OTP 24 overlay: resolving code:which/1 non‑determinism in repeatable development environments

Answer the Question First

1. Enforce OTP 23 resolution order in OTP 24: Set the external code directories before the overlay directory in the VM’s code path. Use the ERL_LIBS environment variable or the -pa flag when invoking erl.

• Example: export ERL_LIBS=$EXTERNAL_DIR:$OVERLAY_DIR (external first).
• Or: erl -pa $EXTERNAL_DIR -pa $OVERLAY_DIR …

2. Disable overlay loading for a specific release: OTP 24 does not provide a dedicated flag, but the overlay is only active when a non‑empty overlay directory exists. To disable it, either:

• Ensure the overlay directory is absent or empty before starting the VM.
• Or set ERL_CODE_PATH_OVERLAY="" to clear the path passed to the runtime.

3. Best‑practice release_handler rollback script for reused containers:

%% 1. Unpack the previous release
release_handler:unpack_release(PrevRelease).
%% 2. Install the previous release
release_handler:install_release(PrevRelease).
%% 3. Roll back the current release
release_handler:rollback_release().
%% 4. Remove any overlay files that were added by the newer release
filelib:delete_dir(OverlayDir),
%% 5. Purge stale modules that might still be loaded
[code:purge(Module) || Module <- code:all_loaded()].

This sequence guarantees that the VM ends up with the code from the previous release, with no overlay artifacts lingering.

Verification Steps

1. After starting the VM, run code:get_path() and confirm the external directory appears before the overlay directory.
2. Use code:which(MyModule) on a module that exists in both locations to verify the file path comes from the external directory.
3. Check overlay status: filelib:is_dir(OverlayDir) should return false or the directory should be empty if you want overlays disabled.
4. After rollback, run release_handler:which_releases() and code:which(MyModule) again to ensure the original module is loaded.

Missing Diagnostic Detail

To fine‑tune the recommendation, could you confirm whether the release you are testing contains an overlay directory and whether it is non‑empty when the VM starts?