Support plugin start argument for tidb instance

[sdojjy]

Support plugin start argument for tidb instance. The default value is disabled

Tests

• enable the plugin, ARGS is appended as expected.
  ARGS="${ARGS} --plugin-dir /pulugins --plugin-load whitelist-1 "
• disable the plugin, no plugin argument is added

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[LinuxGit]

s/pulgin/plugin

[sdojjy]

fixed

[LinuxGit]

s/pulugins//plugins

[sdojjy]

fixed

[LinuxGit]

If there're multiple plugins, is list a comma separated string? I found a example, but not sure it works or not.https://github.com/helm/charts/blob/master/stable/opa/templates/deployment.yaml#L96

[sdojjy]

yes!
From the design document.
https://github.com/pingcap/tidb/blob/master/docs/design/2018-12-10-plugin-framework.md#Management

and tidb uage document
https://github.com/pingcap/tidb/blob/11d7199fcfbb66b1205ffa00f770f90f7b1ee9c1/tidb-server/main.go#L114

and the code that load plugins
https://github.com/pingcap/tidb/blob/11d7199fcfbb66b1205ffa00f770f90f7b1ee9c1/session/session.go#L1305

	if len(cfg.Plugin.Load) > 0 {
		err := plugin.Load(context.Background(), plugin.Config{
			Plugins:        strings.Split(cfg.Plugin.Load, ","),
			PluginDir:      cfg.Plugin.Dir,
			GlobalSysVar:   &variable.SysVars,
			PluginVarNames: &variable.PluginVarNames,
		})
		if err != nil {
			return nil, err
		}
	}

[sdojjy]

the plugin.list is changed to array

[LinuxGit]

LGTM

[tennix]

LGTM