cmd/go: in TestScript, set $GOEXE instead of $exe

$GOEXE exists and is documented in 'go env', so $exe is redundant and
a bit confusing. Notably, mod_modinfo.txt already assumes that GOEXE
is set (even though it isn't), and thus fails on Windows.

After this CL, `go test cmd/go/...` passes on a windows-amd64-2016
builder. However, given that the $PATH on the builder is very minimal
(#32430) and network access is limited, tests that rely on binaries
(such as 'git') or external networking may still be broken.

Updates #25300

Change-Id: I9d80f2a0fbaa8bc35fa2205b6898aeccecda4e94
Reviewed-on: https://go-review.googlesource.com/c/go/+/181542
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/script_test.go

@@ -24,6 +24,7 @@ import (
 	"testing"
 	"time"
 
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/par"
 	"cmd/go/internal/txtar"
@@ -106,6 +107,7 @@ func (ts *testScript) setup() {
 		"CCACHE_DISABLE=1", // ccache breaks with non-existent HOME
 		"GOARCH=" + runtime.GOARCH,
 		"GOCACHE=" + testGOCACHE,
+		"GOEXE=" + cfg.ExeSuffix,
 		"GOOS=" + runtime.GOOS,
 		"GOPATH=" + filepath.Join(ts.workdir, "gopath"),
 		"GOPROXY=" + proxyURL,
@@ -123,11 +125,6 @@ func (ts *testScript) setup() {
 		ts.env = append(ts.env, "path="+testBin+string(filepath.ListSeparator)+os.Getenv("path"))
 	}
 
-	if runtime.GOOS == "windows" {
-		ts.env = append(ts.env, "exe=.exe")
-	} else {
-		ts.env = append(ts.env, "exe=")
-	}
 	for _, key := range extraEnvKeys {
 		if val := os.Getenv(key); val != "" {
 			ts.env = append(ts.env, key+"="+val)

src/cmd/go/testdata/script/README

@@ -28,6 +28,7 @@ Scripts also have access to these other environment variables:
 
 	GOARCH=<target GOARCH>
 	GOCACHE=<actual GOCACHE being used outside the test>
+	GOEXE=<executable file suffix: .exe on Windows, empty on other systems>
 	GOOS=<target GOOS>
 	GOPATH=$WORK/gopath
 	GOPROXY=<local module proxy serving from cmd/go/testdata/mod>
@@ -38,8 +39,6 @@ Scripts also have access to these other environment variables:
 	devnull=<value of os.DevNull>
 	goversion=<current Go version; for example, 1.12>
 
-The environment variable $exe (lowercase) is an empty string on most systems, ".exe" on Windows.
-
 The scripts supporting files are unpacked relative to $GOPATH/src (aka $WORK/gopath/src)
 and then the script begins execution in that directory as well. Thus the example above runs
 in $WORK/gopath/src with GOPATH=$WORK/gopath and $WORK/gopath/src/hello.go

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

@@ -16,9 +16,9 @@ go build -o $devnull -x main.go
 stderr '(link|gccgo)( |\.exe)'
 
 # ... but the output binary can serve as a cache.
-go build -o main$exe -x main.go
+go build -o main$GOEXE -x main.go
 stderr '(link|gccgo)( |\.exe)'
-go build -o main$exe -x main.go
+go build -o main$GOEXE -x main.go
 ! stderr '(link|gccgo)( |\.exe)'
 
 -- main.go --

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

@@ -29,5 +29,5 @@ package pkg1
 -- pkg2/pkg2.go --
 package pkg2
 
--- c1$exe/keep.txt --
+-- c1$GOEXE/keep.txt --
 Create c1 directory.

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

@@ -4,21 +4,21 @@ env GO111MODULE=off
 # 'go install' with no arguments should clean up after go build
 cd mycmd
 go build
-exists mycmd$exe
+exists mycmd$GOEXE
 go install
-! exists mycmd$exe
+! exists mycmd$GOEXE
 
 # 'go install mycmd' does not clean up, even in the mycmd directory
 go build
-exists mycmd$exe
+exists mycmd$GOEXE
 go install mycmd
-exists mycmd$exe
+exists mycmd$GOEXE
 
 # 'go install mycmd' should not clean up in an unrelated current directory either
 cd ..
-cp mycmd/mycmd$exe mycmd$exe
+cp mycmd/mycmd$GOEXE mycmd$GOEXE
 go install mycmd
-exists mycmd$exe
+exists mycmd$GOEXE
 
 -- mycmd/main.go --
 package main

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

@@ -3,15 +3,15 @@ env GO111MODULE=on
 
 go get -d rsc.io/fortune/v2
 
-# The default executable name shouldn't be v2$exe
+# The default executable name shouldn't be v2$GOEXE
 go build rsc.io/fortune/v2
-! exists v2$exe
-exists fortune$exe
+! exists v2$GOEXE
+exists fortune$GOEXE
 
-# The default test binary name shouldn't be v2.test$exe
+# The default test binary name shouldn't be v2.test$GOEXE
 go test -c rsc.io/fortune/v2
-! exists v2.test$exe
-exists fortune.test$exe
+! exists v2.test$GOEXE
+exists fortune.test$GOEXE
 
 -- go.mod --
 module scratch

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

@@ -4,7 +4,7 @@ env GO111MODULE=off
 # should work (see golang.org/issue/28035).
 cd x
 go test -o=$devnull -c
-! exists x.test$exe
+! exists x.test$GOEXE
 
 -- x/x_test.go --
 package x_test