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
diff --git a/response/Android.bp b/response/Android.bp
new file mode 100644
index 0000000..e19981f
--- /dev/null
+++ b/response/Android.bp
@@ -0,0 +1,16 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+bootstrap_go_package {
+    name: "soong-response",
+    pkgPath: "android/soong/response",
+    deps: [
+    ],
+    srcs: [
+        "response.go",
+    ],
+    testSrcs: [
+        "response_test.go",
+    ],
+}
diff --git a/response/response.go b/response/response.go
new file mode 100644
index 0000000..e8ff4b2
--- /dev/null
+++ b/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
+}
diff --git a/response/response_test.go b/response/response_test.go
new file mode 100644
index 0000000..99ce623
--- /dev/null
+++ b/response/response_test.go
@@ -0,0 +1,96 @@
+// 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 (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+func TestReadRspFile(t *testing.T) {
+	testCases := []struct {
+		name, in string
+		out      []string
+	}{
+		{
+			name: "single quoting test case 1",
+			in:   `./cmd '"'-C`,
+			out:  []string{"./cmd", `"-C`},
+		},
+		{
+			name: "single quoting test case 2",
+			in:   `./cmd '-C`,
+			out:  []string{"./cmd", `-C`},
+		},
+		{
+			name: "single quoting test case 3",
+			in:   `./cmd '\"'-C`,
+			out:  []string{"./cmd", `\"-C`},
+		},
+		{
+			name: "single quoting test case 4",
+			in:   `./cmd '\\'-C`,
+			out:  []string{"./cmd", `\\-C`},
+		},
+		{
+			name: "none quoting test case 1",
+			in:   `./cmd \'-C`,
+			out:  []string{"./cmd", `'-C`},
+		},
+		{
+			name: "none quoting test case 2",
+			in:   `./cmd \\-C`,
+			out:  []string{"./cmd", `\-C`},
+		},
+		{
+			name: "none quoting test case 3",
+			in:   `./cmd \"-C`,
+			out:  []string{"./cmd", `"-C`},
+		},
+		{
+			name: "double quoting test case 1",
+			in:   `./cmd "'"-C`,
+			out:  []string{"./cmd", `'-C`},
+		},
+		{
+			name: "double quoting test case 2",
+			in:   `./cmd "\\"-C`,
+			out:  []string{"./cmd", `\-C`},
+		},
+		{
+			name: "double quoting test case 3",
+			in:   `./cmd "\""-C`,
+			out:  []string{"./cmd", `"-C`},
+		},
+		{
+			name: "ninja rsp file",
+			in:   "'a'\nb\n'@'\n'foo'\\''bar'\n'foo\"bar'",
+			out:  []string{"a", "b", "@", "foo'bar", `foo"bar`},
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			got, err := ReadRspFile(bytes.NewBuffer([]byte(testCase.in)))
+			if err != nil {
+				t.Errorf("unexpected error: %q", err)
+			}
+			if !reflect.DeepEqual(got, testCase.out) {
+				t.Errorf("expected %q got %q", testCase.out, got)
+			}
+		})
+	}
+}