Write out error message when plugin seems have a bug

[rmatsuoka]

Mackerel-agent does not write out any error log when fails to parse a broken JSON as a plugin's graph definition.
It makes hard to find out a bug of the plugin.

This PR makes mackerel-agent write out error log when plugin seems have a bug.

Implements

I defined PluginFaultError.

// PluginFaultError may be returned by [PluginGenerator.PrepareGraphDefs].
// This error indicates a bug in a plugin and should be logged for a user.
// Note that [PluginGenerator.PrepareGraphDefs] can also return other error types.
type PluginFaultError struct {
	Err error
}

A method PrepareGraphDefs fetches a graph definition from a plugin. By this PR, PrepareGraphDefs will return PluginFaultError, when it faces following two situation,

• Fails to exec plugin
• Fails to parse JSON as a graph definition.

Mackerel-agent will write out an error log when it catches PluginFaultError.

Behaviour

When plugin provides broken JSON as a graph definition, mackerel-agent write out ERROR log.

2024/11/18 20:58:40 agent.go:105: ERROR <agent> Failed to fetch meta information from plugin &{0x14000140b60 <nil>}; seems that the plugin may have a bug: while reading plugin configuration: invalid character '}' looking for beginning of object key string

When plugin has no graph definition (It should not be a bug), no ERROR log.

DEBUG log is written out if verbose option is on. This behavior is the same as before.

2024/11/18 20:56:38 agent.go:102: DEBUG <agent> Failed to fetch meta information from plugin &{0x140001c9b90 <nil>} (non critical); seems that this plugin does not have meta information: bad format of first line: "x-custom-sh.testmetric2\t16452\t1731930998\n"

[yohfee]

LGTM!

Just out of curiosity, why aren't following situations considered PluginFaultError?
I thought they are also caused when plugin does not satisfy specifications.

• plugin outputs bad header
• plugin outputs unsupported version

[yohfee]

[nit] According to https://pkg.go.dev/errors#As example, maybe it is more idiomatic to check nil first then detect by errors.As.

[rmatsuoka]

hmmm, I feel like it is not idiomatic. I think the way in the example is just for illustration. That requires more nesting.

[rmatsuoka]

@yohfee

• plugin outputs bad header

For mackerel-agent, "Bad header" means "the first line from a plugin is not # mackerel-agent-plugin".
This situation is probably that the plugin has no graph definitions and just yields a metric, which is formatted into {metric_name} {metric_value} {timestamp}. It is permissible for a plugin to have no graph definitions, and it should be not a bug.

• plugin outputs unsupported version

PluginFaultError is yielded when a plugin seems have a bug. Outputting unsupported version is not a bug.

Though, I think it is kind for user to log a message about plugin is unsupported.

[rmatsuoka]

Thanks for your review!