docs(testing): sandbox-doctor — one command makes a cloud sandbox run the suite green

Measured failure taxonomy for Vercel/Conductor sandboxes (missing /dev/fd,
64M /dev/shm, seccomp-supervisor access(2) EACCES under load, uid-1000
processes with FULL capabilities defeating chmod-denial tests, no X server,
no git identity, Conductor git-shim exit-code laundering) plus the
idempotent script that treats all of it and seeds the run recipe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/sandbox-doctor.sh

@@ -0,0 +1,93 @@
+#!/bin/sh
+# sandbox-doctor — make a syscall-supervised cloud sandbox (Vercel sandbox /
+# Conductor cloud workspace) able to run `bun run test` green.
+#
+# Root causes this script treats (all measured on a live Vercel sandbox,
+# Amazon Linux 2023, PID 1 = sandbox-init with a seccomp filter):
+#
+#   1. /dev/fd is missing on fresh boots — every bash process substitution
+#      `<(...)` fails with "/dev/fd/63: No such file or directory".
+#   2. /dev/shm is 64M — concurrent Chromium instances crash.
+#   3. The seccomp supervisor spuriously fails access(2)-family syscalls for
+#      BUSY processes: `git init` dies with "Cannot access work tree:
+#      Permission denied", bun's existsSync returns false for files written
+#      microseconds earlier (statx succeeds while access fails on the same
+#      path). Per-process pressure matters: 1 serial mega-shard and 6-way
+#      sharding both fail hard; 2 shards is the sweet spot. Under blanket
+#      denial the whole /tmp subtree is denied while $HOME stays clean, so
+#      tests run with TMPDIR under HOME.
+#   4. Every process runs with FULL capabilities (CapEff=1ffffffffff) despite
+#      uid 1000 — CAP_DAC_OVERRIDE makes chmod-denial tests unfailable.
+#      Tests must run under `setpriv --ambient-caps=-all --bounding-set=-all`.
+#   5. No X server — headed-browser tests (browse handoff) need Xvfb.
+#   6. No git identity — fixtures that rely on ambient user.name/email fail.
+#   7. Conductor's /conductor/bin/git shim captures $? AFTER its `if`
+#      construct (POSIX resets it to 0 on a false condition with no else), so
+#      every push/pull/fetch/clone/ls-remote FAILURE exits 0. Tests that
+#      inject remote failures (pre-receive hooks) see phantom successes.
+#      Report upstream via Conductor Help -> Send Feedback; patched locally.
+#
+# Idempotent. Run once per sandbox boot (or source ~/.bashrc, which this
+# script also seeds). Then:
+#
+#   DISPLAY=:99 TMPDIR=$HOME/tmp GSTACK_FREE_JOBS=2 GSTACK_FREE_RETRY_FLAKY=1 \
+#     setpriv --ambient-caps=-all --bounding-set=-all bun run test
+set -eu
+
+say() { printf 'sandbox-doctor: %s\n' "$1"; }
+
+# 1. /dev/fd
+if [ ! -e /dev/fd ]; then
+  sudo ln -sfn /proc/self/fd /dev/fd
+  say 'restored /dev/fd -> /proc/self/fd'
+fi
+
+# 2. /dev/shm size
+if [ "$(df -k /dev/shm 2>/dev/null | awk 'NR==2 {print $2}')" -lt 1048576 ]; then
+  sudo mount -o remount,size=4G /dev/shm
+  say 'remounted /dev/shm at 4G'
+fi
+
+# 3. TMPDIR under HOME (persisted via bashrc below; created here)
+mkdir -p "$HOME/tmp"
+
+# 5. Xvfb for headed-browser tests
+command -v Xvfb >/dev/null 2>&1 || sudo dnf install -y xorg-x11-server-Xvfb >/dev/null
+pgrep -x Xvfb >/dev/null 2>&1 || { Xvfb :99 -screen 0 1280x800x24 >/dev/null 2>&1 & say 'started Xvfb on :99'; }
+
+# 6. git identity (only if absent — never clobber a configured one)
+git config --global user.name >/dev/null 2>&1 || {
+  git config --global user.name "$(whoami)"
+  git config --global user.email "$(whoami)@localhost"
+  say 'seeded global git identity'
+}
+
+# 7. Conductor git-shim exit-code bug
+if [ -f /conductor/bin/git ] && grep -q '^status=\$?' /conductor/bin/git 2>/dev/null; then
+  sudo python3 - <<'EOF'
+src = open('/conductor/bin/git').read()
+old = 'exit 0\nfi\nstatus=$?'
+new = 'exit 0\nelse\n\tstatus=$?\nfi'
+if old in src:
+    open('/conductor/bin/git', 'w').write(src.replace(old, new))
+    print('sandbox-doctor: patched /conductor/bin/git exit-code laundering')
+EOF
+fi
+
+# Persist the env recipe for interactive shells.
+if ! grep -q 'GSTACK sandbox test env' "$HOME/.bashrc" 2>/dev/null; then
+  cat >> "$HOME/.bashrc" <<'EOF'
+
+# GSTACK sandbox test env (written by scripts/sandbox-doctor.sh)
+export TMPDIR="$HOME/tmp"
+export GSTACK_FREE_JOBS=2
+export GSTACK_FREE_RETRY_FLAKY=1
+export DISPLAY=:99
+[ -e /dev/fd ] || sudo ln -sfn /proc/self/fd /dev/fd 2>/dev/null
+pgrep -x Xvfb >/dev/null 2>&1 || (Xvfb :99 -screen 0 1280x800x24 >/dev/null 2>&1 &)
+EOF
+  say 'seeded ~/.bashrc test env'
+fi
+
+say 'done. run tests with:'
+say '  setpriv --ambient-caps=-all --bounding-set=-all bun run test'