feat: add resilient server recovery and talent upgrades

go/internal/server/accountstate/writer_lock_unix.go

@@ -0,0 +1,51 @@
+//go:build linux || darwin
+
+package accountstate
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"sync"
+
+	"golang.org/x/sys/unix"
+)
+
+type writerLock struct {
+	file *os.File
+	once sync.Once
+	err  error
+}
+
+func acquireWriterLock(path string) (*writerLock, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return nil, fmt.Errorf("accountstate: open writer lock: %w", err)
+	}
+	if err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB); err != nil {
+		_ = file.Close()
+		if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+			return nil, fmt.Errorf("%w: %s", ErrWriterLocked, path)
+		}
+		return nil, fmt.Errorf("accountstate: acquire writer lock: %w", err)
+	}
+	return &writerLock{file: file}, nil
+}
+
+func (l *writerLock) release() error {
+	if l == nil {
+		return nil
+	}
+	l.once.Do(func() {
+		unlockErr := unix.Flock(int(l.file.Fd()), unix.LOCK_UN)
+		closeErr := l.file.Close()
+		if unlockErr != nil {
+			unlockErr = fmt.Errorf("accountstate: release writer lock: %w", unlockErr)
+		}
+		if closeErr != nil {
+			closeErr = fmt.Errorf("accountstate: close writer lock: %w", closeErr)
+		}
+		l.err = errors.Join(unlockErr, closeErr)
+	})
+	return l.err
+}