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[flutter_tools] Fix UNC path resolution in depfile parsing on Windows - #191265
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On Windows, `Uri.toFilePath()` converts a UNC URI like `file:///UNC/server/share/file` to a root-relative path `\UNC\server\share\file`. When parsed by the build system, this path is resolved relative to the current drive (e.g., `U:\UNC\server\share\file`), which does not exist. This change detects root-relative UNC paths starting with `\UNC\` (case-insensitive) and converts them to standard UNC paths starting with `\\` (e.g., `\\server\share\file`), which are resolved correctly on Windows. Also refactored `SourceVisitor` to use `DepfileService` for parsing depfiles, eliminating redundant parsing logic and ensuring both paths benefit from this fix. Fixes flutter#43594
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Code Review
This pull request refactors depfile parsing in SourceVisitor to utilize DepfileService and introduces support for handling Windows UNC paths within DepfileService._processList. It also adds comprehensive tests for parsing and writing various Windows UNC path formats. Feedback suggests optimizing the Windows UNC path detection in depfile.dart by checking the path length and lowercasing only the prefix substring rather than the entire path string to avoid unnecessary allocations.
| /// If the file is missing, this visitor is marked as [containsNewDepfile]. | ||
| /// This is used by the [Node] class to tell the [BuildSystem] to | ||
| /// defer hash computation until after executing the target. | ||
| // depfile logic adopted from https://github.com/flutter/flutter/blob/7065e4330624a5a216c8ffbace0a462617dc1bf5/dev/devicelab/lib/framework/apk_utils.dart#L390 |
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So all of this is no longer relevant?
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Exactly — the parsing logic (_processList, _separatorExpr, _escapeExpr) was duplicate implementation of what DepfileService.parse already does (splitting inputs/outputs, unescaping spaces, and path normalization).
Delegating to environment.depFileService.parse eliminates the duplication and ensures SourceVisitor stays in sync with any depfile fixes (like this Windows UNC handling). I've restored the doc comment explaining the containsNewDepfile hash deferral mechanism for clarity.
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Normalize root-relative Windows UNC paths starting with
\UNC\in depfiles to standard UNC paths starting with\\. Also unifySourceVisitordepfile parsing to useDepfileService.Fixes #43594
Pre-launch Checklist
///).