Write to rotating ContentReWriter in transformer chain

This commit adds the interface ContentReWriter in the tranformer chain.

This is backed by two pooled byte buffers, alternating between being the reader or the writer.

This keeps the performance characteristic of the old implementation, but in a thread safe way.

Fixes #911

Benchmark old vs new:

benchmark              old ns/op     new ns/op     delta
BenchmarkAbsURL        17614         17384         -1.31%
BenchmarkXMLAbsURL     9431          9248          -1.94%

benchmark              old allocs     new allocs     delta
BenchmarkAbsURL        24             28             +16.67%
BenchmarkXMLAbsURL     12             14             +16.67%

benchmark              old bytes     new bytes     delta
BenchmarkAbsURL        3295          3424          +3.92%
BenchmarkXMLAbsURL     1954          1987          +1.69%

5 files changed, 110 insertions, 45 deletions

diff --git a/transform/absurl.go b/transform/absurl.go
index 6fc62adcf..fa1d99306 100644
--- a/transform/absurl.go
+++ b/transform/absurl.go
@@ -17,8 +17,8 @@ func initAbsURLReplacer(baseURL string) {
 func AbsURL(absURL string) (trs []link, err error) {
 	initAbsURLReplacer(absURL)
 
-	trs = append(trs, func(content []byte) []byte {
-		return ar.replaceInHTML(content)
+	trs = append(trs, func(rw ContentReWriter) {
+		ar.replaceInHTML(rw)
 	})
 	return
 }
@@ -26,8 +26,8 @@ func AbsURL(absURL string) (trs []link, err error) {
 func AbsURLInXML(absURL string) (trs []link, err error) {
 	initAbsURLReplacer(absURL)
 
-	trs = append(trs, func(content []byte) []byte {
-		return ar.replaceInXML(content)
+	trs = append(trs, func(rw ContentReWriter) {
+		ar.replaceInXML(rw)
 	})
 	return
 }
diff --git a/transform/absurlreplacer.go b/transform/absurlreplacer.go
index 2f2a5bd53..66fdaf689 100644
--- a/transform/absurlreplacer.go
+++ b/transform/absurlreplacer.go
@@ -2,7 +2,7 @@ package transform
 
 import (
 	"bytes"
-	bp "github.com/spf13/hugo/bufferpool"
+	"io"
 	"net/url"
 	"strings"
 	"unicode/utf8"
@@ -33,7 +33,7 @@ type contentlexer struct {
 	state        stateFunc
 	prefixLookup *prefixes
 
-	b *bytes.Buffer
+	w io.Writer
 }
 
 type stateFunc func(*contentlexer) stateFunc
@@ -95,7 +95,7 @@ func (l *contentlexer) match(r rune) {
 }
 
 func (l *contentlexer) emit() {
-	l.b.Write(l.content[l.start:l.pos])
+	l.w.Write(l.content[l.start:l.pos])
 	l.start = l.pos
 }
 
@@ -134,7 +134,7 @@ func checkCandidate(l *contentlexer) {
 				l.emit()
 			}
 			l.pos += len(m.match)
-			l.b.Write(m.replacement)
+			l.w.Write(m.replacement)
 			l.start = l.pos
 			return
 
@@ -159,7 +159,6 @@ func (l *contentlexer) replace() {
 		}
 		l.width = width
 		l.pos += l.width
-
 		if r == ' ' {
 			l.prefixLookup.ms = matchStateWhitespace
 		} else if l.prefixLookup.ms != matchStateNone {
@@ -177,18 +176,16 @@ func (l *contentlexer) replace() {
 	}
 }
 
-func doReplace(content []byte, matchers []absURLMatcher) []byte {
-	b := bp.GetBuffer()
-	defer bp.PutBuffer(b)
+func doReplace(rw ContentReWriter, matchers []absURLMatcher) {
 
-	lexer := &contentlexer{content: content,
-		b:            b,
+	lexer := &contentlexer{
+		content:      rw.Content(),
+		w:            rw,
 		prefixLookup: &prefixes{pr: mainPrefixRunes},
 		matchers:     matchers}
 
 	lexer.replace()
 
-	return b.Bytes()
 }
 
 type absURLReplacer struct {
@@ -229,10 +226,10 @@ func newAbsURLReplacer(baseURL string) *absURLReplacer {
 
 }
 
-func (au *absURLReplacer) replaceInHTML(content []byte) []byte {
-	return doReplace(content, au.htmlMatchers)
+func (au *absURLReplacer) replaceInHTML(rw ContentReWriter) {
+	doReplace(rw, au.htmlMatchers)
 }
 
-func (au *absURLReplacer) replaceInXML(content []byte) []byte {
-	return doReplace(content, au.xmlMatchers)
+func (au *absURLReplacer) replaceInXML(rw ContentReWriter) {
+	doReplace(rw, au.xmlMatchers)
 }
diff --git a/transform/chain.go b/transform/chain.go
index c6e56960c..0edcb6971 100644
--- a/transform/chain.go
+++ b/transform/chain.go
@@ -1,12 +1,12 @@
 package transform
 
 import (
-	"io"
-
+	"bytes"
 	bp "github.com/spf13/hugo/bufferpool"
+	"io"
 )
 
