Merge multiple fish history files together using flock (untested)

fish_merge.py

#!/bin/python

import pathlib
import fnctl
import yaml
import time

# Watch for changes in other_history and merge them into local_history
local_history = pathlib.Path('~/.local/share/fish/fish_history').expanduser()
other_history = pathlib.Path('~/.local/share/fish/other_history').expanduser()

# Wait this many seconds between merges
merge_frequency_sec = 5.0

last_mtime = 0
while True:
    # Read the other history file if it's changed since we've last merged it
    last_mtime_now = other_history.stat().st_mtime
    if last_mtime_now > last_mtime:
        # Open and lock the other file (it might work nicely over sshfs)
        with open(other_history, 'r') as f:
            fd = f.fileno()
            fnctl.flock(fd, fnctl.LOCK_EX)
            try:
                last_mtime_now = other_history.stat().st_mtime  # Capture the actual mtime when we had the lock
                other_history_entries = yaml.safe_load(f)
            except:
                fcntl.flock(fd, fcntl.LOCK_UN)
                
        # Filter to new entries (TODO maybe this can start from a last_entry_index for less comparisons)
        other_history_entries_new = [x for x in other_history_entries if int(x['when']) > last_mtime]
        history_to_append = yaml.dump(filtered_entries, default_flow_style=False)
        
        # Now open and lock the local file to append the other history
        with open(local_history, 'a') as f:
            fd = f.fileno()
            fnctl.flock(fd, fnctl.LOCK_EX)
            try:
                f.write(history_to_append)
            except:
                fcntl.flock(fd, fcntl.LOCK_UN)
                
        last_mtime = last_mtime_now
        # last_mtime = max([int(entry['when']) for entry in other_history_enties_new])  # Would something like this be more robust?
    time.sleep(merge_frequency_sec)