cmd/go: add local module proxy to make tests faster

Because "go get" with modules enabled allows use of a proxy,
we can run a proxy in the test and serve modules from local files,
which makes the tests not depend on remote network servers
and run significantly faster, speeding development.

The proxy serves modules from the testdata/mod directory,
which holds one txtar archive per module. See testdata/mod/README.

Reduces time for 'go test -run=^TestMod' from 115s to 11.9s.
Still longer than I would like but significantly better.

Change-Id: I1b8367b208a02549a44e91e4ea5c5fb9003123ae
Reviewed-on: https://go-review.googlesource.com/123361
Reviewed-by: Bryan C. Mills <[email protected]>

Author: rsc

vendor/cmd/go/go_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"debug/elf"
 	"debug/macho"
+	"flag"
 	"fmt"
 	"go/format"
 	"internal/race"
@@ -112,6 +113,12 @@ func TestMain(m *testing.M) {
 	}
 	os.Unsetenv("GOROOT_FINAL")
 
+	flag.Parse()
+	if *proxyAddr != "" {
+		StartProxy()
+		select {}
+	}
+
 	if canRun {
 		args := []string{"build", "-tags", "testgo", "-o", "testgo" + exeSuffix, "../../.."}
 		if race.Enabled {
@@ -417,7 +424,8 @@ func (tg *testgoData) doRun(args []string) error {
 func (tg *testgoData) run(args ...string) {
 	tg.t.Helper()
 	if status := tg.doRun(args); status != nil {
-		tg.t.Logf("go %v failed unexpectedly: %v", args, status)
+		wd, _ := os.Getwd()
+		tg.t.Logf("go %v failed unexpectedly in %s: %v", args, wd, status)
 		tg.t.FailNow()
 	}
 }
@@ -760,16 +768,28 @@ func (tg *testgoData) wantNotStale(pkg, reason, msg string) {
 	}
 }
 
+// If -testwork is specified, the test prints the name of the temp directory
+// and does not remove it when done, so that a programmer can
+// poke at the test file tree afterward.
+var testWork = flag.Bool("testwork", false, "")
+
 // cleanup cleans up a test that runs testgo.
 func (tg *testgoData) cleanup() {
 	tg.t.Helper()
 	if tg.wd != "" {
+		wd, _ := os.Getwd()
+		tg.t.Logf("ended in %s", wd)
+
 		if err := os.Chdir(tg.wd); err != nil {
 			// We are unlikely to be able to continue.
 			fmt.Fprintln(os.Stderr, "could not restore working directory, crashing:", err)
 			os.Exit(2)
 		}
 	}
+	if *testWork {
+		tg.t.Logf("TESTWORK=%s\n", tg.path("."))
+		return
+	}
 	for _, path := range tg.temps {
 		tg.check(removeAll(path))
 	}