--mount-git-worktree-common-dir silently ignored when devcontainer.json sets a custom workspaceMount

[AvishayHirsh]

Summary

--mount-git-worktree-common-dir (added in #1127) has no effect when the devcontainer.json defines a custom workspaceMount. The worktree common-dir bind mount is never added, so in-container git fails with fatal: not a git repository for any git worktree whose devcontainer uses a custom workspaceMount.

Root cause

In src/spec-node/utils.ts (getWorkspaceConfiguration, ~L386) the entire worktree detection + common-dir mount block is guarded by:

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

When workspaceMount is present, the block is skipped — the .git file is never read, the common dir is never resolved, and no mount is added. (Flagged as a possible oversight by Jérémy Lal (@kapouer) in #796, Mar 2026.)

Reproduction (CLI 0.82.0, git 2.52)

1. git init main && (cd main && git commit --allow-empty -m init && git worktree add --relative-paths ../wt)
2. Add wt/.devcontainer/devcontainer.json with a custom workspaceMount, e.g.:

   { "image": "mcr.microsoft.com/devcontainers/base:debian",
     "workspaceMount": "source=${localWorkspaceFolder},target=/workspace,type=bind",
     "workspaceFolder": "/workspace" }

3. devcontainer up --workspace-folder wt --mount-git-worktree-common-dir true
4. devcontainer exec --workspace-folder wt -- git -C /workspace status → fatal: not a git repository: (null)
5. docker inspect on the container shows only the workspace bind mount; the worktree common dir was not mounted.

Removing workspaceMount makes it work — confirming the guard is the cause.

Expected

The common-dir mount should be computed and added even when a custom workspaceMount is set, by resolving the mount target relative to the configured workspaceMount target / workspaceFolder. This is the common case for monorepos that mount a parent directory and set workspaceFolder to a subpath. Relates to #796.

Workaround

Adding the common dir manually via a mounts entry (binding the main repo's .git to the path the worktree's .git file resolves to inside the container) restores in-container git — matching the maintainer's earlier suggestion in #796.

[AvishayHirsh]

Workaround until this is fixed. The CLI skips the worktree common-dir mount whenever workspaceMount is set — so add it yourself with a mounts entry and --override-config. This script derives the correct mount for your worktree layout (sibling or nested) and brings the container up:

#!/usr/bin/env bash
# Worktree + custom workspaceMount: make in-container git work via an extra common-dir mount.
# Run from the folder with your .devcontainer/ :
#   bash fix-worktree-dc.sh                  -> just prints the "mounts" line to add
#   bash fix-worktree-dc.sh --up             -> also brings the container up (devcontainer CLI)
#   bash fix-worktree-dc.sh /workspaces --up -> non-/workspace workspaceMount target + bring up
set -euo pipefail

WMT=/workspace; APPLY=0
for a in "$@"; do case "$a" in --up) APPLY=1;; *) WMT="$a";; esac; done

ROOT="$(git rev-parse --show-toplevel)"
[ -f "$ROOT/.git" ] || { echo "Not a git worktree — nothing to do."; exit 1; }
MAIN_GIT="$(cd "$(git rev-parse --git-common-dir)" && pwd)"
REL="$(sed -n 's/^gitdir: //p' "$ROOT/.git")"
TARGET="$(python3 -c "import posixpath;print(posixpath.normpath('$WMT/$REL/../..'))")"
echo "mounts entry:  source=$MAIN_GIT,target=$TARGET,type=bind"

if [ "$APPLY" = 1 ]; then
  python3 - "$MAIN_GIT" "$TARGET" <<'PY'
import json, re, sys
s = re.sub(r",(\s*[}\]])", r"\1", re.sub(r"//.*", "", open(".devcontainer/devcontainer.json").read()))
c = json.loads(s); c.setdefault("mounts", []).append(f"source={sys.argv[1]},target={sys.argv[2]},type=bind")
json.dump(c, open("/tmp/dc-override.json","w"), indent=2)
PY
  devcontainer up   --workspace-folder . --override-config /tmp/dc-override.json
  devcontainer exec --workspace-folder . -- bash -lc "git config --global --add safe.directory '*' && git status"
fi

CLI steps:

npm i -g @devcontainers/cli                       # if you don't have it
cd path/to/your/worktree/<...>/                   # the folder containing .devcontainer/
bash fix-worktree-dc.sh --up                      # mounts main .git + brings the container up
# then run anything in the container — git now resolves:
devcontainer exec --workspace-folder . -- bash -lc "git status && ./gradlew build"

The override goes to a throwaway file, so your committed devcontainer.json is untouched. (Prefer to make it permanent? run without --up, copy the printed mounts line into devcontainer.json.)

Layout-agnostic — TARGET = normalize(<workspaceMount target> + <relative gitdir from .git> + /../..): nested worktree → /.git, sibling worktree → /<repo>/.git. Requires a relative-paths worktree (git worktree add --relative-paths, git ≥ 2.48).

(VS Code / "Reopen in Container" variant: microsoft/vscode-remote-release#11478 (comment))