Introduce ActionFunction to make it easy to patch and do extra validations before step starts (#365)

Co-authored-by: Shuchang Zheng <wintonzheng0325@gmail.com>

skyvern/forge/agent_functions.py

@@ -0,0 +1,44 @@
+from playwright.async_api import Page
+
+from skyvern.forge import app
+from skyvern.forge.async_operations import AsyncOperation
+from skyvern.forge.sdk.models import Organization, Step, StepStatus
+from skyvern.forge.sdk.schemas.tasks import Task, TaskStatus
+
+
+class AgentFunction:
+    async def validate_step_execution(
+        self,
+        task: Task,
+        step: Step,
+    ) -> None:
+        """
+        Checks if the step can be executed.
+        :return: A tuple of whether the step can be executed and a list of reasons why it can't be executed.
+        """
+        reasons = []
+        # can't execute if task status is not running
+        has_valid_task_status = task.status == TaskStatus.running
+        if not has_valid_task_status:
+            reasons.append(f"invalid_task_status:{task.status}")
+        # can't execute if the step is already running or completed
+        has_valid_step_status = step.status in [StepStatus.created, StepStatus.failed]
+        if not has_valid_step_status:
+            reasons.append(f"invalid_step_status:{step.status}")
+        # can't execute if the task has another step that is running
+        steps = await app.DATABASE.get_task_steps(task_id=task.task_id, organization_id=task.organization_id)
+        has_no_running_steps = not any(step.status == StepStatus.running for step in steps)
+        if not has_no_running_steps:
+            reasons.append(f"another_step_is_running_for_task:{task.task_id}")
+
+        can_execute = has_valid_task_status and has_valid_step_status and has_no_running_steps
+        if not can_execute:
+            raise Exception(f"Cannot execute step. Reasons: {reasons}, Step: {step}")
+
+    def generate_async_operations(
+        self,
+        organization: Organization,
+        task: Task,
+        page: Page,
+    ) -> list[AsyncOperation]:
+        return []