dotcloud/docker

package loggerutils // import "github.com/docker/docker/daemon/logger/loggerutils"

import (
    "context"
    "fmt"
    "io"
    "os"

    "github.com/containerd/log"
    "github.com/docker/docker/daemon/logger"
    "github.com/pkg/errors"
)

type follow struct {
    LogFile   *LogFile
    Watcher   *logger.LogWatcher
    Decoder   Decoder
    Forwarder *forwarder

    log *log.Entry
    c   chan logPos
}

// Do follows the log file as it is written, starting from f at read.
func (fl *follow) Do(f *os.File, read logPos) {
    fl.log = log.G(context.TODO()).WithFields(log.Fields{
        "module": "logger",
        "file":   f.Name(),
    })
    // Optimization: allocate the write-notifications channel only once and
    // reuse it for multiple invocations of nextPos().
    fl.c = make(chan logPos, 1)

    defer func() {
        if err := f.Close(); err != nil && !errors.Is(err, os.ErrClosed) {
            fl.log.WithError(err).Warn("error closing current log file")
        }
    }()

    for {
        wrote, ok := fl.nextPos(read)
        if !ok {
            return
        }

        if wrote.rotation != read.rotation {
            // Flush the current file before moving on to the next.
            if _, err := f.Seek(read.size, io.SeekStart); err != nil {
                fl.Watcher.Err <- err
                return
            }
            if !fl.forward(f) {
                return
            }

            // Open the new file, which has the same name as the old
            // file thanks to file rotation. Make no mistake: they
            // are different files, with distinct identities.
            // Atomically capture the wrote position to make
            // absolutely sure that the position corresponds to the
            // file we have opened; more rotations could have
            // occurred since we previously received it.
            if err := f.Close(); err != nil {
                fl.log.WithError(err).Warn("error closing rotated log file")
            }
            var err error
            func() {
                fl.LogFile.fsopMu.RLock()
                st := <-fl.LogFile.read
                defer func() {
                    fl.LogFile.read <- st
                    fl.LogFile.fsopMu.RUnlock()
                }()
                f, err = open(f.Name())
                wrote = st.pos
            }()
            // We tried to open the file inside a critical section
            // so we shouldn't have been racing the rotation of the
            // file. Any error, even fs.ErrNotFound, is exceptional.
            if err != nil {
                fl.Watcher.Err <- fmt.Errorf("logger: error opening log file for follow after rotation: %w", err)
                return
            }

            if nrot := wrote.rotation - read.rotation; nrot > 1 {
                fl.log.WithField("missed-rotations", nrot).
                    Warn("file rotations were missed while following logs; some log messages have been skipped over")
            }

            // Set up our read position to start from the top of the file.
            read.size = 0
        }

        if !fl.forward(io.NewSectionReader(f, read.size, wrote.size-read.size)) {
            return
        }
        read = wrote
    }
}

// nextPos waits until the write position of the LogFile being followed has
// advanced from current and returns the new position.
func (fl *follow) nextPos(current logPos) (next logPos, ok bool) {
    var st logReadState
    select {
    case <-fl.Watcher.WatchConsumerGone():
        return current, false
    case st = <-fl.LogFile.read:
    }

    // Have any logs been written since we last checked?
    if st.pos == current { // Nope.
        // Add ourself to the notify list.
        st.wait = append(st.wait, fl.c)
    } else { // Yes.
        // "Notify" ourself immediately.
        fl.c <- st.pos
    }
    fl.LogFile.read <- st

    select {
    case <-fl.LogFile.closed: // No more logs will be written.
        select { // Have we followed to the end?
        case next = <-fl.c: // No: received a new position.
        default: // Yes.
            return current, false
        }
    case <-fl.Watcher.WatchConsumerGone():
        return current, false
    case next = <-fl.c:
    }
    return next, true
}

// forward decodes log messages from r and forwards them to the log watcher.
//
// The return value, cont, signals whether following should continue.
func (fl *follow) forward(r io.Reader) (cont bool) {
    fl.Decoder.Reset(r)
    return fl.Forwarder.Do(fl.Watcher, fl.Decoder)
}