How to implement a very simple file-based notification queue using bash?

I'm trying to come up with a very simple implementation for this problem:

• I have 2 environments behaving under a master-slave paradigm
• Environment A manages environment B: configures it, runs scripts in it, etc.
• Environment B cannot initiate communications with environment A
• Sometimes environment A leaves scripts running in environment B's background [./myScript.sh &], and has no way of knowing when those scripts are finished (well, environment A could just poll B with ps -aux, but I'm looking for a more generic system)

I'd like to create a system that satisfies these conditions:

• When a background script myScript.sh finishes, it leaves a message in some mailbox
• Environment A keeps a runner that periodically checks the mailbox in environment B, and consumes all new messages
• Adding and consuming messages to this 'queue' must be resistant to race conditions / concurrency
• The solution must be based on Bash

I thought of 2 options:

• Option A: modeling the mailbox as a file. myScript.sh appends notifications to the file; environment A reads and empties the file via ssh. I guess race conditions could be solved via file locking? I think bash allows this.
• Option B: much simpler, myScript.sh would leave messages as separate files in a well-known folder which would act as mailbox. Environment A would get the list of files, cat them and remove them.

But perhaps the vast know-how of StackOverflow will propose must better / simpler options.

Thanks!