Watch the physical directory of files reached through a symlinked directory

[Joaquín Sorianello (joac)]

Fixes #64448

Problem

Since the file watcher stopped following symlinks during its directory walk (typescript-go#4376), tsc --watch never sees edits to program files that live behind a symlinked directory inside the project, e.g. src/common -> ../../common/src in monorepos that share sources this way. The recursive watch on src covers the logical path, but the OS subscription is on the physical src directory, which does not contain the symlink target. TypeScript 6.0.3 detected these edits; 7.0.2 and @next do not (repro in the issue).

Change (internal/execute/watcher.go)

• In computeDesiredWatches, resolve the real path of each seen file's directory (memoised per directory). When it differs from the logical directory, watch the physical directory, non-recursively like other uncovered directories, and record the physical → logical mapping keyed by canonical path.
• In DoCycle, right after draining events, rewrite events reported under such a physical directory back to the logical path. That is the name the program knows the file by, so the relevance check and the single-file fast path work unchanged.
• Files whose program path is already real (e.g. node_modules resolved through realpath) take the existing code path; no behaviour change for them.

Tests

• New tscWatch scenario "watch detects change in file under symlinked directory" with baseline. It shows the physical shared/src directory being watched and an edit behind the symlink producing a rebuild with the expected new error. With watcher.go stashed, the scenario fails.
• Verified on a real filesystem (macOS 26, arm64) with the standalone repro from tsc --watch 7.x does not detect edits to files under a symlinked directory inside the project (6.0.3 did) #64448, using builds of this commit with and without the change: the unfixed build misses edits made through the real path and through the symlink, the fixed build detects both.

Notes for review

• Cost: one Realpath per distinct seen-file directory on each reconcile after a full build, memoised per directory.
• New entries behind the symlink are handled: creating a new file in the symlinked directory, or a new subdirectory (even two levels deep at once) with a file, raises an event on the watched physical directory. Mapped to the logical path it is a directory change under the wildcard root, which marks the file set dirty, so the next build re-enumerates the includes and picks the file up; the reconcile that follows adds a watch for the new directory, so later edits to that file are detected too (verified on macOS with the repro). The one remaining gap is a new file in a subdirectory that previously contained no program files, which has no watch of its own: it is picked up on the next build triggered by any other change. For comparison, TypeScript 6.0.3 detects edits to existing files behind the symlink but never notices new files or new subdirectories there, so this is strictly better than the previous behaviour.
• Only macOS was exercised on a real filesystem; the in-memory test covers the logic for all backends.

Checklist

• There is an associated issue: tsc --watch 7.x does not detect edits to files under a symlinked directory inside the project (6.0.3 did) #64448
• Code is up-to-date with the main branch
• npx hereby test: all internal/execute packages pass; the only failures in the full run are internal/fswatch/TestFSEventsExpansionAliases sub-cases (FSEvents Unicode-normalisation tests) which fail on this machine on main as well, with a different sub-case each run, and are untouched by this change
• npx hereby lint (0 issues)
• npx hereby check:format
• There are new or updated tests validating the change

[Joaquín Sorianello (joac)]

@microsoft-github-policy-service agree

[Copilot]

Copilot review overview

🔵 Needs a closer look

The cross-platform filesystem watcher behavior and path remapping warrant final human review despite the focused regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

This PR restores watch-mode updates for source files reached through symlinked project directories by adding physical directory watches and translating their events back to logical program paths.

Changes:

• Detects logical source directories whose real paths differ and watches their physical directories.
• Remaps physical watcher events to logical paths before relevance and incremental-update checks.
• Adds a tscWatch regression scenario and reference baseline.

File	Description
tsc/​internal/​execute/​watcher.go	Adds physical-directory watches and symlink event-path remapping.
tsc/​internal/​execute/​tsctests/​tscwatch_test.go	Adds the symlinked-source watch regression scenario.
tsc/​testdata/​baselines/​reference/​tscWatch/​commandLineWatch/​watch-detects-change-in-file-under-symlinked-directory.js	Captures expected watch registration and rebuild behavior.

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[Joaquín Sorianello (joac)]

This was already fixed on #64404