Make logging configurable (dgraph-io#605)

* added Logger interface to attach logging systems.

Closes dgraph-io#556

.gitignore

@@ -0,0 +1 @@
+p/

backup.go

@@ -20,12 +20,10 @@ import (
 	"bufio"
 	"encoding/binary"
 	"io"
-	"log"
 	"sync"
 
-	"github.com/dgraph-io/badger/y"
-
 	"github.com/dgraph-io/badger/protos"
+	"github.com/dgraph-io/badger/y"
 )
 
 func writeTo(entry *protos.KVPair, w io.Writer) error {
@@ -63,7 +61,7 @@ func (db *DB) Backup(w io.Writer, since uint64) (uint64, error) {
 			}
 			valCopy, err := item.ValueCopy(nil)
 			if err != nil {
-				log.Printf("Key [%x]. Error while fetching value [%v]\n", item.Key(), err)
+				Errorf("Key [%x]. Error while fetching value [%v]\n", item.Key(), err)
 				continue
 			}
 

db.go

@@ -20,7 +20,6 @@ import (
 	"bytes"
 	"encoding/binary"
 	"expvar"
-	"log"
 	"math"
 	"os"
 	"path/filepath"
@@ -30,13 +29,11 @@ import (
 	"time"
 
 	"github.com/dgraph-io/badger/options"
-
-	"golang.org/x/net/trace"
-
 	"github.com/dgraph-io/badger/skl"
 	"github.com/dgraph-io/badger/table"
 	"github.com/dgraph-io/badger/y"
 	"github.com/pkg/errors"
+	"golang.org/x/net/trace"
 )
 
 var (
@@ -683,7 +680,7 @@ func (db *DB) doWrites(lc *y.Closer) {
 
 	writeRequests := func(reqs []*request) {
 		if err := db.writeRequests(reqs); err != nil {
-			log.Printf("ERROR in Badger::writeRequests: %v", err)
+			Errorf("writeRequests: %v", err)
 		}
 		<-pendingCh
 	}
@@ -903,7 +900,7 @@ func (db *DB) flushMemtable(lc *y.Closer) error {
 				break
 			}
 			// Encounterd error. Retry indefinitely.
-			log.Printf("Error while flushing memtable to disk: %v. Retrying...\n", err)
+			Errorf("Failure while flushing memtable to disk: %v. Retrying...\n", err)
 			time.Sleep(time.Second)
 		}
 	}
@@ -1260,7 +1257,7 @@ func (op *MergeOperator) runCompactions(dur time.Duration) {
 		case <-ticker.C: // wait for tick
 		}
 		if err := op.compact(); err != nil {
-			log.Printf("Error while running merge operation: %s", err)
+			Errorf("failure while running merge operation: %s", err)
 		}
 		if stop {
 			ticker.Stop()

logger.go

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2018 Dgraph Labs, Inc. and Contributors
+ *
+ * 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 badger
+
+import (
+	"log"
+	"os"
+)
+
+// Logger is implemented by any logging system that is used for standard logs.
+type Logger interface {
+	Errorf(string, ...interface{})
+	Infof(string, ...interface{})
+	Warningf(string, ...interface{})
+}
+
+var badgerLogger Logger
+
+func SetLogger(l Logger) { badgerLogger = l }
+
+func Errorf(format string, v ...interface{}) {
+	badgerLogger.Errorf(format, v...)
+}
+
+func Infof(format string, v ...interface{}) {
+	badgerLogger.Infof(format, v...)
+}
+
+func Warningf(format string, v ...interface{}) {
+	badgerLogger.Warningf(format, v...)
+}
+
+type defaultLog struct {
+	*log.Logger
+}
+
+var defaultLogger = &defaultLog{Logger: log.New(os.Stderr, "badger", log.LstdFlags)}
+
+func UseDefaultLogger() { SetLogger(defaultLogger) }
+
+func (l *defaultLog) Errorf(f string, v ...interface{}) {
+	l.Printf("ERROR: "+f, v...)
+}
+
+func (l *defaultLog) Infof(f string, v ...interface{}) {
+	l.Printf("INFO: "+f, v...)
+}
+
+func (l *defaultLog) Warningf(f string, v ...interface{}) {
+	l.Printf("WARNING: "+f, v...)
+}
+
+func init() {
+	UseDefaultLogger()
+}

value.go

@@ -24,7 +24,6 @@ import (
 	"hash/crc32"
 	"io"
 	"io/ioutil"
-	"log"
 	"math"
 	"math/rand"
 	"os"
@@ -36,7 +35,6 @@ import (
 	"time"
 
 	"github.com/dgraph-io/badger/options"
-
 	"github.com/dgraph-io/badger/y"
 	"github.com/pkg/errors"
 	"golang.org/x/net/trace"
@@ -403,7 +401,7 @@ func (vlog *valueLog) rewrite(f *logFile, tr trace.Trace) error {
 				wb = wb[:0]
 			}
 		} else {
-			log.Printf("WARNING: This entry should have been caught. %+v\n", e)
+			Warningf("This entry should have been caught. %+v\n", e)
 		}
 		return nil
 	}
@@ -421,7 +419,7 @@ func (vlog *valueLog) rewrite(f *logFile, tr trace.Trace) error {
 	for i := 0; i < len(wb); {
 		loops++
 		if batchSize == 0 {
-			log.Printf("WARNING: We shouldn't reach batch size of zero.")
+			Warningf("We shouldn't reach batch size of zero.")
 			return ErrNoRewrite
 		}
 		end := i + batchSize