docs(specs): Add probe as value to startup_error_behavior

docs/specs/tsd-006-startup-error-behavior.md

@@ -75,6 +75,19 @@ must *not* fail on startup errors and should continue running. On startup error,
 Telegraf must ignore the plugin as-if it was not configured at all, i.e. the
 plugin must be completely removed from processing.
 
+### `probe` behavior
+
+When using the `probe` setting for the `startup_error_behavior` option Telegraf
+must *not* fail on startup errors and should continue running. On startup error,
+Telegraf must ignore the plugin as-if it was not configured at all, i.e. the
+plugin must be completely removed from processing, similar to the `ignore`
+behavior. Additionally, Telegraf must probe the plugin (as defined in 
+[TSD-009][tsd_009]) after startup, if it implements the `ProbePlugin` interface.
+If probing is available *and* returns an error Telegraf must *ignore* the
+plugin as-if it was not configured at all.
+
+[tsd_009]: /docs/specs/tsd-009-probe-on-startup.md
+
 ## Plugin Requirements
 
 Plugins participating in handling startup errors must implement the `Start()`

docs/specs/tsd-009-probe-on-startup.md

@@ -0,0 +1,69 @@
+# Probing plugins after startup
+
+## Objective
+
+Allow Telegraf to probe plugins during startup to enable enhanced plugin error 
+detection like availability of hardware or services
+
+## Keywords
+
+inputs, outputs, startup, probe, error, ignore, behavior
+
+## Overview
+
+When plugins are first instantiated, Telegraf will call the plugin's `Start()` 
+method (for inputs) or `Connect()` (for outputs) which will initialize its 
+configuration based off of config options and the running environment. It is 
+sometimes the case that while the initialization step succeeds, the upstream 
+service in which the plugin relies on is not actually running, or is not capable 
+of being communicated with due to incorrect configuration or environmental 
+problems. In situations like this, Telegraf does not detect that the plugin's 
+upstream service is not functioning properly, and thus it will continually call 
+the plugin during each `Gather()` iteration. This often has the effect of 
+polluting journald and system logs with voluminous error messages, which creates 
+issues for system administrators who rely on such logs to identify other 
+unrelated system problems.
+
+More background discussion on this option, including other possible avenues, can 
+be viewed [here](https://github.com/influxdata/telegraf/issues/16028).
+
+## Probing
+
+Probing is an action whereby the plugin should ensure that the plugin will be
+fully functional on a best effort basis. This may comprise communicating with
+its external service, trying to access required devices, entities or executables
+etc to ensure that the plugin will not produce errors during e.g. data collection
+or data output. Probing must *not* produce, process or output any metrics.
+
+Plugins that support probing must implement the `ProbePlugin` interface. Such 
+plugins must behave in the following manner:
+
+1. Return an error if the external dependencies (hardware, services, 
+executables, etc.) of the plugin are not available.
+2. Return an error if information cannot be gathered (in the case of inputs) or 
+sent (in the case of outputs) due to unrecoverable issues. For example, invalid 
+authentication, missing permissions, or non-existent endpoints.
+3. Otherwise, return `nil` indicating the plugin will be fully functional.
+
+## Plugin Requirements
+
+Plugins that allow probing must implement the `ProbePlugin` interface. The
+exact implementation depends on the plugin's functionality and requirements,
+but generally it should take the same actions as it would during normal operation
+e.g. calling `Gather()` or `Write()` and check if errors occur. If probing fails, 
+it must be safe to call the plugin's `Close()` method.
+
+Input plugins must *not* produce metrics, output plugins must *not* send any
+metrics to the service. Plugins must *not* influence the later data processing or
+collection by modifying the internal state of the plugin or the external state of the
+service or hardware. For example, file-offsets or other service states must be
+reset to not lose data during the first gather or write cycle.
+
+Plugins must return `nil` upon successful probing or an error otherwise.
+
+## Related Issues
+
+- [#16028](https://github.com/influxdata/telegraf/issues/16028)
+- [#15916](https://github.com/influxdata/telegraf/pull/15916)
+- [#16001](https://github.com/influxdata/telegraf/pull/16001)
+