conifg, sessionctx: set default value "/data/deploy/plugin" for plugin_dir

[Defined2014]

What problem does this PR solve?

Issue Number: close #28084

Problem Summary: set default value "/data/deploy/plugin" for plugin_dir

Release note

None

[ti-chi-bot]

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• dveeden
• morgo

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[Defined2014]

/cc @morgo @dveeden
PTAL

[dveeden]

Could you add a test?

[morgo]

Could you add a test?

In theory there is already a test for this:

tidb/sessionctx/variable/sysvar_test.go

Lines 592 to 598 in 83e559d

	val, err = GetSessionOrGlobalSystemVar(vars, PluginDir)
	require.NoError(t, err)
	require.Equal(t, config.GetGlobalConfig().Plugin.Dir, val)
	val, err = GetSessionOrGlobalSystemVar(vars, PluginLoad)
	require.NoError(t, err)
	require.Equal(t, config.GetGlobalConfig().Plugin.Load, val)

But it was previously passing, presumably because both values were empty in the tests.

[morgo]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 56c8880

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