container/output: export suspendable writer

This is quite useful for other packages as well. This change prepares internal/hlog for removal.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/hlog/hlog.go

@@ -2,69 +2,17 @@
 package hlog
 
 import (
-	"bytes"
-	"io"
 	"log"
 	"os"
-	"sync"
-	"sync/atomic"
-	"syscall"
+
+	"hakurei.app/container"
 )
 
-const (
-	bufSize    = 4 * 1024
-	bufSizeMax = 16 * 1024 * 1024
-)
-
-var o = &suspendable{w: os.Stderr}
+var o = &container.Suspendable{Downstream: os.Stderr}
 
 // Prepare configures the system logger for [Suspend] and [Resume] to take effect.
 func Prepare(prefix string) { log.SetPrefix(prefix + ": "); log.SetFlags(0); log.SetOutput(o) }
 
-type suspendable struct {
-	w io.Writer
-	s atomic.Bool
-
-	buf     bytes.Buffer
-	bufOnce sync.Once
-	bufMu   sync.Mutex
-	dropped int
-}
-
-func (s *suspendable) Write(p []byte) (n int, err error) {
-	if !s.s.Load() {
-		return s.w.Write(p)
-	}
-	s.bufOnce.Do(func() { s.prepareBuf() })
-
-	s.bufMu.Lock()
-	defer s.bufMu.Unlock()
-
-	if l := len(p); s.buf.Len()+l > bufSizeMax {
-		s.dropped += l
-		return 0, syscall.ENOMEM
-	}
-	return s.buf.Write(p)
-}
-
-func (s *suspendable) prepareBuf()   { s.buf.Grow(bufSize) }
-func (s *suspendable) Suspend() bool { return o.s.CompareAndSwap(false, true) }
-func (s *suspendable) Resume() (resumed bool, dropped uintptr, n int64, err error) {
-	if o.s.CompareAndSwap(true, false) {
-		o.bufMu.Lock()
-		defer o.bufMu.Unlock()
-
-		resumed = true
-		dropped = uintptr(o.dropped)
-
-		o.dropped = 0
-		n, err = io.Copy(s.w, &s.buf)
-		s.buf = bytes.Buffer{}
-		s.prepareBuf()
-	}
-	return
-}
-
 func Suspend() bool { return o.Suspend() }
 func Resume() bool {
 	resumed, dropped, _, err := o.Resume()