Mount worktree common dir when a custom workspaceMount is set

[AvishayHirsh]

Fixes #1243

Problem

--mount-git-worktree-common-dir (added in #1127) is silently ignored when a devcontainer.json sets a custom workspaceMount. Git worktrees then break inside the container:

$ devcontainer exec --workspace-folder wt -- git -C /workspace status
fatal: not a git repository: (null)

This is common in monorepo setups where a custom workspaceMount/workspaceFolder is used.

Root cause

In getWorkspaceConfiguration (src/spec-node/utils.ts), the worktree common-dir computation lives inside the guard that only auto-computes workspaceMount/workspaceFolder when they aren't already set in the config:

if (workspace && (!workspaceFolder || !('workspaceMount' in config))) { ... }

When the config sets both workspaceFolder and workspaceMount, the whole block is skipped, so additionalMountString (the common-dir bind mount) is never built and the main repo's .git is never mounted.

Fix

The common-dir mount is an additive, separate mount — it can't conflict with a user-defined workspaceMount, so it shouldn't be gated by the "don't overwrite user config" guard:

• Widen the guard so the worktree block also runs when the feature is enabled. --mount-git-worktree-common-dir defaults to false, so default behavior is unchanged unless the flag is opted into.
• Resolve the common-dir target relative to the custom workspaceMount target. Because additionalMountString is not part of the later variable-substitution pass, the target is substituted here (handles the canonical target=/workspaces/${localWorkspaceFolderBasename}). Without a custom mount, the previously computed target is used, so existing behavior is unchanged.

Tests

Added unit tests in src/test/workspaceConfiguration.test.ts (across linux/darwin/win32) covering: the reported scenario, ${localWorkspaceFolderBasename} substitution in the target, feature-off no-op, non-worktree no-op, and a worktree nested inside the main repo. yarn lint, yarn type-check, and the test suite all pass locally.

Known limitations (pre-existing, unchanged by this PR)

• Only a relative gitdir is handled (worktrees created with git worktree add --relative-paths); absolute gitdirs were already unsupported.
• Assumes the custom mount's source is the worktree root (the directory holding the .git file).

[AvishayHirsh]

@microsoft-github-policy-service agree

[AvishayHirsh]

Thanks for the worktree common-dir feature in #1127! This is a small follow-up for #1243: the common-dir mount is skipped when devcontainer.json sets a custom workspaceMount (common in monorepos), so worktree git operations break in the container (fatal: not a git repository).

The fix is gated behind the existing --mount-git-worktree-common-dir flag (default off → no behavior change otherwise), and resolves the common-dir target relative to the custom (substituted) workspaceMount target. Added unit tests across linux/darwin/win32. CLA is signed; yarn lint, yarn type-check, and the test suite pass locally.

@Mathiyarasy @chrmarti would appreciate a workflow-run approval and a review whenever you have a moment — thanks!