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read.go
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read.go
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// Copyright 2015 RedHat, Inc.
// Copyright 2015 CoreOS, Inc.
//
// 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 sdjournal
import (
"errors"
"fmt"
"io"
"log"
"strings"
"sync"
"time"
)
var (
// ErrExpired gets returned when the Follow function runs into the
// specified timeout.
ErrExpired = errors.New("Timeout expired")
)
// JournalReaderConfig represents options to drive the behavior of a JournalReader.
type JournalReaderConfig struct {
// The Since, NumFromTail and Cursor options are mutually exclusive and
// determine where the reading begins within the journal. The order in which
// options are written is exactly the order of precedence.
Since time.Duration // start relative to a Duration from now
NumFromTail uint64 // start relative to the tail
Cursor string // start relative to the cursor
// Show only journal entries whose fields match the supplied values. If
// the array is empty, entries will not be filtered.
Matches []Match
// If not empty, the journal instance will point to a journal residing
// in this directory. The supplied path may be relative or absolute.
Path string
// If not nil, Formatter will be used to translate the resulting entries
// into strings. If not set, the default format (timestamp and message field)
// will be used. If Formatter returns an error, Read will stop and return the error.
Formatter func(entry *JournalEntry) (string, error)
}
// JournalReader is an io.ReadCloser which provides a simple interface for iterating through the
// systemd journal. A JournalReader is not safe for concurrent use by multiple goroutines.
type JournalReader struct {
journal *Journal
msgReader *strings.Reader
formatter func(entry *JournalEntry) (string, error)
}
// NewJournalReader creates a new JournalReader with configuration options that are similar to the
// systemd journalctl tool's iteration and filtering features.
func NewJournalReader(config JournalReaderConfig) (*JournalReader, error) {
// use simpleMessageFormatter as default formatter.
if config.Formatter == nil {
config.Formatter = simpleMessageFormatter
}
r := &JournalReader{
formatter: config.Formatter,
}
// Open the journal
var err error
if config.Path != "" {
r.journal, err = NewJournalFromDir(config.Path)
} else {
r.journal, err = NewJournal()
}
if err != nil {
return nil, err
}
// Add any supplied matches
for _, m := range config.Matches {
if err = r.journal.AddMatch(m.String()); err != nil {
return nil, err
}
}
// Set the start position based on options
if config.Since != 0 {
// Start based on a relative time
start := time.Now().Add(config.Since)
if err := r.journal.SeekRealtimeUsec(uint64(start.UnixNano() / 1000)); err != nil {
return nil, err
}
} else if config.NumFromTail != 0 {
// Start based on a number of lines before the tail
if err := r.journal.SeekTail(); err != nil {
return nil, err
}
// Move the read pointer into position near the tail. Go one further than
// the option so that the initial cursor advancement positions us at the
// correct starting point.
skip, err := r.journal.PreviousSkip(config.NumFromTail + 1)
if err != nil {
return nil, err
}
// If we skipped fewer lines than expected, we have reached journal start.
// Thus, we seek to head so that next invocation can read the first line.
if skip != config.NumFromTail+1 {
if err := r.journal.SeekHead(); err != nil {
return nil, err
}
}
} else if config.Cursor != "" {
// Start based on a custom cursor
if err := r.journal.SeekCursor(config.Cursor); err != nil {
return nil, err
}
}
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) {
if r.msgReader == nil {
// Advance the journal cursor. It has to be called at least one time
// before reading
c, err := r.journal.Next()
// An unexpected error
if err != nil {
return 0, err
}
// EOF detection
if c == 0 {
return 0, io.EOF
}
entry, err := r.journal.GetEntry()
if err != nil {
return 0, err
}
// Build a message
msg, err := r.formatter(entry)
if err != nil {
return 0, err
}
r.msgReader = strings.NewReader(msg)
}
// Copy and return the message
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 sz, nil
}
// Close closes the JournalReader's handle to the journal.
func (r *JournalReader) Close() error {
return r.journal.Close()
}
// Rewind attempts to rewind the JournalReader to the first entry.
func (r *JournalReader) Rewind() error {
r.msgReader = nil
return r.journal.SeekHead()
}
// Follow synchronously follows the JournalReader, writing each new journal entry to writer. The
// follow will continue until a single time.Time is received on the until channel.
func (r *JournalReader) Follow(until <-chan time.Time, writer io.Writer) error {
// Process journal entries and events. Entries are flushed until the tail or
// timeout is reached, and then we wait for new events or the timeout.
var msg = make([]byte, 64*1<<(10))
var waitCh = make(chan int, 1)
var waitGroup sync.WaitGroup
defer waitGroup.Wait()
process:
for {
c, err := r.Read(msg)
if err != nil && err != io.EOF {
return err
}
select {
case <-until:
return ErrExpired
default:
}
if c > 0 {
if _, err = writer.Write(msg[:c]); err != nil {
return err
}
continue process
}
// We're at the tail, so wait for new events or time out.
// Holds journal events to process. Tightly bounded for now unless there's a
// reason to unblock the journal watch routine more quickly.
for {
waitGroup.Add(1)
go func() {
status := r.journal.Wait(100 * time.Millisecond)
waitCh <- status
waitGroup.Done()
}()
select {
case <-until:
return ErrExpired
case e := <-waitCh:
switch e {
case SD_JOURNAL_NOP:
// the journal did not change since the last invocation
case SD_JOURNAL_APPEND, SD_JOURNAL_INVALIDATE:
continue process
default:
if e < 0 {
return fmt.Errorf("received error event: %d", e)
}
log.Printf("received unknown event: %d\n", e)
}
}
}
}
}
// simpleMessageFormatter is the default formatter.
// It returns a string representing the current journal entry in a simple format which
// includes the entry timestamp and MESSAGE field.
func simpleMessageFormatter(entry *JournalEntry) (string, error) {
msg, ok := entry.Fields["MESSAGE"]
if !ok {
return "", fmt.Errorf("no MESSAGE field present in journal entry")
}
usec := entry.RealtimeTimestamp
timestamp := time.Unix(0, int64(usec)*int64(time.Microsecond))
return fmt.Sprintf("%s %s\n", timestamp, msg), nil
}