Move response file handling to a separate package

sbox is going to need to read and write response files, move ReadRspFile
to its own package.

Test: response_test.go
Change-Id: Iecb5486b4aaeb2531828743ad8ef784df675e18e

response/response.go

@@ -0,0 +1,70 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package response
+
+import (
+	"io"
+	"io/ioutil"
+	"unicode"
+)
+
+const noQuote = '\x00'
+
+// ReadRspFile reads a file in Ninja's response file format and returns its contents.
+func ReadRspFile(r io.Reader) ([]string, error) {
+	var files []string
+	var file []byte
+
+	buf, err := ioutil.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+
+	isEscaping := false
+	quotingStart := byte(noQuote)
+	for _, c := range buf {
+		switch {
+		case isEscaping:
+			if quotingStart == '"' {
+				if !(c == '"' || c == '\\') {
+					// '\"' or '\\' will be escaped under double quoting.
+					file = append(file, '\\')
+				}
+			}
+			file = append(file, c)
+			isEscaping = false
+		case c == '\\' && quotingStart != '\'':
+			isEscaping = true
+		case quotingStart == noQuote && (c == '\'' || c == '"'):
+			quotingStart = c
+		case quotingStart != noQuote && c == quotingStart:
+			quotingStart = noQuote
+		case quotingStart == noQuote && unicode.IsSpace(rune(c)):
+			// Current character is a space outside quotes
+			if len(file) != 0 {
+				files = append(files, string(file))
+			}
+			file = file[:0]
+		default:
+			file = append(file, c)
+		}
+	}
+
+	if len(file) != 0 {
+		files = append(files, string(file))
+	}
+
+	return files, nil
+}