cmd/go: add -t flag to 'go get' to download test dependencies

Fixes #5126.

R=golang-dev, dsymonds, bradfitz, r, rsc, rogpeppe
CC=golang-dev
https://golang.org/cl/12566046

src/cmd/go/doc.go

@@ -236,7 +236,7 @@ Download and install packages and dependencies
 
 Usage:
 
-	go get [-d] [-fix] [-u] [build flags] [packages]
+	go get [-d] [-fix] [-t] [-u] [build flags] [packages]
 
 Get downloads and installs the packages named by the import paths,
 along with their dependencies.
@@ -247,6 +247,9 @@ it instructs get not to install the packages.
 The -fix flag instructs get to run the fix tool on the downloaded packages
 before resolving dependencies or building the code.
 
+The -t flag instructs get to also download the packages required to build
+the tests for the specified packages.
+
 The -u flag instructs get to use the network to update the named packages
 and their dependencies.  By default, get uses the network to check out
 missing packages but does not use it to look for updates to existing packages.

src/cmd/go/get.go

@@ -18,7 +18,7 @@ import (
 )
 
 var cmdGet = &Command{
-	UsageLine: "get [-d] [-fix] [-u] [build flags] [packages]",
+	UsageLine: "get [-d] [-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,
@@ -30,6 +30,9 @@ it instructs get not to install the packages.
 The -fix flag instructs get to run the fix tool on the downloaded packages
 before resolving dependencies or building the code.
 
+The -t flag instructs get to also download the packages required to build
+the tests for the specified packages.
+
 The -u flag instructs get to use the network to update the named packages
 and their dependencies.  By default, get uses the network to check out
 missing packages but does not use it to look for updates to existing packages.
@@ -53,6 +56,7 @@ See also: go build, go install, go clean.
 }
 
 var getD = cmdGet.Flag.Bool("d", false, "")
+var getT = cmdGet.Flag.Bool("t", false, "")
 var getU = cmdGet.Flag.Bool("u", false, "")
 var getFix = cmdGet.Flag.Bool("fix", false, "")
 
@@ -65,7 +69,7 @@ func runGet(cmd *Command, args []string) {
 	// Phase 1.  Download/update.
 	var stk importStack
 	for _, arg := range downloadPaths(args) {
-		download(arg, &stk)
+		download(arg, &stk, *getT)
 	}
 	exitIfErrors()
 
@@ -137,7 +141,7 @@ var downloadRootCache = map[string]bool{}
 
 // download runs the download half of the get command
 // for the package named by the argument.
-func download(arg string, stk *importStack) {
+func download(arg string, stk *importStack, getTestDeps bool) {
 	p := loadPackage(arg, stk)
 
 	// There's nothing to do if this is a package in the standard library.
@@ -216,7 +220,18 @@ func download(arg string, stk *importStack) {
 
 		// Process dependencies, now that we know what they are.
 		for _, dep := range p.deps {
-			download(dep.ImportPath, stk)
+			// Don't get test dependencies recursively.
+			download(dep.ImportPath, stk, false)
+		}
+		if getTestDeps {
+			// Process test dependencies when -t is specified.
+			// (Don't get test dependencies for test dependencies.)
+			for _, path := range p.TestImports {
+				download(path, stk, false)
+			}
+			for _, path := range p.XTestImports {
+				download(path, stk, false)
+			}
 		}
 	}
 }