cmd/go: encode backslash and newline in response files

Fixes #42295

Change-Id: Ie324bc99a74c1d864c6c2da2e7b929b338c2e033
Reviewed-on: https://go-review.googlesource.com/c/go/+/272870
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

2 files changed, 65 insertions, 0 deletions

diff --git a/src/cmd/internal/objabi/flag.go b/src/cmd/internal/objabi/flag.go
index 79ad2ccf74..3fd73f3c57 100644
--- a/src/cmd/internal/objabi/flag.go
+++ b/src/cmd/internal/objabi/flag.go
@@ -5,6 +5,7 @@
 package objabi
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io"
@@ -59,6 +60,9 @@ func expandArgs(in []string) (out []string) {
 				log.Fatal(err)
 			}
 			args := strings.Split(strings.TrimSpace(strings.Replace(string(slurp), "\r", "", -1)), "\n")
+			for i, arg := range args {
+				args[i] = DecodeArg(arg)
+			}
 			out = append(out, expandArgs(args)...)
 		} else if out != nil {
 			out = append(out, s)
@@ -160,3 +164,38 @@ func (f fn1) Set(s string) error {
 }
 
 func (f fn1) String() string { return "" }
+
+// DecodeArg decodes an argument.
+//
+// This function is public for testing with the parallel encoder.
+func DecodeArg(arg string) string {
+	// If no encoding, fastpath out.
+	if !strings.ContainsAny(arg, "\\\n") {
+		return arg
+	}
+
+	// We can't use strings.Builder as this must work at bootstrap.
+	var b bytes.Buffer
+	var wasBS bool
+	for _, r := range arg {
+		if wasBS {
+			switch r {
+			case '\\':
+				b.WriteByte('\\')
+			case 'n':
+				b.WriteByte('\n')
+			default:
+				// This shouldn't happen. The only backslashes that reach here
+				// should encode '\n' and '\\' exclusively.
+				panic("badly formatted input")
+			}
+		} else if r == '\\' {
+			wasBS = true
+			continue
+		} else {
+			b.WriteRune(r)
+		}
+		wasBS = false
+	}
+	return b.String()
+}
diff --git a/src/cmd/internal/objabi/flag_test.go b/src/cmd/internal/objabi/flag_test.go
new file mode 100644
index 0000000000..935b9c2193
--- /dev/null
+++ b/src/cmd/internal/objabi/flag_test.go
@@ -0,0 +1,26 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package objabi
+
+import "testing"
+
+func TestDecodeArg(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		arg, want string
+	}{
+		{"", ""},
+		{"hello", "hello"},
+		{"hello\\n", "hello\n"},
+		{"hello\\nthere", "hello\nthere"},
+		{"hello\\\\there", "hello\\there"},
+		{"\\\\\\n", "\\\n"},
+	}
+	for _, test := range tests {
+		if got := DecodeArg(test.arg); got != test.want {
+			t.Errorf("decodoeArg(%q) = %q, want %q", test.arg, got, test.want)
+		}
+	}
+}