Description

This pull request addresses an error code inconsistency in the handling of empty stacks for the OP_IF and OP_NOTIF Bitcoin Script opcodes.

Currently, if the stack is empty when evaluating these opcodes, the script returns SCRIPT_ERR_UNBALANCED_CONDITIONAL, which is misleading. The correct error code for this scenario should be SCRIPT_ERR_INVALID_STACK_OPERATION, as the issue lies in insufficient stack elements rather than mismatched conditionals.

This change modifies the script execution engine to return SCRIPT_ERR_INVALID_STACK_OPERATION when an empty stack is encountered during OP_IF and OP_NOTIF processing.

Motivation and Improvement

• Accuracy: The updated error code aligns with the actual nature of the problem, providing more accurate feedback during script validation.
• Debugging and Analysis: This correction will make it easier for developers and users to diagnose script failures accurately, leading to improved debugging and troubleshooting.
• Consistency: It brings the error handling for empty stack conditions in line with other parts of the Bitcoin Script interpreter, enhancing overall code consistency.

Testing

This change is accompanied by unit tests in the src/test/ directory that specifically verify the corrected behavior for OP_IF and OP_NOTIF with empty stacks.

Additional Considerations

While this change might appear minor, its impact on error reporting and debugging is significant. It contributes to a more robust and accurate Bitcoin Script validation process.

Note: Please note that no functional tests have been added as this is a minor fix in the script interpreter's logic, not affecting the overall Bitcoin functionality.

I hope this pull request description is helpful. Please let me know if you have any other questions or need further refinements.