Does make-file-logger guarantee log persistence during abrupt process termination?

No. make-file-logger does not guarantee that buffered log entries reach disk if the process dies abruptly. The file port it creates uses block buffering by default, so entries are written only when the buffer fills, the port is explicitly flushed, or the port is closed during a normal shutdown. An uncaught exception that escapes the runtime's cleanup, a segfault, or SIGKILL will silently drop whatever is still in the buffer — typically the exact error messages you wanted for post-mortem analysis.

What Racket does and does not give you

There is no built-in "flush on fatal crash" hook in the logging system itself. You have three realistic options, in increasing order of reliability:

• Line buffering: open the logger's file with 'line buffering so each newline-terminated entry is flushed immediately. This survives uncaught exceptions and most signals, but not SIGKILL or a hard crash, and it adds per-line I/O overhead.
• A custodian shutdown handler that calls flush-output on the logger's port (or logger-flush where available) before the custodian tears down. This catches normal exits and most exception-driven terminations, because Racket's runtime still gets to run cleanup code.
• Explicit flush after fatal-level messages: keep block buffering for throughput, but flush after log-fatal/log-error calls. This is usually the best balance for deployment diagnostics.

A practical compromise

(define log-port
  (open-output-file "deploy.log" #:exists 'append))

(define logger (make-file-logger "deploy" log-port))

;; Flush when the custodian shuts down (normal exit, most exceptions)
(define shutdown-cust (make-custodian))
(parameterize ([current-custodian shutdown-cust])
  (thread (lambda ()
            (sync (custodian-shutdown-event shutdown-cust))
            (flush-output log-port))))

;; For the messages that matter most:
(define (log-critical! msg)
  (log-message logger 'fatal msg)
  (flush-output log-port))

The exact event API varies by Racket version; verify against your installed racket/logging and custodian docs rather than assuming this snippet compiles unchanged.

Verify the failure modes you actually expect

Do not trust the configuration without testing it against your real termination paths:

1. Log a few messages and exit normally — everything should appear.
2. Log, then raise an uncaught exception or send SIGTERM — with default buffering the tail is usually missing; with a shutdown handler or line buffering it should be present.
3. Send SIGKILL (kill -9) — messages will be lost regardless of buffering mode or handlers, because no Racket code runs. No software-side flush can fix this.

The honest limit

If your deployment process can be killed with SIGKILL or can crash below the runtime (segfault in a foreign library, OOM killer), the only robust answer is to move persistence out of the process: log to stdout and let a supervisor, systemd journal, or a separate collector own the file. For everything short of that, line buffering plus a custodian flush handler covers the realistic failure set at acceptable cost.