cmd/go/internal/vcs: error out if the requested repo does not support a secure protocol

Fixes golang#63845.

Change-Id: If86d6b13d3b55877b35c087112bd76388c9404b8
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/539321
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

src/cmd/go/internal/vcs/vcs.go

@@ -1171,18 +1171,31 @@ func repoRootFromVCSPaths(importPath string, security web.SecurityMode, vcsPaths
 			var ok bool
 			repoURL, ok = interceptVCSTest(repo, vcs, security)
 			if !ok {
-				scheme := vcs.Scheme[0] // default to first scheme
-				if vcs.PingCmd != "" {
-					// If we know how to test schemes, scan to find one.
+				scheme, err := func() (string, error) {
 					for _, s := range vcs.Scheme {
 						if security == web.SecureOnly && !vcs.isSecureScheme(s) {
 							continue
 						}
-						if vcs.Ping(s, repo) == nil {
-							scheme = s
-							break
+
+						// If we know how to ping URL schemes for this VCS,
+						// check that this repo works.
+						// Otherwise, default to the first scheme
+						// that meets the requested security level.
+						if vcs.PingCmd == "" {
+							return s, nil
+						}
+						if err := vcs.Ping(s, repo); err == nil {
+							return s, nil
 						}
 					}
+					securityFrag := ""
+					if security == web.SecureOnly {
+						securityFrag = "secure "
+					}
+					return "", fmt.Errorf("no %sprotocol found for repository", securityFrag)
+				}()
+				if err != nil {
+					return nil, err
 				}
 				repoURL = scheme + "://" + repo
 			}

src/cmd/go/testdata/script/mod_insecure_issue63845.txt

@@ -0,0 +1,28 @@
+# Regression test for https://go.dev/issue/63845:
+# If 'git ls-remote' fails for all secure protocols,
+# we should fail instead of falling back to an arbitrary protocol.
+#
+# Note that this test does not use the local vcweb test server
+# (vcs-test.golang.org), because the hook for redirecting to that
+# server bypasses the "ping to determine protocol" logic
+# in cmd/go/internal/vcs.
+
+[!net:golang.org] skip
+[!git] skip
+[short] skip 'tries to access a nonexistent external Git repo'
+
+env GOPRIVATE=golang.org
+env CURLOPT_TIMEOUT_MS=100
+env GIT_SSH_COMMAND=false
+
+! go get -x golang.org/nonexist.git@latest
+stderr '^git ls-remote https://golang.org/nonexist$'
+stderr '^git ls-remote git\+ssh://golang.org/nonexist'
+stderr '^git ls-remote ssh://golang.org/nonexist$'
+! stderr 'git://'
+stderr '^go: golang.org/nonexist.git@latest: no secure protocol found for repository$'
+
+-- go.mod --
+module example
+
+go 1.19