plugin docs: stdin/stdout uses type:"body" inside callbacks

for-developers/plugin.md

@@ -22,6 +22,7 @@ Configured callbacks use replacements for specific URL to receive and send data.
 | `%_input.url%`  | The URL pointing to an HTTP endpoint sending out the input data for the plugin. This can be mapped to STDIN or read directly by the plugin.                                                                                                                                                                                                                                                              |
 | `%_output.url%` | The URL to write the data back to. This can be mapped to STDOUT or written directly by the plugin.                                                                                                                                                                                                                                                                                                       |
 | `%info.json%`   | A map containing context information about the current callback. This includes the URL requested, its query in a parsed form, as well as the HTTP Headers of the request. In addition it includes the plugin's base config as returned by `/api/config`. Since version 6.17, the info also includes the languages as configured in the base config. Individual callbacks may add additional information. |
+| `%_exec.pluginDir%` | The absolute filesystem path to the plugin's directory. Use this to reference scripts and other plugin resources by their absolute path — e.g. `value: "%_exec.pluginDir%/src/handler.js"`. Without this prefix the path is resolved relative to the per-job working directory (a transient subdirectory under fylr's `tempDir`), where your script does not exist; the process exits with the runtime's "module not found" error. |
 
 When a plugin wants to return an error, it needs to exit with a non zero exit code. If called as an extension plugin, FYLR sets the `X-Execserver-Error` header, unless more than 4K of data have already been produce and sent out to the response body.
 
@@ -64,12 +65,12 @@ callbacks:
         commands:
           - prog: "node"
             stdin:
-              url: "%_input.url%"
+              type: "body"
             stdout:
-              url: "%_output.url%"
+              type: "body"
             args:
               - type: "value"
-                value: "set_comment.js"
+                value: "%_exec.pluginDir%/set_comment.js"
 
   db_pre_save:
     steps:
@@ -86,12 +87,12 @@ callbacks:
           commands:
             - prog: "node"
               stdin:
-                url: "%_input.url%"
+                type: "body"
               stdout:
-                url: "%_output.url%"
+                type: "body"
               args:
                 - type: "value"
-                  value: "set_comment.js"
+                  value: "%_exec.pluginDir%/set_comment.js"
                 - type: "value"
                   value: "%info.json%"
 
@@ -152,6 +153,10 @@ In case of an error, fylr tries to parse errors using the api error format. If a
 
 Callbacks are predefined hooks in the API of FYLR. Each hook is different and might use its own format for in and output of the data.
 
+{% hint style="warning" %}
+**Stdin/stdout transport differs between extensions and callbacks.** Extensions use the URL-replacement form (`stdin.url: "%_input.url%"` / `stdout.url: "%_output.url%"`) because fylr exposes the input/output as on-the-fly HTTP endpoints. Callbacks (e.g. `db_pre_save`, `transition_db_pre_save`) connect the plugin's process stdin/stdout directly via `type: "body"`. Using the URL form inside a callback definition fails at job dispatch with `Could not prepare stdin "%_input.url%": parse "%_input.url%": invalid URL escape "%_i"` because the placeholder is not substituted in that path.
+{% endhint %}
+
 Errors can be generated using the api error JSON format which looks like this:
 
 ```json