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follow.go
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follow.go
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// Copyright 2017 Marcus Heese
//
// 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 journal
import (
"io"
"time"
"github.com/coreos/go-systemd/sdjournal"
"github.com/elastic/beats/libbeat/logp"
)
// SD_JOURNAL_FIELD_CATALOG_ENTRY stores the name of the JournalEntry field to export Catalog entry to.
const SD_JOURNAL_FIELD_CATALOG_ENTRY = "CATALOG_ENTRY"
// Follow follows the journald and writes the entries to the output channel
// It is a slightly reworked version of sdjournal.Follow to fit our needs.
func Follow(journal *sdjournal.Journal, stop <-chan struct{}) <-chan *sdjournal.JournalEntry {
readEntry := func(journal *sdjournal.Journal) (*sdjournal.JournalEntry, error) {
c, err := journal.Next()
if err != nil {
return nil, err
}
if c == 0 {
return nil, io.EOF
}
entry, err := journal.GetEntry()
if err != nil {
return nil, err
}
return entry, nil
}
out := make(chan *sdjournal.JournalEntry)
go func(journal *sdjournal.Journal, stop <-chan struct{}, out chan<- *sdjournal.JournalEntry) {
defer close(out)
eventWaitCh := make(chan int)
process:
for {
select {
case <-stop:
return
default:
}
entry, err := readEntry(journal)
if err != nil && err != io.EOF {
if cursor, cerr := journal.GetCursor(); cerr != nil {
logp.Warn("Received unknown error when reading a new entry: %v, cursor read error: %v", err, cerr)
} else {
logp.Warn("Received unknown error when reading a new entry: %v, cursor: %s", err, cursor)
}
continue
}
if entry != nil {
if _, ok := entry.Fields[sdjournal.SD_JOURNAL_FIELD_MESSAGE_ID]; ok {
if catalogEntry, err := journal.GetCatalog(); err == nil {
entry.Fields[SD_JOURNAL_FIELD_CATALOG_ENTRY] = catalogEntry
}
}
// non-blocking return
select {
case <-stop:
return
case out <- entry:
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 {
go func() {
select {
case <-stop:
case eventWaitCh <- journal.Wait(100 * time.Millisecond):
}
}()
select {
case <-stop:
return
case e := <-eventWaitCh:
switch e {
case sdjournal.SD_JOURNAL_NOP:
// the journal did not change since the last invocation
case sdjournal.SD_JOURNAL_APPEND, sdjournal.SD_JOURNAL_INVALIDATE:
continue process
default:
logp.Err("Received unknown event: %d", e)
}
}
}
}
}(journal, stop, out)
return out
}