[release-branch.go1.8] cmd/go: reject update of VCS inside VCS

Cherry-pick of CL 68110.

Change-Id: Iae84c6404ab5eeb6950faa2364f97a017c67c506
Reviewed-on: https://go-review.googlesource.com/68190
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
diff --git a/src/cmd/go/get.go b/src/cmd/go/get.go
index 6fb4235..14da0bd 100644
--- a/src/cmd/go/get.go
+++ b/src/cmd/go/get.go
@@ -439,6 +439,11 @@
 		p.build.PkgRoot = filepath.Join(list[0], "pkg")
 	}
 	root := filepath.Join(p.build.SrcRoot, filepath.FromSlash(rootPath))
+
+	if err := checkNestedVCS(vcs, root, p.build.SrcRoot); err != nil {
+		return err
+	}
+
 	// If we've considered this repository already, don't do it again.
 	if downloadRootCache[root] {
 		return nil
diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index fa78578..00fd746 100644
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -1273,6 +1273,25 @@
 	tg.grepStdout(`\* another-branch`, "not on correct default branch")
 }
 
+func TestAccidentalGitCheckout(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("skipping because git binary not found")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src")
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.runFail("get", "-u", "vcs-test.golang.org/go/test1-svn-git")
+	tg.grepStderr("src[\\\\/]vcs-test.* uses git, but parent .*src[\\\\/]vcs-test.* uses svn", "get did not fail for right reason")
+
+	tg.runFail("get", "-u", "vcs-test.golang.org/go/test2-svn-git/test2main")
+	tg.grepStderr("src[\\\\/]vcs-test.* uses git, but parent .*src[\\\\/]vcs-test.* uses svn", "get did not fail for right reason")
+}
+
 func TestErrorMessageForSyntaxErrorInTestGoFileSaysFAIL(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
diff --git a/src/cmd/go/vcs.go b/src/cmd/go/vcs.go
index fcdce22..2b9f52f 100644
--- a/src/cmd/go/vcs.go
+++ b/src/cmd/go/vcs.go
@@ -497,11 +497,28 @@
 		return nil, "", fmt.Errorf("directory %q is outside source root %q", dir, srcRoot)
 	}
 
+	var vcsRet *vcsCmd
+	var rootRet string
+
 	origDir := dir
 	for len(dir) > len(srcRoot) {
 		for _, vcs := range vcsList {
 			if _, err := os.Stat(filepath.Join(dir, "."+vcs.cmd)); err == nil {
-				return vcs, filepath.ToSlash(dir[len(srcRoot)+1:]), nil
+				root := filepath.ToSlash(dir[len(srcRoot)+1:])
+				// Record first VCS we find, but keep looking,
+				// to detect mistakes like one kind of VCS inside another.
+				if vcsRet == nil {
+					vcsRet = vcs
+					rootRet = root
+					continue
+				}
+				// Allow .git inside .git, which can arise due to submodules.
+				if vcsRet == vcs && vcs.cmd == "git" {
+					continue
+				}
+				// Otherwise, we have one VCS inside a different VCS.
+				return nil, "", fmt.Errorf("directory %q uses %s, but parent %q uses %s",
+					filepath.Join(srcRoot, rootRet), vcsRet.cmd, filepath.Join(srcRoot, root), vcs.cmd)
 			}
 		}
 
@@ -514,9 +531,48 @@
 		dir = ndir
 	}
 
+	if vcsRet != nil {
+		return vcsRet, rootRet, nil
+	}
+
 	return nil, "", fmt.Errorf("directory %q is not using a known version control system", origDir)
 }
 
+// checkNestedVCS checks for an incorrectly-nested VCS-inside-VCS
+// situation for dir, checking parents up until srcRoot.
+func checkNestedVCS(vcs *vcsCmd, dir, srcRoot string) error {
+	if len(dir) <= len(srcRoot) || dir[len(srcRoot)] != filepath.Separator {
+		return fmt.Errorf("directory %q is outside source root %q", dir, srcRoot)
+	}
+
+	otherDir := dir
+	for len(otherDir) > len(srcRoot) {
+		for _, otherVCS := range vcsList {
+			if _, err := os.Stat(filepath.Join(dir, "."+otherVCS.cmd)); err == nil {
+				// Allow expected vcs in original dir.
+				if otherDir == dir && otherVCS == vcs {
+					continue
+				}
+				// Allow .git inside .git, which can arise due to submodules.
+				if otherVCS == vcs && vcs.cmd == "git" {
+					continue
+				}
+				// Otherwise, we have one VCS inside a different VCS.
+				return fmt.Errorf("directory %q uses %s, but parent %q uses %s", dir, vcs.cmd, otherDir, otherVCS.cmd)
+			}
+		}
+		// Move to parent.
+		newDir := filepath.Dir(otherDir)
+		if len(newDir) >= len(otherDir) {
+			// Shouldn't happen, but just in case, stop.
+			break
+		}
+		otherDir = newDir
+	}
+
+	return nil
+}
+
 // repoRoot represents a version control system, a repo, and a root of
 // where to put it on disk.
 type repoRoot struct {