feat: Add optional runner.post_task_script hook after task cleanup (#1026)

- Adds `runner.post_task_script` and `runner.post_task_script_timeout` (default `5m`) to run a host executable after each task’s built-in cleanup (post-steps, container teardown, bind-workdir removal).
- Stops task heartbeats via `Reporter.StopHeartbeats()` while the script runs so Gitea won’t assign overlapping work; the final task acknowledgement still happens in `reporter.Close()`.
- Script output goes to the runner process log; non-zero exits are warned only and do not change the job result.
- Documents lifecycle, offline behavior, timeouts, and Windows limits (`.ps1` not supported yet) in `docs/post-task-script.md`.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1026
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

docs/post-task-script.md

@@ -0,0 +1,155 @@
+# Post-task script
+
+The post-task script is an optional host hook that runs **once after every task**, after the runner has already finished its normal per-task cleanup. Typical uses include pruning Docker images, vacuuming ephemeral disks, or resetting VM state between jobs.
+
+It is configured under `runner.post_task_script` in the runner YAML config (see [config.example.yaml](../internal/pkg/config/config.example.yaml)).
+
+## When it runs
+
+For each task, execution order is:
+
+1. Workflow runs (steps, actions, containers).
+2. In-job cleanup (action `post:` steps, container stop/remove).
+3. Job outputs are reported to Gitea.
+4. Bind-workdir workspace removal, when `container.bind_workdir` is enabled.
+5. **Post-task script** (this hook).
+6. Final task acknowledgement to Gitea (`reporter.Close()`).
+
+The script is **additive**: it does not replace any built-in cleanup. When `container.bind_workdir` is enabled, the task workspace directory has usually already been deleted before the script starts. `GITEA_WORKSPACE` is still set to the path the job used, for reference.
+
+## Runner stays offline until the script finishes
+
+This is the most important operational detail.
+
+When the post-task script starts, the runner **stops sending task heartbeats** to Gitea (the same mechanism used during cancel/cleanup). From Gitea's perspective, the runner is **not available for new work** until:
+
+1. The script exits (success or failure), **and**
+2. The runner sends the final task flush to Gitea.
+
+While the script runs:
+
+- **Gitea will not assign another task** to this runner for the current job slot (heartbeats are stopped).
+- **The runner capacity slot stays occupied** locally — with `capacity: 1`, the poller will not start another task until the script completes.
+- **Runner shutdown** (`shutdown_timeout`) counts this phase as part of the in-flight task; a long or stuck script delays graceful shutdown.
+
+If the script **never exits**, the runner remains in this state until `runner.post_task_script_timeout` elapses (default **5 minutes** when a script is configured). The runner then kills the script process and proceeds to the final acknowledgement. Until that timeout fires, **the runner effectively stays offline**.
+
+Set `post_task_script_timeout` to a value that matches how long your housekeeping is allowed to take — not how long you wish it could take. Prefer short, bounded scripts.
+
+### Recommendations
+
+- Keep scripts **fast and bounded** (seconds, not minutes).
+- Avoid interactive prompts, blocking network calls without timeouts, or waiting on user input.
+- Use **idempotent** operations (the script may run after success, failure, or cancellation).
+- Test failure modes: hung script, non-zero exit, missing executable.
+- Watch the **runner process log** for script output (it is not written to the Gitea job log).
+- On shutdown, ensure scripts respond to process termination within `post_task_script_timeout`.
+
+## Configuration
+
+```yaml
+runner:
+  # Path to an executable on the host. Empty or omitted disables the hook.
+  post_task_script: /usr/local/bin/gitea-post-task.sh
+
+  # Hard limit on script runtime. Default when post_task_script is set: 5m.
+  # If the script exceeds this, it is killed and the runner continues.
+  post_task_script_timeout: 2m
