1 files changed, 86 insertions, 0 deletions
diff --git a/internal/hlog/hlog.go b/internal/hlog/hlog.go
new file mode 100644
index 00000000..415d16bf
--- /dev/null
+++ b/internal/hlog/hlog.go
@@ -0,0 +1,86 @@
+// Package hlog provides various functions for output messages.
+package hlog
+
+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")
+	}
+}