cmd/go: add get -f flag

get -u now checks that remote repo paths match the
ones predicted by the import paths: if you are get -u'ing
rsc.io/pdf, it has to be checked out from the right location.
This is important in case the rsc.io/pdf redirect changes.

In some cases, people have good reasons to use
non-standard remote repos. Add -f flag to allow that.
The f can stand for force or fork, as you see fit.

Fixes #8850.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/164120043

src/cmd/go/get.go

@@ -16,7 +16,7 @@ import (
 )
 
 var cmdGet = &Command{
-	UsageLine: "get [-d] [-fix] [-t] [-u] [build flags] [packages]",
+	UsageLine: "get [-d] [-f] [-fix] [-t] [-u] [build flags] [packages]",
 	Short:     "download and install packages and dependencies",
 	Long: `
 Get downloads and installs the packages named by the import paths,
@@ -25,6 +25,11 @@ along with their dependencies.
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.
 
+The -f flag, valid only when -u is set, forces get -u not to verify that
+each package has been checked out from the source control repository
+implied by its import path. This can be useful if the source is a local fork
+of the original.
+
 The -fix flag instructs get to run the fix tool on the downloaded packages
 before resolving dependencies or building the code.
 
@@ -53,6 +58,7 @@ See also: go build, go install, go clean.
 }
 
 var getD = cmdGet.Flag.Bool("d", false, "")
+var getF = cmdGet.Flag.Bool("f", false, "")
 var getT = cmdGet.Flag.Bool("t", false, "")
 var getU = cmdGet.Flag.Bool("u", false, "")
 var getFix = cmdGet.Flag.Bool("fix", false, "")
@@ -63,6 +69,10 @@ func init() {
 }
 
 func runGet(cmd *Command, args []string) {
+	if *getF && !*getU {
+		fatalf("go get: cannot use -f flag without -u")
+	}
+
 	// Phase 1.  Download/update.
 	var stk importStack
 	for _, arg := range downloadPaths(args) {
@@ -268,7 +278,7 @@ func downloadPackage(p *Package) error {
 		repo = "<local>" // should be unused; make distinctive
 
 		// Double-check where it came from.
-		if *getU && vcs.remoteRepo != nil {
+		if *getU && vcs.remoteRepo != nil && !*getF {
 			dir := filepath.Join(p.build.SrcRoot, rootPath)
 			if remote, err := vcs.remoteRepo(vcs, dir); err == nil {
 				if rr, err := repoRootForImportPath(p.ImportPath); err == nil {

src/cmd/go/test.bash

@@ -219,6 +219,16 @@ q' | ed $d/src/$config >/dev/null 2>&1
 			cat $d/err
 			ok=false
 		fi
+		
+		if GOPATH=$d ./testgo get -d -f -u $url 2>$d/err; then
+			echo "go get -d -u $url succeeded with wrong remote repo"
+			cat $d/err
+			ok=false
+		elif ! egrep -i 'validating server certificate|not found' $d/err >/dev/null; then
+			echo "go get -d -f -u $url failed for wrong reason"
+			cat $d/err
+			ok=false
+		fi
 	fi
 	rm -rf $d
 }