Fill in IDBBatchAtomicVFS blocks on writes past EOF

[rhashimoto]

No description provided.

[gemini-code-assist]

Hello @rhashimoto, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

This pull request addresses an issue in the IDBBatchAtomicVFS where writes past the end of the file (EOF) were not correctly handled. Specifically, it ensures that when a write occurs beyond the current file size, the skipped space is filled with zeroed blocks. This is crucial for maintaining data integrity and preventing unexpected behavior when reading from these extended regions. Additionally, a console error is added to prevent changing the page size after the database file has been created.

Highlights

• EOF Handling: The primary change involves filling in the gaps when writing past the end of a file in IDBBatchAtomicVFS. This ensures that reads in the extended region return expected zeroed data.
• Page Size Restriction: A console error is added to prevent changing the page size after the database file has been created, which could lead to data corruption.
• Data Integrity: The changes improve the overall data integrity of the VFS by ensuring that writes past EOF are handled correctly, preventing potential data corruption or unexpected behavior.

Changelog

• src/examples/IDBBatchAtomicVFS.js
  • Removes the initial file size declaration.
  • Introduces logic to fill skipped space with zeroed blocks when writing past EOF (lines 269-283).
  • Removes a redundant block write (lines 284-286).
  • Adds a console error to prevent changing the page size after file creation (lines 471-473).

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---

Past the file's end,
A zeroed space now appears,
Data made complete.

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[gemini-code-assist]

Code Review

The pull request addresses an important issue in the IDBBatchAtomicVFS, specifically handling writes past the end of the file. The changes introduce logic to fill in the skipped space with zeroed blocks, ensuring data integrity. The addition of a console error when attempting to change the page size is also a good preventative measure. Overall, the changes seem well-considered and necessary for the correct functioning of the VFS.

Summary of Findings

• Redundant file.changedPages.add(iOffset) call: The file.changedPages.add(iOffset) call is present in both the conditional block (when writing past EOF) and the unconditional block. This redundancy can be removed by placing the call outside the conditional block.
• Potential performance issue with large skips: When writing far past the end of the file, the loop inserting zeroed blocks could become a performance bottleneck. Consider alternative strategies for large skips, such as creating a single large zeroed block or using a sparse representation.

Merge Readiness

The pull request introduces a critical fix for handling writes past EOF in the IDBBatchAtomicVFS. While the changes appear to be well-implemented, there are a couple of potential improvements that could be considered. I recommend addressing the redundant file.changedPages.add(iOffset) call before merging. The potential performance issue with large skips is a lower priority but should be considered for future optimization. I am unable to approve this pull request, and recommend that others review and approve this code before merging.

[gemini-code-assist]

Consider using data.slice() here to avoid potential modification of the original pData buffer. While the current implementation works, explicitly creating a copy ensures that any subsequent changes to pData do not affect the data stored in the block.

Suggested change

	data
	data: data.slice()

[gemini-code-assist]

The file.changedPages.add(iOffset) call is present in both the conditional block (when writing past EOF) and the unconditional block. This redundancy can be removed by placing the call outside the conditional block to avoid unnecessary duplication.

Suggested change

	blocks.put(block);
	file.changedPages.add(iOffset);
	blocks.put(block);
	}, 'rw', file.txOptions);
	file.changedPages.add(iOffset);

[gemini-code-assist]

When writing far past the end of the file, this loop inserting zeroed blocks could become a performance bottleneck. Consider alternative strategies for large skips, such as creating a single large zeroed block or using a sparse representation.