Backport Bug#28949700: SUBOPTIMAL USE OF STRING::RESERVE() IN WRAPPER_TO_STRING() to 5.7

[annirudh]

Backports the following fix into 5.7: 8bd55fc

Fetching rows with large JSON columns (> 1MB) can cause CPU usage to spike to 100% CPU. Stack traces captured from mysqld when it is in this state show that wrapper_to_string calls are stuck in realloc.

This can cause serious performance problems in production MySQL instances that make use of JSON columns.

[mysql-oca-bot]

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[annirudh]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
Please make sure to include your MySQL bug system user (email) in the returned form.
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This is done.

[mysql-oca-bot]

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[annirudh]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

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bug http://bugs.mysql.com/bug.php?id=103790 for updates.
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