Add a dependency fixer for proto deps

protoc dependency files, at least for C++ outputs, uses the form of:

  a/b.c \
  a/b.h: <dep1> <dep2>...

Ninja will fail the command when it parses a dep file and there's more
than one output file (even though it doesn't care what the output file
name is). So this tool will parse the original file, and output a
version with only a single output file.

Bug: 67329638
Test: NINJA_ARGS="-t deps ...pb.c" m
Test: NINJA_ARGS="-t deps ...srcjar" m
Test: NINJA_ARGS="-t deps ...srcszip" m
Test: Run dep_fixer across all of taimen's dep files, no failures.
Test: Run dep_fixer against the processed files, no changes.
Test: Run androidmk across all of our Android.mk files, inspect the diffs
Change-Id: I4263b7d5faea37285afa6b24dedf5964aa7d19dc
diff --git a/cmd/dep_fixer/deps.go b/cmd/dep_fixer/deps.go
new file mode 100644
index 0000000..64c97f5
--- /dev/null
+++ b/cmd/dep_fixer/deps.go
@@ -0,0 +1,95 @@
+// Copyright 2018 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 main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"strings"
+
+	"android/soong/androidmk/parser"
+)
+
+type Deps struct {
+	Output string
+	Inputs []string
+}
+
+func Parse(filename string, r io.Reader) (*Deps, error) {
+	p := parser.NewParser(filename, r)
+	nodes, errs := p.Parse()
+
+	if len(errs) == 1 {
+		return nil, errs[0]
+	} else if len(errs) > 1 {
+		return nil, fmt.Errorf("many errors: %v", errs)
+	}
+
+	pos := func(node parser.Node) string {
+		return p.Unpack(node.Pos()).String() + ": "
+	}
+
+	ret := &Deps{}
+
+	for _, node := range nodes {
+		switch x := node.(type) {
+		case *parser.Comment:
+			// Do nothing
+		case *parser.Rule:
+			if x.Recipe != "" {
+				return nil, fmt.Errorf("%sunexpected recipe in rule: %v", pos(node), x)
+			}
+
+			if !x.Target.Const() {
+				return nil, fmt.Errorf("%sunsupported variable expansion: %v", pos(node), x.Target.Dump())
+			}
+			outputs := x.Target.Words()
+			if len(outputs) == 0 {
+				return nil, fmt.Errorf("%smissing output: %v", pos(node), x)
+			}
+			ret.Output = outputs[0].Value(nil)
+
+			if !x.Prerequisites.Const() {
+				return nil, fmt.Errorf("%sunsupported variable expansion: %v", pos(node), x.Prerequisites.Dump())
+			}
+			for _, input := range x.Prerequisites.Words() {
+				ret.Inputs = append(ret.Inputs, input.Value(nil))
+			}
+		default:
+			return nil, fmt.Errorf("%sunexpected line: %#v", pos(node), node)
+		}
+	}
+
+	return ret, nil
+}
+
+func (d *Deps) Print() []byte {
+	// We don't really have to escape every \, but it's simpler,
+	// and ninja will handle it.
+	replacer := strings.NewReplacer(" ", "\\ ",
+		":", "\\:",
+		"#", "\\#",
+		"$", "$$",
+		"\\", "\\\\")
+
+	b := &bytes.Buffer{}
+	fmt.Fprintf(b, "%s:", replacer.Replace(d.Output))
+	for _, input := range d.Inputs {
+		fmt.Fprintf(b, " %s", replacer.Replace(input))
+	}
+	fmt.Fprintln(b)
+	return b.Bytes()
+}