Add per-app flock locking around deploy, config change, rollback, new-app

Nothing previously serialized these against each other, so concurrent
invocations for the same app (a manual config change racing an
in-flight deploy, a double git push, a rollback mid-deploy) could
interleave snapshot/apply/restart/rollback sequences against the same
dataset and jail. The lock file lives under a new LocksDir, outside
any per-app ZFS dataset, since those datasets get rolled back on a
failed health check or a manual rollback.

1 files changed, 43 insertions, 0 deletions

diff --git a/internal/lock/lock.go b/internal/lock/lock.go
new file mode 100644
index 0000000..af24882
--- /dev/null
+++ b/internal/lock/lock.go
@@ -0,0 +1,43 @@
+// Package lock provides per-app exclusive locking, so that concurrent skiff
+// invocations against the same app - a deploy racing a config change, either
+// racing a manual rollback - can't interleave their snapshot/apply/restart
+// sequences against the same dataset and jail.
+package lock
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// Handle is a held lock; call Release to give it up.
+type Handle struct {
+	f *os.File
+}
+
+// Acquire blocks until it holds an exclusive lock on path, creating the file
+// (and its parent directory) if needed. The lock is held via flock(2) on the
+// open file descriptor, so it's released automatically if the process dies -
+// a crash never leaves a stale lock behind.
+func Acquire(path string) (*Handle, error) {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return nil, fmt.Errorf("create lock dir: %w", err)
+	}
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDONLY, 0644)
+	if err != nil {
+		return nil, fmt.Errorf("open lock file: %w", err)
+	}
+	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
+		f.Close()
+		return nil, fmt.Errorf("flock: %w", err)
+	}
+	return &Handle{f: f}, nil
+}
+
+// Release gives up the lock. Safe to call via defer immediately after a
+// successful Acquire.
+func (h *Handle) Release() error {
+	defer h.f.Close()
+	return syscall.Flock(int(h.f.Fd()), syscall.LOCK_UN)
+}