cmd/go: make get -u upgrade only modules providing packages

Currently, 'go get -u' upgrades modules matching command line
arguments and any modules they transitively require. 'go get -u' with
no positional arguments upgrades all modules transitively required by
the main module. This usually adds a large number of indirect
requirements, which is surprising to users.

With this change, 'go get' will load packages specified by
its arguments using a similar process to other commands
('go build', etc). Only modules providing packages will be upgraded.

'go get -u' now upgrades modules providing packages transitively
imported by the command-line arguments. 'go get -u' without arguments
will only upgrade modules needed by the package in the current
directory.

'go get -m' will load all packages within a module. 'go get -m -u'
without arguments will upgrade modules needed by the main module. It
is equivalent to 'go get -u all'. Neither command will upgrade modules
that are required but not used.

Note that 'go get -m' and 'go get -d' both download modules in order
to load packages.

Fixes #26902

Change-Id: I2bad686b3ca8c9de985a81fb42b16a36bb4cc3ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/174099
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index 74847a2912..a195b76fa1 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -442,3 +442,13 @@ func (e *packageNotInModuleError) Error() string {
 	}
 	return fmt.Sprintf("module %s@%s%s found, but does not contain package %s", e.mod.Path, e.query, found, e.pattern)
 }
+
+// ModuleHasRootPackage returns whether module m contains a package m.Path.
+func ModuleHasRootPackage(m module.Version) (bool, error) {
+	root, isLocal, err := fetch(m)
+	if err != nil {
+		return false, err
+	}
+	_, ok := dirInModule(m.Path, m.Path, root, isLocal)
+	return ok, nil
+}