+```
+
+| Option | Default | Description |
+| --- | --- | --- |
+| `runner.post_task_script` | *(disabled)* | Host path to the script or binary. Relative paths are resolved from the runner process working directory. |
+| `runner.post_task_script_timeout` | `5m` (only when script is set) | Maximum time the script may run before the runner kills it and moves on. |
+
+The script must be **executable** on the host (shebang on Linux/macOS, or a native `.exe` / `.bat` / `.cmd` on Windows). **PowerShell (`.ps1`) is not supported yet** as the value of `post_task_script`; the runner executes the configured path directly and does not invoke `powershell.exe` for you.
+
+`gitea-runner exec` does **not** load runner YAML and will not run this hook.
+
+## Environment variables
+
+The script receives `runner.envs` / `runner.env_file` values plus:
+
+| Variable | Description |
+| --- | --- |
+| `GITEA_TASK_ID` | Numeric task ID. |
+| `GITEA_RUN_ID` | Workflow run ID, when provided by the server. |
+| `GITEA_REPOSITORY` | Repository slug (`owner/name`). |
+| `GITEA_WORKSPACE` | Workspace path used for the job (may already be deleted). |
+| `GITEA_JOB_RESULT` | `success`, `failure`, `cancelled`, `skipped`, or `unknown`. |
+
+The script environment is **not** a full copy of the job container environment. System variables such as `PATH` are only present if you define them in `runner.envs` or `runner.env_file`.
+
+## Output and errors
+
+- **Stdout/stderr** are written to the **runner process log** (logrus), prefixed with `post-task script stdout:` / `post-task script stderr:`.
+- **Non-zero exit codes** are logged as warnings only. They do **not** change the job result already reported to Gitea.
+- **Timeouts and start failures** are logged as warnings; the runner still completes the task acknowledgement.
+
+## Interaction with other timeouts
+
+| Timeout | Effect on post-task script |
+| --- | --- |
+| `runner.post_task_script_timeout` | Kills the script if it runs too long. This is the **only** timeout that bounds the script. |
+| `runner.timeout` | Caps the task **up to** the script. The script detaches from the task deadline, so a job near the runner timeout limit does **not** cut the script short — it still gets its full `post_task_script_timeout`. |
+| `runner.shutdown_timeout` | On SIGINT/SIGTERM, bounds how long the runner waits for the **task** to finish. The post-task script detaches from cancellation, so it is **not** interrupted by this window and may extend shutdown until its own `post_task_script_timeout` elapses. |
+
+## Examples
+
+### Linux — prune dangling Docker resources
+
+`/usr/local/bin/gitea-post-task.sh`:
+
+```sh
+#!/bin/sh
+set -eu
+docker image prune -f
+docker builder prune -f --filter 'until=24h'
+```
+
+`config.yaml`:
+
+```yaml
+runner:
+  post_task_script: /usr/local/bin/gitea-post-task.sh
+  post_task_script_timeout: 3m
+```
+
+### Windows — batch file (`.cmd`)
+
+Use a `.cmd` or `.bat` file. PowerShell scripts are **not supported yet** as `post_task_script`; call PowerShell from a batch wrapper if needed:
+
+`C:\gitea-runner\scripts\post-task.cmd`:
+
+```bat
+@echo off
+docker image prune -f
+```
+
+```yaml
+runner:
+  post_task_script: C:\gitea-runner\scripts\post-task.cmd
+  post_task_script_timeout: 3m
+```
+
+PowerShell workaround until native `.ps1` support exists:
+
+`C:\gitea-runner\scripts\post-task.cmd`:
+
+```bat
+@echo off
+powershell.exe -NoProfile -NonInteractive -ExecutionPolicy Bypass -File "%~dp0post-task.ps1"
+```
+
+## Windows notes
+
+- Supported as `post_task_script`: `.exe`, `.bat`, `.cmd`.
+- **Not supported yet:** `.ps1` as the configured path (use a `.cmd` wrapper; see above).
+- `.sh` files require a Unix shell on the PATH unless you point `post_task_script` at the interpreter.
+- Use backslashes or forward slashes in YAML paths; both work in Go on Windows.
+
+## See also
+
+- [Configuration](../README.md#configuration) — generating and loading `config.yaml`
+- [config.example.yaml](../internal/pkg/config/config.example.yaml) — all runner options
+- Bind-workdir idle cleanup (`runner.workdir_cleanup_age`) — separate from this hook; runs only when the runner is idle