JournalReader: fixes in JournalReader.Read()

This patch fixes two issues in JournalReader.Read():

1. The return value of copy() was ignored and JournalReader.Read() was
returning the wrong amount of bytes copied.

2. JournalReader implements the Reader interface so it must be able to read a
specific amount of bytes. Journald on the other hand only allows us to read
full entries of arbitrary size (without byte granularity). JournalReader
therefore needs to internally buffer entries that don't fit in the read buffer.
Callers should keep calling until 0 and/or an error is returned.

Fixes #172

Author: alban

sdjournal/read.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"strings"
 	"time"
 )
 
@@ -46,9 +47,10 @@ type JournalReaderConfig struct {
 }
 
 // JournalReader is an io.ReadCloser which provides a simple interface for iterating through the
-// systemd journal.
+// systemd journal. A JournalReader is not safe for concurrent use by multiple goroutines.
 type JournalReader struct {
-	journal *Journal
+	journal   *Journal
+	msgReader *strings.Reader
 }
 
 // NewJournalReader creates a new JournalReader with configuration options that are similar to the
@@ -101,35 +103,60 @@ func NewJournalReader(config JournalReaderConfig) (*JournalReader, error) {
 	return r, nil
 }
 
+// Read reads entries from the journal. Read follows the Reader interface so
+// it must be able to read a specific amount of bytes. Journald on the other
+// hand only allows us to read full entries of arbitrary size (without byte
+// granularity). JournalReader is therefore internally buffering entries that
+// don't fit in the read buffer. Callers should keep calling until 0 and/or an
+// error is returned.
 func (r *JournalReader) Read(b []byte) (int, error) {
 	var err error
-	var c int
 
-	// Advance the journal cursor
-	c, err = r.journal.Next()
+	if r.msgReader == nil {
+		var c int
 
-	// An unexpected error
-	if err != nil {
-		return 0, err
-	}
+		// Advance the journal cursor. It has to be called at least one time
+		// before reading
+		c, err = r.journal.Next()
 
-	// EOF detection
-	if c == 0 {
-		return 0, io.EOF
-	}
+		// An unexpected error
+		if err != nil {
+			return 0, err
+		}
 
-	// Build a message
-	var msg string
-	msg, err = r.buildMessage()
+		// EOF detection
+		if c == 0 {
+			return 0, io.EOF
+		}
 
-	if err != nil {
-		return 0, err
+		// Build a message
+		var msg string
+		msg, err = r.buildMessage()
+
+		if err != nil {
+			return 0, err
+		}
+		r.msgReader = strings.NewReader(msg)
 	}
 
 	// Copy and return the message
-	copy(b, []byte(msg))
+	var sz int
+	sz, err = r.msgReader.Read(b)
+	if err == io.EOF {
+		// The current entry has been fully read. Don't propagate this
+		// EOF, so the next entry can be read at the next Read()
+		// iteration.
+		r.msgReader = nil
+		return sz, nil
+	}
+	if err != nil {
+		return sz, err
+	}
+	if r.msgReader.Len() == 0 {
+		r.msgReader = nil
+	}
 
-	return len(msg), nil
+	return sz, nil
 }
 
 // Close closes the JournalReader's handle to the journal.
@@ -139,6 +166,7 @@ func (r *JournalReader) Close() error {
 
 // Rewind attempts to rewind the JournalReader to the first entry.
 func (r *JournalReader) Rewind() error {
+	r.msgReader = nil
 	return r.journal.SeekHead()
 }