cmd/go: redirect vcs-test.golang.org repo URLs to a test-local server

The new server reconstructs the vcs-test repos on the fly using
scripts that run the actual version-control binaries.

This allows those repos to be code-reviewed using our normal tools —
and, crucially, allows contributors to add new vcs-test contents
as part of a contributed CL.

It also prevents failures due to network errors reaching
vcs-test.golang.org (such as when developing offline), and allows us
to iterate on the repo contents without dealing with annoying and
unpredictable GCS caching behavior.

We can't quite turn down vcs-test.golang.org yet — this server doesn't
yet handle "go-import" metadata (and related authentication behaviors),
and doesn't serve Subversion repos.

But we're getting much closer!

For golang#27494.

Change-Id: I233fc718617aed287b0f7248bd8cfe1e5cebe96b
Reviewed-on: https://go-review.googlesource.com/c/go/+/421455
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills

src/cmd/go/go_test.go

@@ -33,6 +33,8 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/robustio"
 	"cmd/go/internal/search"
+	"cmd/go/internal/vcs"
+	"cmd/go/internal/vcweb/vcstest"
 	"cmd/go/internal/work"
 	"cmd/internal/sys"
 
@@ -129,6 +131,12 @@ func TestMain(m *testing.M) {
 				return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s)", callerPos, filepath.Join("GOROOT", rel))
 			}
 		}
+
+		if vcsTest := os.Getenv("TESTGO_VCSTEST_URL"); vcsTest != "" {
+			vcs.VCSTestRepoURL = vcsTest
+			vcs.VCSTestHosts = vcstest.Hosts
+		}
+
 		cmdgo.Main()
 		os.Exit(0)
 	}

src/cmd/go/internal/get/get.go

@@ -495,21 +495,21 @@ func downloadPackage(p *load.Package) error {
 		vcsCmd, repo, rootPath = rr.VCS, rr.Repo, rr.Root
 	}
 	if !blindRepo && !vcsCmd.IsSecure(repo) && security != web.Insecure {
-		return fmt.Errorf("cannot download, %v uses insecure protocol", repo)
+		return fmt.Errorf("cannot download: %v uses insecure protocol", repo)
 	}
 
 	if p.Internal.Build.SrcRoot == "" {
 		// Package not found. Put in first directory of $GOPATH.
 		list := filepath.SplitList(cfg.BuildContext.GOPATH)
 		if len(list) == 0 {
-			return fmt.Errorf("cannot download, $GOPATH not set. For more details see: 'go help gopath'")
+			return fmt.Errorf("cannot download: $GOPATH not set. For more details see: 'go help gopath'")
 		}
 		// Guard against people setting GOPATH=$GOROOT.
 		if filepath.Clean(list[0]) == filepath.Clean(cfg.GOROOT) {
-			return fmt.Errorf("cannot download, $GOPATH must not be set to $GOROOT. For more details see: 'go help gopath'")
+			return fmt.Errorf("cannot download: $GOPATH must not be set to $GOROOT. For more details see: 'go help gopath'")
 		}
 		if _, err := os.Stat(filepath.Join(list[0], "src/cmd/go/alldocs.go")); err == nil {
-			return fmt.Errorf("cannot download, %s is a GOROOT, not a GOPATH. For more details see: 'go help gopath'", list[0])
+			return fmt.Errorf("cannot download: %s is a GOROOT, not a GOPATH. For more details see: 'go help gopath'", list[0])
 		}
 		p.Internal.Build.Root = list[0]
 		p.Internal.Build.SrcRoot = filepath.Join(list[0], "src")