Document how to deal with repeatedly triggered bugs

CONTRIBUTING.md

@@ -30,6 +30,10 @@ For a permissive DBMS, implementing the expression generator is easier, since th
 
 For a strict DBMS, the better approach is typically to attempt to generate expressions of the expected type. For PostgreSQL, the expression generator thus expects an additional type argument (see [PostgreSQLExpressionGenerator](https://github.com/sqlancer/sqlancer/blob/86647df8aa2dd8d167b5c3ce3297290f5b0b2bcd/src/sqlancer/postgres/gen/PostgresExpressionGenerator.java#L251)). This type is propagated recursively. For example, if we require a predicate for the `WHERE` clause, we pass boolean as a type. The expression generator then calls a method `generateBooleanExpression` that attempts to produce a boolean expression, by, for example, generating a comparison (e.g., `<=`). For the comparison's operands, a random type is then selected and propagated. For example, if an integer type is selected, then `generateExpression` is called with this type once for the left operand, and once for the right operand. Note that this process does not guarantee that the expression will indeed have the expected type. It might happen, for example, that the expression generator attempts to produce an integer value, but that it produces a double value instead, namely when an integer overflow occurs, which, depending on the DBMS, implicitly converts the result to a floating-point value.
 
+### Unfixed Bugs
+
+Often, some bugs are fixed only after an extended period, meaning that SQLancer will repeatedly report the same bug. In such cases, it might be possible to avoid generating the problematic pattern, or adding an expected error with the internal error message. Rather than, for example, commenting out the code with the bug-inducing pattern, a pattern implemented by the [TiDBBugs class](https://github.com/sqlancer/sqlancer/blob/4c20a94b3ad2c037e1a66c0b637184f8c20faa7e/src/sqlancer/tidb/TiDBBugs.java) should be applied. The core idea is to use a public, static flag for each issue, which is set to true as long as the issue persists (e.g., see [bug35652](https://github.com/sqlancer/sqlancer/blob/4c20a94b3ad2c037e1a66c0b637184f8c20faa7e/src/sqlancer/tidb/TiDBBugs.java#L55)). The work-around code is then executed—or the problematic pattern should not be generated—if the flag is set to true (e.g., [an expected error is added for bug35652](https://github.com/sqlancer/sqlancer/blob/59564d818d991d54b32fa5a79c9f733799c090f2/src/sqlancer/tidb/TiDBErrors.java#L47)). This makes it easy to later on identify and remove all such work-around code once the issue has been fixed.
+
 ## Options
 
 SQLancer uses [JCommander](https://jcommander.org/) for handling options. The `MainOptions` class contains options that are expected to be supported by all DBMS-testing implementations. Furthermore, each `*Provider` class provides a method to return an additional set of supported options.