Write ninja file directly to the output file

Writing the ninja file to a byte buffer causes a significant amount
of time to be spent in memmove when growing the byte slice.  Write
the file directly to disk instead.

This also fixes some unhandled error warnings, which become more
likely when doing disk IO instead of byte buffer writes.

Change-Id: I5094e4c45cab4012713037f60c5a4fb00718f92e

bootstrap/command.go

@@ -15,9 +15,10 @@
 package bootstrap
 
 import (
-	"bytes"
+	"bufio"
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -173,20 +174,39 @@ func Main(ctx *blueprint.Context, config interface{}, extraNinjaFileDeps ...stri
 	}
 	deps = append(deps, extraDeps...)
 
-	buf := bytes.NewBuffer(nil)
-	err = ctx.WriteBuildFile(buf)
-	if err != nil {
-		fatalf("error generating Ninja file contents: %s", err)
-	}
-
-	if stage == StageMain && emptyNinjaFile {
-		buf.Reset()
-	}
-
 	const outFilePermissions = 0666
-	err = ioutil.WriteFile(outFile, buf.Bytes(), outFilePermissions)
+	var out io.Writer
+	var f *os.File
+	var buf *bufio.Writer
+
+	if stage != StageMain || !emptyNinjaFile {
+		f, err = os.OpenFile(outFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, outFilePermissions)
 		if err != nil {
-		fatalf("error writing %s: %s", outFile, err)
+			fatalf("error opening Ninja file: %s", err)
+		}
+		buf = bufio.NewWriter(f)
+		out = buf
+	} else {
+		out = ioutil.Discard
+	}
+
+	err = ctx.WriteBuildFile(out)
+	if err != nil {
+		fatalf("error writing Ninja file contents: %s", err)
+	}
+
+	if buf != nil {
+		err = buf.Flush()
+		if err != nil {
+			fatalf("error flushing Ninja file contents: %s", err)
+		}
+	}
+
+	if f != nil {
+		err = f.Close()
+		if err != nil {
+			fatalf("error closing Ninja file: %s", err)
+		}
 	}
 
 	if globFile != "" {

context.go

@@ -3113,7 +3113,10 @@ func (c *Context) writeNinjaRequiredVersion(nw *ninjaWriter) error {
 
 func (c *Context) writeSubninjas(nw *ninjaWriter) error {
 	for _, subninja := range c.subninjas {
-		nw.Subninja(subninja)
+		err := nw.Subninja(subninja)
+		if err != nil {
+			return err
+		}
 	}
 	return nw.BlankLine()
 }

ninja_defs.go

@@ -410,7 +410,10 @@ func (b *buildDef) WriteTo(nw *ninjaWriter, pkgNames map[*packageContext]string)
 	}
 
 	if !b.Optional {
-		nw.Default(outputs...)
+		err = nw.Default(outputs...)
+		if err != nil {
+			return err
+		}
 	}
 
 	return nw.BlankLine()

ninja_writer.go

@@ -117,7 +117,10 @@ func (n *ninjaWriter) Build(comment string, rule string, outputs, implicitOuts,
 	}
 
 	if comment != "" {
-		wrapper.Comment(comment)
+		err := wrapper.Comment(comment)
+		if err != nil {
+			return err
+		}
 	}
 
 	wrapper.WriteString("build")
@@ -237,7 +240,10 @@ func (n *ninjaWriterWithWrap) writeString(s string, space bool) {
 		n.writtenLen = indentWidth * 2
 		s = strings.TrimLeftFunc(s, unicode.IsSpace)
 	} else if space {
-		io.WriteString(n.writer, " ")
+		_, n.err = io.WriteString(n.writer, " ")
+		if n.err != nil {
+			return
+		}
 		n.writtenLen++
 	}