[QuadStingray]

Summary

Fixes the CI failures where the "Start MongoDB" step aborts with Error response from daemon: container ... is not running, while docker logs shows a completely clean, error-free MongoDB startup.

Root cause: PID=$(pgrep mongod) runs under set -euo pipefail. When pgrep matches nothing it exits 1, which aborts the entire script — and since entrypoint.sh is PID 1, the container goes down with it, killing the mongod that had just started successfully. The if [ -z "$PID" ] guard in stop_mongod was never reachable for the same reason, and multiple matches broke ps -p the same way.

Change: a mongod_pid() helper that cannot abort the script and never returns more than one PID, plus wait_for_mongod_shutdown() which briefly retries for the forked process to appear and — if mongod really is gone — fails loudly with an explicit error instead of silently ending the script. Replaces all three affected call sites (stop_mongod and both keep-alive loops).

MONGO_LOG=STDOUT is the Dockerfile default, so the changed branch is the production path; the tail -f branch for a custom log file is untouched.

Test plan

Verified without Docker (no daemon available in the dev environment), via faked mongod/mongosh/pgrep binaries driving the real script:

• Unit tests against the real functions (8/8): empty pgrep result no longer aborts the script; multiple PIDs no longer break ps -p; stop_mongod now actually reaches its No mongod process found branch. As a regression proof, the old line dies before its own -z check under identical conditions.

• End-to-end simulation of the whole script across four configurations (CI default, with password, with replica set, race case):

  	before	after
  Normal operation — container stays up	✅	✅
  Container exits cleanly when mongod dies	✅	✅
  Race (pgrep sees mongod with a delay)	❌ container dead	✅

• Real CI run on this branch / next tag build.

Note: if some second cause is still hiding behind the same symptom, the container now writes an explicit ERROR: mongod is not running after startup line, which surfaces in CI via the docker logs dump added in MongoCamp/mongodb-github-action#2.