encoding/csv, encoding/xml: report write errors

Fixes #3773.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6327053

src/pkg/encoding/csv/writer.go

diff --git a/src/pkg/encoding/csv/writer.go b/src/pkg/encoding/csv/writer.go
index c4dcba5..324944c 100644
--- a/src/pkg/encoding/csv/writer.go
+++ b/src/pkg/encoding/csv/writer.go
@@ -101,11 +101,10 @@
 	for _, record := range records {
 		err = w.Write(record)
 		if err != nil {
-			break
+			return err
 		}
 	}
-	w.Flush()
-	return nil
+	return w.w.Flush()
 }
 
 // fieldNeedsQuotes returns true if our field must be enclosed in quotes.

src/pkg/encoding/xml/marshal.go

diff --git a/src/pkg/encoding/xml/marshal.go b/src/pkg/encoding/xml/marshal.go
index 51e1dc8..8592a0c 100644
--- a/src/pkg/encoding/xml/marshal.go
+++ b/src/pkg/encoding/xml/marshal.go
@@ -83,9 +83,7 @@
 	enc := NewEncoder(&b)
 	enc.prefix = prefix
 	enc.indent = indent
-	err := enc.marshalValue(reflect.ValueOf(v), nil)
-	enc.Flush()
-	if err != nil {
+	if err := enc.Encode(v); err != nil {
 		return nil, err
 	}
 	return b.Bytes(), nil
@@ -107,8 +105,10 @@
 // of Go values to XML.
 func (enc *Encoder) Encode(v interface{}) error {
 	err := enc.marshalValue(reflect.ValueOf(v), nil)
-	enc.Flush()
-	return err
+	if err != nil {
+		return err
+	}
+	return enc.Flush()
 }
 
 type printer struct {
@@ -224,7 +224,7 @@
 	p.WriteString(name)
 	p.WriteByte('>')
 
-	return nil
+	return p.cachedWriteError()
 }
 
 var timeType = reflect.TypeOf(time.Time{})
@@ -260,15 +260,15 @@
 	default:
 		return &UnsupportedTypeError{typ}
 	}
-	return nil
+	return p.cachedWriteError()
 }
 
 var ddBytes = []byte("--")
 
 func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 	if val.Type() == timeType {
-		p.WriteString(val.Interface().(time.Time).Format(time.RFC3339Nano))
-		return nil
+		_, err := p.WriteString(val.Interface().(time.Time).Format(time.RFC3339Nano))
+		return err
 	}
 	s := parentStack{printer: p}
 	for i := range tinfo.fields {
@@ -353,7 +353,13 @@
 		}
 	}
 	s.trim(nil)
-	return nil
+	return p.cachedWriteError()
+}
+
+// return the bufio Writer's cached write error
+func (p *printer) cachedWriteError() error {
+	_, err := p.Write(nil)
+	return err
 }
 
 func (p *printer) writeIndent(depthDelta int) {

src/pkg/encoding/xml/marshal_test.go

diff --git a/src/pkg/encoding/xml/marshal_test.go b/src/pkg/encoding/xml/marshal_test.go
index 90b4925..e729a24 100644
--- a/src/pkg/encoding/xml/marshal_test.go
+++ b/src/pkg/encoding/xml/marshal_test.go
@@ -5,6 +5,9 @@
 package xml
 
 import (
+	"bytes"
+	"errors"
+	"io"
 	"reflect"
 	"strconv"
 	"strings"
@@ -779,6 +782,55 @@
 	}
 }
 
+type limitedBytesWriter struct {
+	w      io.Writer
+	remain int // until writes fail
+}
+
+func (lw *limitedBytesWriter) Write(p []byte) (n int, err error) {
+	if lw.remain <= 0 {
+		println("error")
+		return 0, errors.New("write limit hit")
+	}
+	if len(p) > lw.remain {
+		p = p[:lw.remain]
+		n, _ = lw.w.Write(p)
+		lw.remain = 0
+		return n, errors.New("write limit hit")
+	}
+	n, err = lw.w.Write(p)
+	lw.remain -= n
+	return n, err
+}
+
+func TestMarshalWriteErrors(t *testing.T) {
+	var buf bytes.Buffer
+	const writeCap = 1024
+	w := &limitedBytesWriter{&buf, writeCap}
+	enc := NewEncoder(w)
+	var err error
+	var i int
+	const n = 4000
+	for i = 1; i <= n; i++ {
+		err = enc.Encode(&Passenger{
+			Name:   []string{"Alice", "Bob"},
+			Weight: 5,
+		})
+		if err != nil {
+			break
+		}
+	}
+	if err == nil {
+		t.Error("expected an error")
+	}
+	if i == n {
+		t.Errorf("expected to fail before the end")
+	}
+	if buf.Len() != writeCap {
+		t.Errorf("buf.Len() = %d; want %d", buf.Len(), writeCap)
+	}
+}
+
 func BenchmarkMarshal(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Marshal(atomValue)