2 files changed, 54 insertions, 0 deletions

diff --git a/internal/layout/layout.go b/internal/layout/layout.go
index d5888dc..5c2cf30 100644
--- a/internal/layout/layout.go
+++ b/internal/layout/layout.go
@@ -22,6 +22,7 @@ type Config struct {
 	JailsMount   string // parent mountpoint for per-app datasets
 	JailConfDir  string // /etc/jail.conf.d
 	NginxConfDir string // nginx include dir regenerated on every deploy
+	LocksDir     string // per-app flock files live here, outside any per-app dataset
 }
 
 func Default() Config {
@@ -32,6 +33,7 @@ func Default() Config {
 		JailsMount:   "/skiff/apps",
 		JailConfDir:  "/etc/jail.conf.d",
 		NginxConfDir: "/usr/local/etc/nginx/skiff.d",
+		LocksDir:     "/skiff/locks",
 	}
 }
 
@@ -47,6 +49,15 @@ func (c Config) RepoPath(app string) string {
 	return filepath.Join(c.ReposDir, app+".git")
 }
 
+// LockPath is the per-app flock file that serializes deploy, config change,
+// and rollback against each other. It deliberately lives outside
+// AppDataset(app): that dataset gets `zfs rollback`ed on a failed health
+// check or a manual rollback, which would discard a lock file living inside
+// it along with everything else written since the snapshot.
+func (c Config) LockPath(app string) string {
+	return filepath.Join(c.LocksDir, app+".lock")
+}
+
 func (c Config) JailConfPath(app string) string {
 	return filepath.Join(c.JailConfDir, app+".conf")
 }
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)
+}