fix(filesystem): name a broken symlink instead of blaming its parent

[v0ropaev]

Description

A symlink pointing at nothing makes the filesystem server report a missing parent directory for a directory that is plainly there.

Server Details

• Server: filesystem
• Changes to: path validation (src/filesystem/lib.ts), one new test file

Motivation and Context

validatePath catches ENOENT out of resolveUnicodeEquivalentPath and turns it into

Parent directory does not exist: <dirname>

That message never fits the case it names. When a path component is genuinely missing, the resolver does not throw: it returns the joined tail, with a comment saying so, precisely so create_directory can mkdir -p it. What does raise ENOENT in there is fs.realpath on an entry that appears in the directory listing but points nowhere.

So with an allowed directory R holding a dangling link.txt:

Parent directory does not exist: /var/folders/.../mcp-broken-symlink-N9OgbP

R exists, fs.stat(R) succeeds, and the caller is sent looking at the wrong thing. A stale node_modules/.bin entry gives the same shape.

The fix classifies the error where it happens, at the realpath call, rather than guessing from an errno two frames up:

Broken symlink: <path>

I left the existing fallback in place. The other ENOENT sources in that function are a vanished allowed directory and an intermediate directory removed mid-walk, where "parent directory does not exist" reads closer to true.

I first tried putting an fs.stat(parent) check in the catch to tell the two apart. That is worse: __tests__/lib.test.ts auto-mocks fs/promises, so fs.stat resolves to undefined and the check passes for both cases. Classifying at the source needs no such guess.

How Has This Been Tested?

New src/filesystem/__tests__/broken-symlink.test.ts, two cases against a real temp directory: a dangling symlink now reports Broken symlink while its parent still stats fine, and a genuinely missing path still resolves to the joined tail so create_directory keeps working. The first fails on current main with the message quoted above.

npx vitest run     167 passed, 3 failed
npx tsc --noEmit   clean

The three failures are in __tests__/unicode-paths.test.ts and are the same three on unmodified main in this checkout; they look like the composed/decomposed distinction not surviving on APFS. Not touched by this change.

Not tested through an LLM client, this is an error-message path.

Breaking Changes

None. No client configuration changes, and the only difference a caller sees is the wording of an error that was already being raised.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly (no user-facing behaviour to document, only the error text)
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options (none added)

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