add ReadWithCallback.

* ReadWithCallback can be used to plug custom deserialization
  logic.

read.go

@@ -48,8 +48,8 @@ func unquote(s string) string {
 	return string(u)
 }
 
-func readIntoPass(c *warnings.Collector, config interface{}, fset *token.FileSet,
-	file *token.File, src []byte, subsectPass bool) error {
+func read(c *warnings.Collector, callback func(string,string,string,string,bool)error,
+	fset *token.FileSet, file *token.File, src []byte) error {
 	//
 	var s scanner.Scanner
 	var errs scanner.ErrorList
@@ -122,7 +122,7 @@ func readIntoPass(c *warnings.Collector, config interface{}, fset *token.FileSet
 			// If a section/subsection header was found, ensure a
 			// container object is created, even if there are no
 			// variables further down.
-			err := c.Collect(set(c, config, sect, sectsub, "", true, "", subsectPass))
+			err := c.Collect(callback(sect, sectsub, "", "", true))
 			if err != nil {
 				return err
 			}
@@ -168,7 +168,7 @@ func readIntoPass(c *warnings.Collector, config interface{}, fset *token.FileSet
 					}
 				}
 			}
-			err := set(c, config, sect, sectsub, n, blank, v, subsectPass)
+			err := c.Collect(callback(sect, sectsub, n, v, blank))
 			if err != nil {
 				return err
 			}
@@ -190,17 +190,52 @@ func readInto(config interface{}, fset *token.FileSet, file *token.File,
 	src []byte) error {
 	//
 	c := warnings.NewCollector(isFatal)
-	err := readIntoPass(c, config, fset, file, src, false)
+	firstPassCallback := func(s string, ss string, k string, v string, bv bool) error {
+		return set(c, config, s, ss, k, v, bv, false)
+	}
+	err := read(c, firstPassCallback, fset, file, src)
 	if err != nil {
 		return err
 	}
-	err = readIntoPass(c, config, fset, file, src, true)
+	secondPassCallback := func(s string, ss string, k string, v string, bv bool) error {
+		return set(c, config, s, ss, k, v, bv, true)
+	}
+	err = read(c, secondPassCallback, fset, file, src)
 	if err != nil {
 		return err
 	}
 	return c.Done()
 }
 
+// ReadWithCallback reads gcfg formatted data from reader and calls
+// callback with each section and option found.
+//
+// Callback is called with section, subsection, option key, option value
+// and blank value flag as arguments.
+//
+// When a section is found, callback is called with nil subsection, option key
+// and option value.
+//
+// When a subsection is found, callback is called with nil option key and
+// option value.
+//
+// If blank value flag is true, it means that the value was not set for an option
+// (as opposed to set to empty string).
+//
+// If callback returns an error, ReadWithCallback terminates with an error too.
+func ReadWithCallback(reader io.Reader, callback func(string,string,string,string,bool)error) error {
+	src, err := ioutil.ReadAll(reader)
+	if err != nil {
+		return err
+	}
+
+	fset := token.NewFileSet()
+	file := fset.AddFile("", fset.Base(), len(src))
+	c := warnings.NewCollector(isFatal)
+
+	return read(c, callback, fset, file, src)
+}
+
 // ReadInto reads gcfg formatted data from reader and sets the values into the
 // corresponding fields in config.
 func ReadInto(config interface{}, reader io.Reader) error {

read_test.go

@@ -6,6 +6,9 @@ import (
 	"os"
 	"reflect"
 	"testing"
+	"bytes"
+	"strconv"
+	"github.com/pkg/errors"
 )
 
