cmd/go: fix go get -u handling of changing dependencies

Fixes #9224.

Change-Id: Ie0f4f14407099e4fa7ebe361a95b6492012928a2
Reviewed-on: https://go-review.googlesource.com/12192Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/get.go

@@ -277,9 +277,9 @@ func download(arg string, parent *Package, stk *importStack, getTestDeps bool) {
 		}
 
 		// Process dependencies, now that we know what they are.
-		for _, dep := range p.deps {
+		for _, path := range p.Imports {
 			// Don't get test dependencies recursively.
-			download(dep.ImportPath, p, stk, false)
+			download(path, p, stk, false)
 		}
 		if getTestDeps {
 			// Process test dependencies when -t is specified.

src/cmd/go/go_test.go

@@ -2128,3 +2128,32 @@ func TestGoTestImportErrorStack(t *testing.T) {
 		t.Fatal("did not give full import stack:\n\n%s", tg.stderr.String())
 	}
 }
+
+func TestGoGetUpdate(t *testing.T) {
+	// golang.org/issue/9224.
+	// The recursive updating was trying to walk to
+	// former dependencies, not current ones.
+	testenv.MustHaveExternalNetwork(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+
+	rewind := func() {
+		tg.run("get", "github.com/rsc/go-get-issue-9224-cmd")
+		cmd := exec.Command("git", "reset", "--hard", "HEAD~")
+		cmd.Dir = tg.path("src/github.com/rsc/go-get-issue-9224-lib")
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Fatalf("git: %v\n%s", err, out)
+		}
+	}
+
+	rewind()
+	tg.run("get", "-u", "github.com/rsc/go-get-issue-9224-cmd")
+
+	// Again with -d -u.
+	rewind()
+	tg.run("get", "-d", "-u", "github.com/rsc/go-get-issue-9224-cmd")
+}