go/+/411397
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

The go list -m and go mod download commands now have a -reuse flag,
which is passed the name of a file containing the JSON output from a
previous run of the same command. (It is up to the caller to ensure
that flags such as -versions or -retracted, which affect the output,
are consistent between the old and new run.)

The new run uses the old JSON to evaluate whether the answer is
unchanged since the old run. If so, it reuses that information,
avoiding a costly 'git fetch', and sets a new Reuse: true field in its
own JSON output.

This dance with saving the JSON output and passing it back to -reuse
is not necessary on most systems, because the go command caches
version control checkouts in the module cache. That cache means that a
new 'git fetch' would only download the commits that are new since the
previous one (often none at all).

The dance becomes important only on systems that do not preserve the
module cache, for example by running 'go clean -modcache' aggressively
or by running in some environment that starts with an empty file
system.

For #53644.

Change-Id: I447960abf8055f83cc6dbc699a9fde9931130004
Reviewed-on: https://go-review.googlesource.com/c/go/+/411398
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

This change adds an "Origin" JSON key to the output of
go list -json -m and go mod download -json. The associated value is a
JSON object with metadata about the source control system. For Git,
that metadata is sufficient to evaluate whether the remote server has
changed in any interesting way that might invalidate the cached data.
In most cases, it will not have, and a fetch could then avoid
downloading a full repo from the server.

This origin metadata is also now recorded in the .info file for a
given module@version, for informational and debugging purposes.

This change only adds the metadata. It does not use it to optimize
away unnecessary git fetch operations. (That's the next change.)

For golang#53644.

Change-Id: I4a1712a2386d1d8ab4e02ffdf0f72ba75d556115
Reviewed-on: https://go-review.googlesource.com/c/go/+/411397
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

The go list -m and go mod download commands now have a -reuse flag,
which is passed the name of a file containing the JSON output from a
previous run of the same command. (It is up to the caller to ensure
that flags such as -versions or -retracted, which affect the output,
are consistent between the old and new run.)

The new run uses the old JSON to evaluate whether the answer is
unchanged since the old run. If so, it reuses that information,
avoiding a costly 'git fetch', and sets a new Reuse: true field in its
own JSON output.

This dance with saving the JSON output and passing it back to -reuse
is not necessary on most systems, because the go command caches
version control checkouts in the module cache. That cache means that a
new 'git fetch' would only download the commits that are new since the
previous one (often none at all).

The dance becomes important only on systems that do not preserve the
module cache, for example by running 'go clean -modcache' aggressively
or by running in some environment that starts with an empty file
system.

For golang#53644.

Change-Id: I447960abf8055f83cc6dbc699a9fde9931130004
Reviewed-on: https://go-review.googlesource.com/c/go/+/411398
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

CL 411398 added the -reuse flag for reusing cached JSON output
when the remote Git repository has not changed. One case that was
not yet cached is a lookup of a nonexistent version.

This CL adds caching of failed lookups of nonexistent versions,
by saving a checksum of all the heads and tags refs on the remote
server (we never consider other kinds of refs). If none of those have
changed, then we don't need to download the full server.

Fixes golang#53644.

Change-Id: I428bbc8ec8475bd7d03788934d643e1e2be3add0
Reviewed-on: https://go-review.googlesource.com/c/go/+/415678
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>