all: add -mod=vendor to GOFLAGS in tests that execute 'go' commands within std or cmd

Updates #30228
Updates #30240
Updates #30241

Change-Id: Idc311ba77e99909318b5b86f8ef82d4878f73e47
Reviewed-on: https://go-review.googlesource.com/c/go/+/165378
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/src/internal/testenv/testenv.go b/src/internal/testenv/testenv.go
index 8f69fe0da5..72e4d803cb 100644
--- a/src/internal/testenv/testenv.go
+++ b/src/internal/testenv/testenv.go
@@ -19,6 +19,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -77,6 +78,31 @@ func MustHaveGoRun(t testing.TB) {
 	}
 }
 
+var modVendorOnce sync.Once
+
+// SetModVendor adds the "-mod=vendor" flag to the GOFLAGS environment variable.
+// This allows tests whose working directories are within the cmd and std
+// modules to run ``go'' commands without accessing the network to load
+// dependencies modules.
+//
+// SetModVendor must be called before any test may read the GOFLAGS environment
+// variable.
+//
+// TODO(golang.org/issue/30240): If we load go.mod files from vendor/
+// automatically, this will probably no longer be necessary.
+func SetModVendor() {
+	modVendorOnce.Do(func() {
+		var goflags []string
+		for _, f := range strings.Fields(os.Getenv("GOFLAGS")) {
+			if !strings.HasPrefix(f, "-mod=") && !strings.HasPrefix(f, "--mod=") {
+				goflags = append(goflags, f)
+			}
+		}
+		goflags = append(goflags, "-mod=vendor")
+		os.Setenv("GOFLAGS", strings.Join(goflags, " "))
+	})
+}
+
 // GoToolPath reports the path to the Go tool.
 // It is a convenience wrapper around GoTool.
 // If the tool is unavailable GoToolPath calls t.Skip.