Ensured calculation of a negative repost count does not happen

[mike182uk]

ref https://linear.app/ghost/issue/PROD-2332

It was possible to get a negative repost count if the post repost count had already been updated via another process whilst the post was being updated

[coderabbitai]

Walkthrough

A new integration test was added to the test suite for the KnexPostRepository to validate that the repost_count field for internal posts does not become negative, even in the presence of inconsistent database states. Additionally, the repository implementation was updated to use the SQL GREATEST function when updating the repost_count, ensuring that it cannot drop below zero. No changes were made to the public API or exported entities; all modifications are internal to the repository logic and its tests.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

• The changes involve a focused update to a SQL expression and the addition of a single, self-contained integration test. The logic is straightforward, with no changes to public interfaces or complex refactoring.

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📒 Files selected for processing (2)

• src/post/post.repository.knex.integration.test.ts (1 hunks)
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[mike182uk]

Why use CAST? From the AI:

 In MySQL, the repost_count column is defined as BIGINT UNSIGNED. When MySQL performs arithmetic operations on unsigned integers, it tries to keep the result unsigned as well.

  Here's what happens:

  1. Without CAST:
    - repost_count is 1 (unsigned)
    - We try to calculate: 1 + (-2)
    - MySQL attempts to calculate this as unsigned arithmetic
    - Since unsigned integers can't be negative, MySQL throws an error: "BIGINT UNSIGNED value is out of range"
  2. With CAST AS SIGNED:
    - repost_count is cast to a signed integer first
    - The calculation becomes: 1 + (-2) = -1 (as a signed integer)
    - Then GREATEST(0, -1) evaluates to 0
    - The final result 0 is valid for an unsigned column

  The CAST is necessary because MySQL's type system is strict about unsigned arithmetic. It won't allow intermediate calculations that would result in negative values, even if the final
  result (after applying GREATEST) would be non-negative.