improvement: fall back to disk storage when shared memory cannot allocate

[mscolnick]

SharedMemoryStorage.store() had no error handling around shared_memory.SharedMemory(create=True, size=...), so an OSError (e.g. /dev/shm full) would crash the kernel and drop the virtual file. Add a FallbackStorage composite that tries each backend in order and falls back on OSError, plus a cross-process-safe DiskStorage backend; wire EDIT mode and the manager's ad-hoc cross-process reader to use both tiers.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 2, 2026 0:57am

[cubic-dev-ai]

1 issue found across 4 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="marimo/_runtime/virtual_file/storage.py">

<violation number="1" location="marimo/_runtime/virtual_file/storage.py:397">
P2: When all backends are stale, `last_err` stays `None` and `assert last_err is not None` raises an uninformative `AssertionError` (or, with `python -O`, `raise None` causes a `TypeError`). Replace the assertion with an explicit error that describes the failure mode.</violation>
</file>

Architecture diagram

sequenceDiagram
    participant K as Kernel Subprocess
    participant FS_K as FallbackStorage (Kernel)
    participant SMS as Shared Memory (/dev/shm)
    participant DS as DiskStorage (/tmp)
    participant S as Server Main Process
    participant MGR as VirtualFileStorageManager (Server)
    participant FS_S as NEW: Ad-hoc FallbackReader

    Note over K, DS: Write Path (Kernel)
    K->>FS_K: store(key, bytes)
    FS_K->>SMS: store(key, bytes)
    
    alt OSError (e.g. /dev/shm full)
        SMS-->>FS_K: Raise OSError
        FS_K->>FS_K: Catch error & log warning
        FS_K->>DS: NEW: store(key, bytes)
        Note right of DS: Atomic write & rename
        DS-->>FS_K: Success
    else Success
        SMS-->>FS_K: Success
    end

    Note over DS, S: Read Path (Cross-Process)
    S->>MGR: read(key)
    MGR->>FS_S: NEW: _cross_process_storage().read()
    
    FS_S->>SMS: CHANGED: has(key)
    Note right of SMS: Probes segment name by key
    
    alt Key found in SHM
        SMS-->>FS_S: True
        FS_S->>SMS: read(key)
        SMS-->>FS_S: bytes
    else Key not in SHM
        SMS-->>FS_S: False
        FS_S->>DS: NEW: has(key)
        Note right of DS: Check if file exists in /tmp/marimo-vfs
        DS-->>FS_S: True
        FS_S->>DS: NEW: read(key)
        DS-->>FS_S: bytes
    end
    
    FS_S-->>MGR: bytes
    MGR-->>S: bytes (served via /@file)

    Note over K, DS: Cleanup
    K->>FS_K: shutdown()
    FS_K->>SMS: shutdown()
    FS_K->>DS: shutdown()
    Note right of DS: Only removes keys owned by this kernel instance

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_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review, or fix all with cubic.}

[Copilot]

Pull request overview

Adds a resilient, cross-process-capable virtual file storage stack for EDIT mode by introducing a disk-backed fallback when shared memory allocation fails (e.g., /dev/shm full), and wiring cross-process readers to probe both tiers.

Changes:

• Add DiskStorage (temp-dir backed) and FallbackStorage (tiered composite) virtual file storage backends.
• Update SharedMemoryStorage.has() to support cross-process existence probing by segment name.
• Use the fallback stack in kernel EDIT mode and in VirtualFileStorageManager’s ad-hoc cross-process reader; add comprehensive tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
marimo/_runtime/virtual_file/storage.py	Implements DiskStorage + FallbackStorage, adds cross-process probing, and updates manager cross-process reads to probe both tiers.
marimo/_runtime/context/kernel_context.py	Switches EDIT mode storage to FallbackStorage([SharedMemoryStorage(), DiskStorage()]).
marimo/_runtime/virtual_file/__init__.py	Exports the new storage backends from the virtual_file package.
tests/_runtime/test_storage.py	Adds unit tests covering disk storage behavior and fallback routing/probing semantics.

[cubic-dev-ai]

3 issues found across 4 files (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="tests/_runtime/test_storage.py">

<violation number="1" location="tests/_runtime/test_storage.py:614">
P2: Use a valid marimo-shaped filename here; `any-name` is rejected before storage access, so this test never exercises the OSError fallback.</violation>

<violation number="2" location="tests/_runtime/test_storage.py:627">
P2: Use a valid marimo-shaped filename here; `any-name` is rejected before storage access, so this test never exercises the chunked OSError fallback.</violation>

<violation number="3" location="tests/_runtime/test_storage.py:767">
P3: Guard symlink creation here and skip when the platform/environment does not allow symlinks.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review, or fix all with cubic.}

[dmadisetti]

I think it might be verifying that this gets mapped to tmpfs on molab, otherwise this will be very slow when actually used. Currently maps to /tmp (expected) but our mounts should this to be on disk

[dmadisetti]

Left a comment but not addressable here pinged @mchav