internal/literals: add fuzzer

To inevstigate #721, I wrote this fuzzer to see if any particular
combination of string literals and literal obfuscators would result
in a broken program.

I didn't find anything, but I reckon this fuzzer can still be useful.

internal/literals/fuzz_test.go

@@ -0,0 +1,115 @@
+package literals_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"go/types"
+	mathrand "math/rand"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"mvdan.cc/garble/internal/literals"
+)
+
+// The fuzzing string is passed in as a string and []byte literal.
+var fuzzTemplate = `
+package main
+
+var str string = %#[1]v
+var strFold string = "x" + %#[1]v + "y"
+var byt []byte = %#[2]v
+var bytPtr *[]byte = &%#[2]v
+
+func main() {
+	println(str)
+	println(strFold)
+	println("--")
+	println(string(byt))
+	println(string(*bytPtr))
+}
+`[1:]
+
+func FuzzObfuscate(f *testing.F) {
+	initialRandSeed := int64(123)
+	f.Add("", initialRandSeed)
+	f.Add("short", initialRandSeed)
+	f.Add("long_enough_string", initialRandSeed)
+	f.Add("binary_\x00\x01\x02", initialRandSeed)
+	f.Add("whitespace    \n\t\t", initialRandSeed)
+	f.Add(strings.Repeat("x", (2<<10)+1), initialRandSeed) // past maxSize
+
+	tdir := f.TempDir()
+	var tdirCounter atomic.Int64
+	f.Fuzz(func(t *testing.T, in string, randSeed int64) {
+		// The code below is an extreme simplification of what "garble build" does,
+		// but it does significantly less, allowing the fuzz function to be faster.
+		// For example, we only obfuscate the literals, not any identifiers.
+		// Note that the fuzzer is still quite slow, as it still builds a binary.
+
+		// Create the source, parse it, and typecheck it.
+		srcText := fmt.Sprintf(fuzzTemplate, in, []byte(in))
+		t.Log(srcText) // shown on failures
+		fset := token.NewFileSet()
+		srcSyntax, err := parser.ParseFile(fset, "", srcText, parser.SkipObjectResolution)
+		if err != nil {
+			t.Fatal(err)
+		}
+		info := types.Info{
+			Types: make(map[ast.Expr]types.TypeAndValue),
+			Defs:  make(map[*ast.Ident]types.Object),
+			Uses:  make(map[*ast.Ident]types.Object),
+		}
+		var conf types.Config
+		if _, err := conf.Check("p", fset, []*ast.File{srcSyntax}, &info); err != nil {
+			t.Fatal(err)
+		}
+
+		// Obfuscate the literals and print the source back.
+		rand := mathrand.New(mathrand.NewSource(randSeed))
+		srcSyntax = literals.Obfuscate(rand, srcSyntax, &info, nil)
+		count := tdirCounter.Add(1)
+		f, err := os.Create(filepath.Join(tdir, fmt.Sprintf("src_%d.go", count)))
+		if err != nil {
+			t.Fatal(err)
+		}
+		srcPath := f.Name()
+		t.Cleanup(func() {
+			f.Close()
+			os.Remove(srcPath)
+		})
+		if err := printer.Fprint(f, fset, srcSyntax); err != nil {
+			t.Fatal(err)
+		}
+
+		// Build the main package. Use some flags to avoid work.
+		binPath := strings.TrimSuffix(srcPath, ".go")
+		if runtime.GOOS == "windows" {
+			binPath += ".exe"
+		}
+		if out, err := exec.Command(
+			"go", "build", "-trimpath", "-ldflags=-w -s", "-p", "1",
+			"-o", binPath, srcPath,
+		).CombinedOutput(); err != nil {
+			t.Fatalf("%v: %s", err, out)
+		}
+		t.Cleanup(func() { os.Remove(binPath) })
+
+		// Run the binary, expecting the output to match.
+		out, err := exec.Command(binPath).CombinedOutput()
+		if err != nil {
+			t.Fatalf("%v: %s", err, out)
+		}
+		want := fmt.Sprintf("%[1]s\nx%[1]sy\n--\n%[1]s\n%[1]s\n", in)
+		if got := string(out); got != want {
+			t.Fatalf("got:  %q\nwant: %q", got, want)
+		}
+	})
+}