[Project516]

Eleven OpenJDK tests that write multi-GB files now pass instead of running out of memory: FileChannel/Transfer2GPlus, Transfer4GBFile, TransferTo6GBFile, LoopingTruncate, Channels/TransferTo_2GB_transferTo, TransferTo_2GB_transferFrom, Files/SBC, FileInputStream/LargeFileAvailable, ZipFile/CenSizeTooLarge, ZipFile/EndOfCenValidation and zipfs/EndOfCenValidation.

All of them write after a large seek, the way sparse files are made. #47 moved a file to the host once it grew past 4 MiB, but only after the write: a write at 2 GB first allocated the whole 2 GB gap in memory. Fixes:

• A write that would take a file past 4 MiB switches it to the host first, so a gap is never allocated.
• Append mode: every write on a descriptor opened for append goes to the end of the file, as O_APPEND does. Positional writes and mapped-buffer write-back keep their position. The transfer tests found this once the allocation was fixed.
• A directory can be opened read-only as a channel, as on Linux (SBC does this); reading it fails with Is a directory, and a snapshot is refused while one is open.
• Large files on read-only Node mounts also use host handles.

Tests:

• test/java/StreamingFile.java adds a write far past the end and appends after a seek, matching HotSpot. test/nio.test.ts covers the append and directory cases.
• The eleven tests passed on GitHub Actions, peak RSS about 258 to 497 MB, the slowest 105 s, and go into passing.txt with their durations in times.tsv.
• pnpm typecheck and pnpm test pass.

Noticed, not fixed:

• An explicit file mapping still takes its whole length from the VM's linear memory, which tops out at 512 MB and does not shrink after unmapping.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 9b4490b and 6a1d88b.

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Files selected for processing: 6

• src/fs.ts
• src/natives/io.ts
• src/natives/nio.ts
• test/java/StreamingFile.java
• test/jtreg/passing.txt
• test/nio.test.ts

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[project516-review-bot]

This PR fixes sparse file handling by switching to host-backed storage before allocating large gaps in memory. It adds append-mode semantics (writes always go to end), allows read-only directory channels, and ensures large files on read-only mounts use host handles. Eleven previously failing OpenJDK tests now pass with peak RSS under 500 MB.

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_{review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict approve}

[project516-review-bot]

The PR fixes sparse file handling by switching to host handles before allocating gap memory, adds append-mode semantics matching O_APPEND, and supports read-only directory channels. Eleven previously failing OpenJDK tests now pass. The changes span VFS, native I/O, NIO, and tests.

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_{review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict approve}

[project516-review-bot]

The PR fixes sparse file handling by switching to host handles before allocating gap memory (preventing OOM on multi-GB writes), adds O_APPEND semantics for append-mode descriptors, allows read-only directory channel opens (failing reads with 'Is a directory'), and makes large files on read-only mounts use host handles. Eleven previously failing OpenJDK tests now pass with peak RSS 258-497 MB.

Not approved: 1 earlier point is still open in the threads.

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_{review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict comment}

[project516-review-bot]

The PR fixes sparse file handling so that writes past 4 MiB switch to host-based storage, preventing memory allocation for gaps. This enables eleven OpenJDK multi-GB file tests to pass. Changes include modifying the write method to stream to host before allocating gaps, adding DirectoryFile class for read-only directory support, updating canOpenFile and openFile for proper writability checks, and adding append mode and mapped buffer write-back support.

Not approved: 1 earlier point is still open in the threads.

---

_{review-bot, model nvidia/nemotron-3.5-lightning:free, verdict comment}

[project516-review-bot]

All points from the last review are settled in the threads.

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_{review-bot, model nvidia/nemotron-3.5-lightning:free, verdict approve}