cmd/go/internal/modget: synchronize writes to modOnly map in runGet

Fixes #35317

Change-Id: Id858a25dc16a1bbff1802d25bcd4aca31c1133bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/205067
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modget/get.go

@@ -454,6 +454,7 @@ func runGet(cmd *base.Command, args []string) {
 	// This includes explicitly requested modules that don't have a root package
 	// and modules with a target version of "none".
 	var wg sync.WaitGroup
+	var modOnlyMu sync.Mutex
 	modOnly := make(map[string]*query)
 	for _, q := range queries {
 		if q.m.Version == "none" {
@@ -466,7 +467,9 @@ func runGet(cmd *base.Command, args []string) {
 				if hasPkg, err := modload.ModuleHasRootPackage(q.m); err != nil {
 					base.Errorf("go get: %v", err)
 				} else if !hasPkg {
+					modOnlyMu.Lock()
 					modOnly[q.m.Path] = q
+					modOnlyMu.Unlock()
 				}
 				wg.Done()
 			}(q)

src/cmd/go/testdata/script/mod_issue35317.txt

@@ -0,0 +1,8 @@
+# Regression test for golang.org/issue/35317:
+# 'go get' with multiple module-only arguments was racy.
+
+env GO111MODULE=on
+[short] skip
+
+go mod init example.com
+go get golang.org/x/text@v0.3.0 golang.org/x/internal@v0.1.0