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moby--moby/engine/streams.go

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package engine
import (
"bufio"
"container/ring"
"fmt"
"io"
"sync"
)
type Output struct {
sync.Mutex
dests []io.Writer
tasks sync.WaitGroup
}
// NewOutput returns a new Output object with no destinations attached.
// Writing to an empty Output will cause the written data to be discarded.
func NewOutput() *Output {
return &Output{}
}
// Add attaches a new destination to the Output. Any data subsequently written
// to the output will be written to the new destination in addition to all the others.
// This method is thread-safe.
// FIXME: Add cannot fail
func (o *Output) Add(dst io.Writer) error {
o.Mutex.Lock()
defer o.Mutex.Unlock()
o.dests = append(o.dests, dst)
return nil
}
// AddPipe creates an in-memory pipe with io.Pipe(), adds its writing end as a destination,
// and returns its reading end for consumption by the caller.
// This is a rough equivalent similar to Cmd.StdoutPipe() in the standard os/exec package.
// This method is thread-safe.
func (o *Output) AddPipe() (io.Reader, error) {
r, w := io.Pipe()
o.Add(w)
return r, nil
}
// AddTail starts a new goroutine which will read all subsequent data written to the output,
// line by line, and append the last `n` lines to `dst`.
func (o *Output) AddTail(dst *[]string, n int) error {
src, err := o.AddPipe()
if err != nil {
return err
}
o.tasks.Add(1)
go func() {
defer o.tasks.Done()
Tail(src, n, dst)
}()
return nil
}
// AddString starts a new goroutine which will read all subsequent data written to the output,
// line by line, and store the last line into `dst`.
func (o *Output) AddString(dst *string) error {
src, err := o.AddPipe()
if err != nil {
return err
}
o.tasks.Add(1)
go func() {
defer o.tasks.Done()
lines := make([]string, 0, 1)
Tail(src, 1, &lines)
if len(lines) == 0 {
*dst = ""
} else {
*dst = lines[0]
}
}()
return nil
}
// Write writes the same data to all registered destinations.
// This method is thread-safe.
func (o *Output) Write(p []byte) (n int, err error) {
o.Mutex.Lock()
defer o.Mutex.Unlock()
var firstErr error
for _, dst := range o.dests {
_, err := dst.Write(p)
if err != nil && firstErr == nil {
firstErr = err
}
}
return len(p), firstErr
}
// Close unregisters all destinations and waits for all background
// AddTail and AddString tasks to complete.
// The Close method of each destination is called if it exists.
func (o *Output) Close() error {
o.Mutex.Lock()
defer o.Mutex.Unlock()
var firstErr error
for _, dst := range o.dests {
if closer, ok := dst.(io.WriteCloser); ok {
err := closer.Close()
if err != nil && firstErr == nil {
firstErr = err
}
}
}
o.tasks.Wait()
return firstErr
}
type Input struct {
src io.Reader
sync.Mutex
}
// NewInput returns a new Input object with no source attached.
// Reading to an empty Input will return io.EOF.
func NewInput() *Input {
return &Input{}
}
// Read reads from the input in a thread-safe way.
func (i *Input) Read(p []byte) (n int, err error) {
i.Mutex.Lock()
defer i.Mutex.Unlock()
if i.src == nil {
return 0, io.EOF
}
return i.src.Read(p)
}
// Add attaches a new source to the input.
// Add can only be called once per input. Subsequent calls will
// return an error.
func (i *Input) Add(src io.Reader) error {
i.Mutex.Lock()
defer i.Mutex.Unlock()
if i.src != nil {
return fmt.Errorf("Maximum number of sources reached: 1")
}
i.src = src
return nil
}
// Tail reads from `src` line per line, and returns the last `n` lines as an array.
// A ring buffer is used to only store `n` lines at any time.
func Tail(src io.Reader, n int, dst *[]string) {
scanner := bufio.NewScanner(src)
r := ring.New(n)
for scanner.Scan() {
if n == 0 {
continue
}
r.Value = scanner.Text()
r = r.Next()
}
r.Do(func(v interface{}) {
if v == nil {
return
}
*dst = append(*dst, v.(string))
})
}
// AddEnv starts a new goroutine which will decode all subsequent data
// as a stream of json-encoded objects, and point `dst` to the last
// decoded object.
// The result `env` can be queried using the type-neutral Env interface.
// It is not safe to query `env` until the Output is closed.
func (o *Output) AddEnv() (dst *Env, err error) {
src, err := o.AddPipe()
if err != nil {
return nil, err
}
dst = &Env{}
o.tasks.Add(1)
go func() {
defer o.tasks.Done()
decoder := NewDecoder(src)
for {
env, err := decoder.Decode()
if err != nil {
return
}
*dst = *env
}
}()
return dst, nil
}