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// Package fmsg provides various functions for output messages.
package fmsg
import (
"bytes"
"io"
"log"
"os"
"sync"
"sync/atomic"
"syscall"
)
const (
bufSize = 4 * 1024
bufSizeMax = 16 * 1024 * 1024
)
var o = &suspendable{w: 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()
if err != nil {
// probably going to result in an error as well,
// so this call is as good as unreachable
log.Printf("cannot dump buffer on resume: %v", err)
}
if resumed && dropped > 0 {
log.Fatalf("dropped %d bytes while output is suspended", dropped)
}
return resumed
}
func BeforeExit() {
if Resume() {
log.Printf("beforeExit reached on suspended output")
}
}
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