 const (
@@ -376,3 +379,96 @@ func TestReadStringIntoExtraData(t *testing.T) {
 		t.Errorf("res.Section.Name=%q; want %q", res.Section.Name, "value")
 	}
 }
+
+func TestReadWithCallback(t *testing.T) {
+	results := [][]string{}
+	cb := func(s string, ss string, k string, v string, bv bool) error {
+		results = append(results, []string{s, ss, k, v, strconv.FormatBool(bv)})
+		return nil
+	}
+	text := `
+	[sect1]
+	key1=value1
+	[sect1 "subsect1"]
+	key2=value2
+	key3=value3
+	key4
+	key5=
+	[sect1 "subsect2"]
+	[sect2]
+	`
+	expected := [][]string{
+		[]string{"sect1", "", "", "", "true"},
+		[]string{"sect1", "", "key1", "value1", "false"},
+		[]string{"sect1", "subsect1", "", "", "true"},
+		[]string{"sect1", "subsect1", "key2", "value2", "false"},
+		[]string{"sect1", "subsect1", "key3", "value3", "false"},
+		[]string{"sect1", "subsect1", "key4", "", "true"},
+		[]string{"sect1", "subsect1", "key5", "", "false"},
+		[]string{"sect1", "subsect2", "", "", "true"},
+		[]string{"sect2", "", "", "", "true"},
+	}
+	err := ReadWithCallback(bytes.NewReader([]byte(text)), cb)
+	if err != nil {
+		t.Error(err)
+	}
+	if !reflect.DeepEqual(results, expected) {
+		t.Errorf("expected %+v, got %+v", expected, results)
+	}
+
+	i := 0
+	expectedErr := errors.New("FATAL ERROR")
+	results = [][]string{}
+	cbWithError := func(s string, ss string, k string, v string, bv bool) error {
+		results = append(results, []string{s, ss, k, v, strconv.FormatBool(bv)})
+		i += 1
+		if i == 3 {
+			return expectedErr
+		}
+		return nil
+	}
+	err = ReadWithCallback(bytes.NewReader([]byte(text)), cbWithError)
+	if err != expectedErr {
+		t.Errorf("expected error: %+v", err)
+	}
+	if !reflect.DeepEqual(results, expected[:3]) {
+		t.Errorf("expected %+v, got %+v", expected, results[:3])
+	}
+}
+
+func TestReadWithCallback_WithError(t *testing.T) {
+	results := [][]string{}
+	cb := func(s string, ss string, k string, v string, bv bool) error {
+		results = append(results, []string{s, ss, k, v, strconv.FormatBool(bv)})
+		return nil
+	}
+	text := `
+	[sect1]
+	key1=value1
+	[sect1 "subsect1"]
+	key2=value2
+	key3=value3
+	key4
+	key5=
+	[sect1 "subsect2"]
+	[sect2]
+	`
+	expected := [][]string{
+		[]string{"sect1", "", "", "", "true"},
+		[]string{"sect1", "", "key1", "value1", "false"},
+		[]string{"sect1", "subsect1", "", "", "true"},
+		[]string{"sect1", "subsect1", "key2", "value2", "false"},
+		[]string{"sect1", "subsect1", "key3", "value3", "false"},
+		[]string{"sect1", "subsect1", "key4", "", "true"},
+		[]string{"sect1", "subsect1", "key5", "", "false"},
+		[]string{"sect1", "subsect2", "", "", "true"},
+		[]string{"sect2", "", "", "", "true"},
+	}
+	err := ReadWithCallback(bytes.NewReader([]byte(text)), cb)
+	if err != nil {
+		t.Error(err)
+	}
+	if !reflect.DeepEqual(results, expected) {
+		t.Errorf("expected %+v, got %+v", expected, results)
+	}
+}

set.go

@@ -214,7 +214,7 @@ func newValue(c *warnings.Collector, sect string, vCfg reflect.Value,
 }
 
 func set(c *warnings.Collector, cfg interface{}, sect, sub, name string,
-	blank bool, value string, subsectPass bool) error {
+	 value string, blankValue bool, subsectPass bool) error {
 	//
 	vPCfg := reflect.ValueOf(cfg)
 	if vPCfg.Kind() != reflect.Ptr || vPCfg.Elem().Kind() != reflect.Struct {
@@ -285,7 +285,7 @@ func set(c *warnings.Collector, cfg interface{}, sect, sub, name string,
 		}
 		vVar = vVar.Elem()
 	}
-	if isMulti && blank {
+	if isMulti && blankValue {
 		vVar.Set(reflect.Zero(vVar.Type()))
 		return nil
 	}
@@ -309,7 +309,7 @@ func set(c *warnings.Collector, cfg interface{}, sect, sub, name string,
 	vAddrI := vAddr.Interface()
 	err, ok := error(nil), false
 	for _, s := range setters {
-		err = s(vAddrI, blank, value, t)
+		err = s(vAddrI, blankValue, value, t)
 		if err == nil {
 			ok = true
 			break