cmd/go: allow GOPROXY to elide the "https://" prefix

Fixes #32191

Change-Id: I6eebe1d4975e904c906e6b839cd6cab9447cbb34
Reviewed-on: https://go-review.googlesource.com/c/go/+/181019
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modfetch/proxy.go

@@ -12,7 +12,9 @@ import (
 	"io/ioutil"
 	"net/url"
 	"os"
+	"path"
 	pathpkg "path"
+	"path/filepath"
 	"strings"
 	"sync"
 	"time"
@@ -110,6 +112,13 @@ func proxyURLs() ([]string, error) {
 				break
 			}
 
+			// Single-word tokens are reserved for built-in behaviors, and anything
+			// containing the string ":/" or matching an absolute file path must be a
+			// complete URL. For all other paths, implicitly add "https://".
+			if strings.ContainsAny(proxyURL, ".:/") && !strings.Contains(proxyURL, ":/") && !filepath.IsAbs(proxyURL) && !path.IsAbs(proxyURL) {
+				proxyURL = "https://" + proxyURL
+			}
+
 			// Check that newProxyRepo accepts the URL.
 			// It won't do anything with the path.
 			_, err := newProxyRepo(proxyURL, "golang.org/x/text")

src/cmd/go/internal/modfetch/sumdb.go

@@ -145,7 +145,7 @@ func (c *dbClient) initBase() {
 		if proxyURL == "noproxy" {
 			continue
 		}
-		if proxyURL == "direct" {
+		if proxyURL == "direct" || proxyURL == "off" {
 			break
 		}
 		proxy, err := url.Parse(proxyURL)

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

@@ -0,0 +1,19 @@
+env GO111MODULE=on
+
+# GOPROXY file paths must provide the "file://" prefix explicitly.
+env GOPROXY=$WORK/proxydir
+! go list -versions -m golang.org/x/text
+stderr 'invalid proxy URL.*proxydir'
+
+[!net] stop
+
+# GOPROXY HTTPS paths may elide the "https://" prefix.
+# (See golang.org/issue/32191.)
+env GOPROXY=proxy.golang.org
+go list -versions -m golang.org/x/text
+
+-- go.mod --
+module example.com
+go 1.13
+-- $WORK/proxydir/README.md --
+This proxy contains no data.