3 files changed, 76 insertions, 11 deletions

diff --git a/pkg/database/locks.go b/pkg/database/locks.go
new file mode 100644
index 0000000..53c6f6e
--- /dev/null
+++ b/pkg/database/locks.go
@@ -0,0 +1,50 @@
+package database
+
+import (
+	"encoding/json"
+)
+
+// Atomically check if there is an existing lock in place on the state. Returns
+// true if it can be set, otherwise returns false and lock is set to the value
+// of the existing lock
+func (db *DB) SetLockOrGetExistingLock(name string, lock any) (bool, error) {
+	tx, err := db.Begin()
+	if err != nil {
+		return false, err
+	}
+	defer func() {
+		if err != nil {
+			_ = tx.Rollback()
+		}
+	}()
+	var data []byte
+	if err = tx.QueryRowContext(db.ctx, `SELECT lock FROM states WHERE name = ?;`, name).Scan(&data); err != nil {
+		return false, err
+	}
+	if data != nil {
+		err = json.Unmarshal(data, lock)
+		return false, err
+	}
+	if data, err = json.Marshal(lock); err != nil {
+		return false, err
+	}
+	_, err = tx.Exec(`UPDATE states SET lock = json(?) WHERE name = ?;`, data, name)
+	if err != nil {
+		return false, err
+	}
+	err = tx.Commit()
+	return true, err
+}
+
+func (db *DB) Unlock(name, lock any) (bool, error) {
+	data, err := json.Marshal(lock)
+	if err != nil {
+		return false, err
+	}
+	result, err := db.Exec(`UPDATE states SET lock = NULL WHERE name = ? and lock = json(?);`, name, data)
+	if err != nil {
+		return false, err
+	}
+	n, err := result.RowsAffected()
+	return n == 1, err
+}
diff --git a/pkg/database/sql/000_init.sql b/pkg/database/sql/000_init.sql
index 0248896..6ec2ef7 100644
--- a/pkg/database/sql/000_init.sql
+++ b/pkg/database/sql/000_init.sql
@@ -5,6 +5,7 @@ CREATE TABLE schema_version (
 CREATE TABLE states (
   id INTEGER PRIMARY KEY,
   name TEXT NOT NULL,
-  data BLOB NOT NULL
+  data BLOB NOT NULL,
+  lock TEXT
 ) STRICT;
 CREATE UNIQUE INDEX states_name on states(name);
diff --git a/pkg/database/states.go b/pkg/database/states.go
index 3a0b0b9..e848924 100644
--- a/pkg/database/states.go
+++ b/pkg/database/states.go
@@ -2,6 +2,7 @@ package database
 
 import (
 	"database/sql"
+	"fmt"
 )
 
 func (db *DB) DeleteState(name string) error {
@@ -18,18 +19,31 @@ func (db *DB) GetState(name string) ([]byte, error) {
 	return db.dataEncryptionKey.DecryptAES256(encryptedData)
 }
 
-func (db *DB) SetState(name string, data []byte) error {
+// returns true in case of id mismatch
+func (db *DB) SetState(name string, data []byte, id string) (bool, error) {
 	encryptedData, err := db.dataEncryptionKey.EncryptAES256(data)
 	if err != nil {
-		return err
+		return false, err
 	}
-	_, err = db.Exec(
-		`INSERT INTO states(name, data) VALUES (:name, :data) ON CONFLICT DO UPDATE SET data = :data WHERE name = :name;`,
-		sql.Named("data", encryptedData),
-		sql.Named("name", name),
-	)
-	if err != nil {
-		return err
+	if id == "" {
+		_, err = db.Exec(
+			`INSERT INTO states(name, data) VALUES (:name, :data) ON CONFLICT DO UPDATE SET data = :data WHERE name = :name;`,
+			sql.Named("data", encryptedData),
+			sql.Named("name", name),
+		)
+		return false, err
+	} else {
+		result, err := db.Exec(`UPDATE states SET data = ? WHERE name = ? and lock->>'ID' = ?;`, encryptedData, name, id)
+		if err != nil {
+			return false, err
+		}
+		n, err := result.RowsAffected()
+		if err != nil {
+			return false, err
+		}
+		if n != 1 {
+			return true, fmt.Errorf("failed to update state, lock ID does not match")
+		}
+		return false, nil
 	}
-	return nil
 }