fix a data race with the global cachedBinary mechanism (#413)

Spotted by our friend "go test -race":

	WARNING: DATA RACE
	Write at 0x0000010522d8 by goroutine 69:
	  mvdan.cc/garble.readFile()
		  garble/main_test.go:124 +0x23a
	  mvdan.cc/garble.binsubstr()
		  garble/main_test.go:141 +0xc4
	  github.com/rogpeppe/go-internal/testscript.(*TestScript).run()
		  github.com/rogpeppe/go-internal@v1.8.1-0.20211023094830-115ce09fd6b4/testscript/testscript.go:496 +0x9e8
	  [...]

	Previous write at 0x0000010522d8 by goroutine 60:
	  mvdan.cc/garble.readFile()
		  garble/main_test.go:124 +0x23a
	  mvdan.cc/garble.binsubstr()
		  garble/main_test.go:141 +0xc4
	  github.com/rogpeppe/go-internal/testscript.(*TestScript).run()
		  github.com/rogpeppe/go-internal@v1.8.1-0.20211023094830-115ce09fd6b4/testscript/testscript.go:496 +0x9e8
	  [...]

This wasn't a data race that we spotted via failures in practice,
as it only affected test code since July.

The race is due to the fact that each test script runs as a parallel
sub-test within the same Go program, sharing all globals.
As such, a single "cached binary" global is read and written with races.

Moreover, note that the caching always missed.
I briefly rewrote the code to avoid the race via a sync.Map keyed by
absolute filenames, and while that removed the data race,
the caching never actually hit.

To have a cache hit, we need an absolute path to already be in the cache
and for it to not have been modified since it was last cached. That is:

	modify-bin-1 foo
	binsubstr foo 'abc' # miss
	binsubstr foo 'def' # hit; use the cached "/tmp/[...]/foo" entry

	modify-bin-2 foo
	binsubstr foo 'abc' # miss

However, the test scripts don't do contiguous binsubstr calls like
these. Instead, they join repeated binsubstr calls:

	modify-bin-1 foo
	binsubstr foo 'abc' 'def' # miss

	modify-bin-2 foo
	binsubstr foo 'abc' # miss

For that reason, remove the extra code entirely.
I didn't notice any change to the performance of "go test -short"
with a warm build cache, with:

	go test -c
	./garble.test -test.short #warm cache
	benchcmd -n 5 TestShort ./garble.test -test.short

	name       old time/op         new time/op         delta
	TestShort          4.62s ±12%          4.35s ±12%   ~     (p=0.310 n=5+5)

	name       old user-time/op    new user-time/op    delta
	TestShort          16.8s ± 3%          16.7s ± 3%   ~     (p=0.690 n=5+5)

	name       old sys-time/op     new sys-time/op     delta
	TestShort          7.28s ± 1%          7.26s ± 2%   ~     (p=0.841 n=5+5)

	name       old peak-RSS-bytes  new peak-RSS-bytes  delta
	TestShort          305MB ± 0%          306MB ± 0%   ~     (p=0.421 n=5+5)

Finally, start using "go test -race" on Linux on CI,
which should have made the PR back in July red before merging.

.github/workflows/test.yml

@@ -11,8 +11,8 @@ jobs:
     strategy:
       matrix:
         go-version: [1.17.x]
-        platform: [ubuntu-latest, macos-latest, windows-latest]
-    runs-on: ${{ matrix.platform }}
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     steps:
     - name: Install Go
       uses: actions/setup-go@v2
@@ -24,6 +24,12 @@ jobs:
       run: |
         go version
         go test ./...
+    - name: Test with -race
+      # macos and windows tend to be a bit slower,
+      # and it's rare that a race in garble would be OS-specific.
+      if: matrix.os == 'ubuntu-latest'
+      run: |
+        go test -race ./...
 
   test-gotip:
     if: ${{ false }} # TODO: support Go 1.18, see https://github.com/burrowers/garble/issues/385

main_test.go

@@ -16,7 +16,6 @@ import (
 	"runtime"
 	"strings"
 	"testing"
-	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/rogpeppe/go-internal/goproxytest"
@@ -101,31 +100,6 @@ func TestScripts(t *testing.T) {
 	testscript.Run(t, p)
 }
 
-type binaryCache struct {
-	name    string
-	modtime time.Time
-	content string
-}
-
-var cachedBinary binaryCache
-
-func readFile(ts *testscript.TestScript, file string) string {
-	file = ts.MkAbs(file)
-	info, err := os.Stat(file)
-	if err != nil {
-		ts.Fatalf("%v", err)
-	}
-
-	if cachedBinary.modtime == info.ModTime() && cachedBinary.name == file {
-		return cachedBinary.content
-	}
-
-	cachedBinary.name = file
-	cachedBinary.modtime = info.ModTime()
-	cachedBinary.content = ts.ReadFile(file)
-	return cachedBinary.content
-}
-
 func createFile(ts *testscript.TestScript, path string) *os.File {
 	file, err := os.Create(ts.MkAbs(path))
 	if err != nil {
@@ -138,7 +112,7 @@ func binsubstr(ts *testscript.TestScript, neg bool, args []string) {
 	if len(args) < 2 {
 		ts.Fatalf("usage: binsubstr file substr...")
 	}
-	data := readFile(ts, args[0])
+	data := ts.ReadFile(args[0])
 	var failed []string
 	for _, substr := range args[1:] {
 		match := strings.Contains(data, substr)