Solving File Caching Issue With LSP After Deletion
Introduction
Hey guys! Today, we're diving deep into a tricky file caching issue that some of you might have encountered while using the Language Server Protocol (LSP) with Nickel. This issue arises after a file is deleted, but the LSP still holds onto a cached version, leading to incorrect diagnostics. It's like the LSP is living in the past, unaware that the file is no longer there. We'll break down the bug, how to reproduce it, what the expected behavior should be, the environment where this issue occurs, and the potential solution. So, let's get started!
Understanding the Bug
The file caching issue we're tackling today revolves around how the LSP handles files after they've been deleted. Imagine you're working on a project with multiple files, and one file imports another. When you open a file in your editor that uses the LSP, the LSP caches that file. Now, here's the problem: if you delete the imported file, the LSP doesn't always recognize this change. It continues to use the cached version, which can lead to misleading diagnostics. For instance, if b.ncl imports a.ncl, and you delete a.ncl, the LSP might not immediately flag b.ncl as having an issue. This is because the LSP still thinks a.ncl exists in its cache. This caching mechanism, while generally helpful for performance, can cause headaches when it doesn't reflect the actual state of your filesystem. The main issue is that once a file is opened in the editor using the LSP, the file remains cached even after deletion. So if another file imports that file, it will fail publish diagnostics indicating that it's importing a missing file. This can lead to confusion and wasted time, especially in larger projects where file dependencies are intricate. The crux of the problem lies in how the LSP manages the SourceKind of the file, which we'll discuss in more detail later. For now, just remember that this caching hiccup can prevent the LSP from providing accurate feedback about missing files.
How to Reproduce the Issue
Reproducing this file caching issue is pretty straightforward, guys. Let's walk through the steps so you can see it in action. First, you'll need an editor that uses the LSP, such as VSCode or Neovim. Then, create two files, a.ncl and b.ncl, with the following content:
# a.ncl
1
# b.ncl
import "a.ncl"
Here, b.ncl imports a.ncl. Now, delete a.ncl. You can do this either through your editor or directly from the filesystem. Here’s where the issue surfaces: even after deleting a.ncl, you won't see any diagnostics in b.ncl indicating a problem. The LSP doesn't seem to notice that a.ncl is gone. This is despite the fact that b.ncl now has a broken import. Even if you try actions that usually trigger a refresh of diagnostics, like editing or saving b.ncl, the problem persists. The LSP stubbornly sticks to its cached version of a.ncl. I've personally reproduced this in both VSCode and Neovim, so it seems to be a consistent issue across different editors. This reproducibility is crucial because it means we can reliably test potential solutions. By following these steps, you can confirm the bug on your system and understand the exact conditions under which it occurs. This hands-on experience will make the subsequent discussion of the root cause and solution much clearer.
Expected Behavior
So, what should happen when a file is deleted? The expected behavior is that the LSP should detect the missing file and immediately publish appropriate diagnostics. This means that if b.ncl imports a.ncl, and a.ncl is deleted, the LSP should flag b.ncl with an error or warning. This is crucial for maintaining code integrity and preventing runtime errors. Imagine working on a large project where files are frequently moved, renamed, or deleted. Without proper diagnostics, you could easily introduce broken dependencies and not realize it until much later in the development process. The LSP's primary job is to provide real-time feedback about your code, and that includes alerting you to missing dependencies. The real-time feedback is essential for a smooth development workflow. The LSP should act like a vigilant assistant, constantly checking that all the pieces of your project fit together. When a file is deleted, it's a significant event that can impact the entire project. The LSP needs to recognize this and promptly inform you of any consequences. By ensuring that the LSP behaves as expected, we can prevent a whole class of errors and make the development process much more robust. In essence, the LSP should be a reliable source of truth about the state of your project, and that means accurately reflecting the presence or absence of files.
Environment
This file caching issue has been observed in a specific environment, which helps us narrow down the potential causes. The setup where this bug manifests is: Operating System: Ubuntu 24.04.1 on Windows with WSL. WSL, or Windows Subsystem for Linux, allows developers to run a Linux environment directly on Windows, which is a popular setup for many. The version of the Nickel code in use is 1.12.2. This specific version information is crucial because it tells us exactly which codebase is affected. Bugs can sometimes be introduced or fixed in certain versions, so knowing the version helps us understand if the issue is already addressed in a later release or if it's a regression. This environmental context is vital for debugging. It allows developers to replicate the issue in the same conditions and test potential fixes. Different operating systems and versions can have different behaviors due to variations in file system handling, caching mechanisms, and other low-level details. By identifying the environment where the bug occurs, we can focus our investigation on the specific components and interactions that might be involved. For instance, WSL has its own set of nuances in how it handles file system events, which could be a factor in this case. Knowing that the bug is reproducible in this particular setup is a key piece of the puzzle.
Root Cause Analysis
Okay, guys, let's dive into the nitty-gritty and figure out the root cause of this pesky file caching issue. The core of the problem seems to lie in how the LSP handles the SourceKind of files. When a file is opened in the editor using the LSP, its SourceKind is updated to SourceKind::Memory. This is a crucial detail because, according to the Nickel codebase, files with SourceKind::Memory are not invalidated by filesystem changes. This means that even if the file is deleted from the filesystem, the LSP continues to hold onto the cached version in memory. Now, here's the kicker: the SourceKind doesn't get changed back to Filesystem in any way, even if the file is no longer open in the editor. This is the key reason why the LSP doesn't detect the deletion of the file. The LSP is essentially stuck with the cached version, unaware of the external changes. You can check out the relevant code in the Nickel repository here to see how SourceKind affects caching behavior. This in-depth analysis reveals a critical flaw in the LSP's handling of file deletions. It highlights the importance of properly managing file metadata and ensuring that the cache reflects the actual state of the filesystem. The issue is not just about caching; it's about how the LSP reacts to external events and keeps its internal state consistent with the outside world. Understanding this root cause is the first step towards developing an effective solution.
Proposed Solution
Alright, let's talk solutions! The proposed solution to this file caching issue revolves around implementing the textDocument/didClose method in the LSP. This method is specifically designed to be called when a text document is closed in the editor. Our plan is to leverage this method to change the SourceKind back to Filesystem when a file is closed. This way, the LSP will no longer treat the file as being solely in memory and will be more responsive to filesystem changes. This is a crucial step in ensuring that the cache remains synchronized with the actual state of the files. By implementing textDocument/didClose, we can effectively tell the LSP,