Production Bugs

How to resolve “No such device or file” error when opening files on a pfs-formatted disk with custom fio’s pfs ioengine (while pfs tools works)?

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Summary The failure to open files on a pfs‑formatted disk through a custom fio ioengine occurs because the fio engine is invoking pfs_open() without initializing the PFS environment, without mounting a PFS instance, and without providing a valid PFS work directory. The official pfs CLI tools succeed because they implicitly perform these initialization steps, while … Read more

lessThan() in subclassed QSortFilterProxyModel being called too many times and I can’t figure why

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Summary The issue arises from excessive calls to the lessThan() method in a subclassed QSortFilterProxyModel, leading to significant performance degradation when sorting large datasets. The root cause is the interaction between manual invalidation and the proxy model’s internal sorting mechanisms, resulting in redundant comparisons. Root Cause Manual invalidation via invalidate() triggers a full resort, causing … Read more

Design and implement a thread-safe in-memory cache in Java with the following features

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Summary The thread-safe in-memory cache implementation in Java exhibited race conditions and inefficient cleanup, leading to inconsistent data retrieval and performance degradation. The root cause was lack of proper synchronization and inefficient expiration handling. Root Cause Race conditions in get() and cleanUp() methods due to unsynchronized access to the cache map. Inefficient cleanup caused by … Read more

VBox bridge 2 internal Networks

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Summary This incident involved a failed attempt to bridge two isolated VirtualBox internal networks using a third VM configured with two NICs. The bridging VM never forwarded traffic between the networks, leaving VM1 and VM2 unable to communicate. The failure stemmed from misunderstanding how VirtualBox internal networks work and what is required to route between … Read more

Iteratively get a mut reference from a nested structure

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Summary Iteratively obtaining a mutable reference from a nested structure, such as a binary tree, in Rust requires careful handling of lifetimes and borrowing rules. The challenge arises when attempting to create an iterator-like structure that allows dynamic traversal (e.g., left or right branches) while maintaining mutable access to the final node. The root cause … Read more

WPF – ICommand: disable-enable button during long process run with Task.Run()

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Summary This incident involved a WPF MVVM application where a button bound to an ICommand correctly disabled during a long-running operation, but failed to re-enable once the operation completed—until the user interacted with the UI. The root cause was that the command’s CanExecute state was never refreshed after IsBusy changed. Root Cause The underlying issue … Read more

Cant install package

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Summary The issue arises from a missing or misconfigured CBLAS (Basic Linear Algebra Subprograms) library during the installation of a neural network package on a Debian server. The error message indicates that the linker cannot find the cblas library, causing the build process to fail. Root Cause Missing Library: The cblas library, essential for linear … Read more

How to annotate a function that returns a dataclass field so that type checkers treat it correctly?

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Summary This incident examines why a seemingly simple wrapper around dataclasses.field() causes type checkers to misinterpret constructor signatures. Although the runtime behavior is correct, type checkers treat the wrapper as opaque, leading to incorrect __init__ parameter inference. The postmortem explains why this happens, how it affects real systems, and how senior engineers typically resolve it. … Read more