-type trans func([]byte) []byte
+type trans func(rw ContentReWriter)
 
 type link trans
 
@@ -20,17 +20,62 @@ func NewEmptyTransforms() []link {
 	return make([]link, 0, 20)
 }
 
-func (c *chain) Apply(w io.Writer, r io.Reader) (err error) {
-	buffer := bp.GetBuffer()
-	defer bp.PutBuffer(buffer)
+// ContentReWriter is an interface that enables rotation
+// of pooled buffers in the transformer chain.
+type ContentReWriter interface {
+	Content() []byte
+	io.Writer
+}
+
+// Implements ContentReWriter
+// Content is read from the from-buffer,
+// and rewritten to to the to-buffer.
+type fromToBuffer struct {
+	from *bytes.Buffer
+	to   *bytes.Buffer
+}
+
+func (ft fromToBuffer) Write(p []byte) (n int, err error) {
+	return ft.to.Write(p)
+}
+
+func (ft fromToBuffer) Content() []byte {
+	return ft.from.Bytes()
+}
 
-	buffer.ReadFrom(r)
-	b := buffer.Bytes()
-	for _, tr := range *c {
-		b = tr(b)
+func (c *chain) Apply(w io.Writer, r io.Reader) error {
+
+	b1 := bp.GetBuffer()
+	defer bp.PutBuffer(b1)
+
+	b1.ReadFrom(r)
+
+	if len(*c) == 0 {
+		b1.WriteTo(w)
+		return nil
 	}
-	buffer.Reset()
-	buffer.Write(b)
-	buffer.WriteTo(w)
-	return
+
+	b2 := bp.GetBuffer()
+	defer bp.PutBuffer(b2)
+
+	fb := &fromToBuffer{from: b1, to: b2}
+
+	for i, tr := range *c {
+		if i > 0 {
+			if fb.from == b1 {
+				fb.from = b2
+				fb.to = b1
+				fb.to.Reset()
+			} else {
+				fb.from = b1
+				fb.to = b2
+				fb.to.Reset()
+			}
+		}
+
+		tr(fb)
+	}
+
+	fb.to.WriteTo(w)
+	return nil
 }
diff --git a/transform/chain_test.go b/transform/chain_test.go
index 8fa45f5a9..2477c3abf 100644
--- a/transform/chain_test.go
+++ b/transform/chain_test.go
@@ -2,6 +2,7 @@ package transform
 
 import (
 	"bytes"
+	"github.com/spf13/hugo/helpers"
 	"strings"
 	"testing"
 )
@@ -54,6 +55,35 @@ func TestChainZeroTransformers(t *testing.T) {
 	}
 }
 
+func TestChaingMultipleTransformers(t *testing.T) {
+	f1 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f1"), []byte("f1r"), -1))
+	}
+	f2 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f2"), []byte("f2r"), -1))
+	}
+	f3 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f3"), []byte("f3r"), -1))
+	}
+
+	f4 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f4"), []byte("f4r"), -1))
+	}
+
+	tr := NewChain(f1, f2, f3, f4)
+
+	out := new(bytes.Buffer)
+	if err := tr.Apply(out, helpers.StringToReader("Test: f4 f3 f1 f2 f1 The End.")); err != nil {
+		t.Errorf("Multi transformer chain returned an error: %s", err)
+	}
+
+	expected := "Test: f4r f3r f1r f2r f1r The End."
+
+	if string(out.Bytes()) != expected {
+		t.Errorf("Expected %s got %s", expected, string(out.Bytes()))
+	}
+}
+
 func BenchmarkAbsURL(b *testing.B) {
 	absURL, _ := AbsURL("http://base")
 	tr := NewChain(absURL...)
diff --git a/transform/livereloadinject.go b/transform/livereloadinject.go
index eb431f14a..bffedf040 100644
--- a/transform/livereloadinject.go
+++ b/transform/livereloadinject.go
@@ -2,29 +2,22 @@ package transform
 
 import (
 	"bytes"
-	jww "github.com/spf13/jwalterweatherman"
 	"github.com/spf13/viper"
 )
 
-func LiveReloadInject(content []byte) (injected []byte) {
-	defer func() {
-		if r := recover(); r != nil {
-			jww.ERROR.Println("Recovered in LiveReloadInject", r)
-			injected = content
-		}
-	}()
+func LiveReloadInject(rw ContentReWriter) {
 	match := []byte("</body>")
 	port := viper.GetString("port")
 	replace := []byte(`<script>document.write('<script src="http://'
         + (location.host || 'localhost').split(':')[0]
 		+ ':` + port + `/livereload.js?mindelay=10"></'
         + 'script>')</script></body>`)
-	newcontent := bytes.Replace(content, match, replace, -1)
+	newcontent := bytes.Replace(rw.Content(), match, replace, -1)
 
-	if len(newcontent) == len(content) {
+	if len(newcontent) == len(rw.Content()) {
 		match := []byte("</BODY>")
-		newcontent = bytes.Replace(content, match, replace, -1)
+		newcontent = bytes.Replace(rw.Content(), match, replace, -1)
 	}
 
-	return newcontent
+	rw.Write(newcontent)
 }