cmd/go: make TestScript/gotoolchain more realistic

- Build the fake go1.999testpath binary from Go source instead of
  special-casing a fake command on Windows.

- Skip the part of the test that uses shell scripts served from the
  test GOPROXY if /bin/sh is not present.

This makes the test more expensive, but also more realistic: notably,
it does not require test hooks to determine whether to run a real or
fake binary.

Change-Id: If14fec52186631d7833eba653c91ec5198dede58
Reviewed-on: https://go-review.googlesource.com/c/go/+/486400
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/gotoolchain.go

@@ -189,10 +189,6 @@ func execGoToolchain(gotoolchain, dir, exe string) {
 	// to allow testing this code even when not on Windows.
 	if godebug.New("#gotoolchainexec").Value() == "0" || runtime.GOOS == "windows" {
 		cmd := exec.Command(exe, os.Args[1:]...)
-		if runtime.GOOS == "windows" && strings.Contains(exe, "go1.999test") {
-			// See testdata/script/gotoolchain.txt.
-			cmd = exec.Command("cmd", "/c", "echo pretend we ran "+exe)
-		}
 		cmd.Stdin = os.Stdin
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr

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

@@ -1,8 +1,9 @@
-[!GOOS:windows] [!exec:/bin/sh] skip
-[!GOOS:windows] chmod 0755 $WORK/bin/go1.999testpath
+[short] skip
+
+mkdir $WORK/bin
 [!GOOS:plan9] env PATH=$WORK/bin${:}$PATH
 [GOOS:plan9] env path=$WORK/bin${:}$path
-[GOOS:plan9] replace /bin/sh /bin/rc $WORK/bin/go1.999testpath
+go build -o $WORK/bin/ ./go1.999testpath.go  # adds .exe extension implicitly on Windows
 
 # Plain go version
 go version
@@ -11,42 +12,31 @@ go version
 # GOTOOLCHAIN from PATH
 env GOTOOLCHAIN=go1.999testpath
 go version
-[!GOOS:windows] stdout 'go1.999testpath here!'
-[GOOS:windows] stdout 'pretend we ran .*go1.999testpath'
+stdout 'go1.999testpath here!'
 
 # GOTOOLCHAIN from PATH, with forced subprocess
 env GOTOOLCHAIN=go1.999testpath
 env GODEBUG=gotoolchainexec=0
 go version
-[!GOOS:windows] stdout 'go1.999testpath here!'
-[GOOS:windows] stdout 'pretend we ran .*go1.999testpath'
+stdout 'go1.999testpath here!'
 env GODEBUG=
 
-# GOTOOLCHAIN from network
-env GOTOOLCHAIN=go1.999testmod
-go version
-stderr 'go: downloading go1.999testmod \(.*/.*\)'
-[!GOOS:windows] stdout 'go1.999testmod here!'
-[GOOS:windows] stdout 'pretend we ran .*go1.999testmod.*\\bin\\go'
-
 # GOTOOLCHAIN from network, does not exist
 env GOTOOLCHAIN=go1.9999x
 ! go version
 stderr 'go: download go1.9999x for .*: toolchain not available'
 
--- $WORK/bin/go1.999testpath --
-#!/bin/sh
-echo go1.999testpath here!
--- $WORK/bin/go1.999testpath.bat --
-This should say:
-	@echo go1.999testpath here!
-but exec.Command does not directly support batch files.
-execGoToolchain in cmd/go/toolchain.go picks off versions
-named go1.999test and instead of running them just runs
-cmd /c "echo pretend we ran <file>".
+# GOTOOLCHAIN from network
+[!exec:/bin/sh] stop 'the fake proxy serves shell scripts instead of binaries'
+env GOTOOLCHAIN=go1.999testmod
+go version
+stderr 'go: downloading go1.999testmod \(.*/.*\)'
 
-Since the real toolchain will have an exe file and cmd is an
-exe file, this seems like a good enough test.
-Changing execGoToolchain to use cmd /c to run the batch file
-hangs for unknown reasons.
+-- go1.999testpath.go --
+package main
 
+import "os"
+
+func main() {
+	os.Stdout.WriteString("go1.999testpath here!")
+}