Original bug ID: 7209
Reporter: lcorrenson
Assigned to: @diml
Status: closed (set by @xavierleroy on 2017-09-24T15:32:12Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: standard library
Monitored by: bobot @dbuenzli

Bug description

Originally, we discover the « bug » when creating a process with a non-existing command :

• Pervasive.at_exit some job
• Unix.create_process with a wrong command
• The job is executed twice

Diagnosis : Unix.create_process forks the original process, which is an ocaml runtime with Pervasive.at_exit registered job(s). Then, Unix.create_process fails without cleaning the duplicated runtime, hence the job is executed in both processes. When Unix.create_process succeed, the registered jobs are executed once as expected.

Less surprising, the « bug » is also visible when using Unix.fork -- is it a bug or a feature ?
A work around is to check within jobs registered via Pervasives.at_exit whether the pid has changed or not.
Though, we would suggest that any forking operation in the standard library starts by cleaning the daemons in the child processes.

Steps to reproduce

See attached files proc.ml and fork.ml :

$ ocamlbuild -package unix proc.native && ./proc.native ocamlc # Existing process (normal behavior)
$ ocamlbuild -package unix proc.native && ./proc.native wrong # Wrong process (unexpected)
$ ocamlbuild -package unix fork.native && ./fork.native # Fork case (non intuitive)

Additional information

As a work-around, we simply use:

let mypid = Unix.getpid ()
let at_exit f = Persasives.at_exit
(fun () -> if Unix.getpid () = mypid then f ())

File attachments

• bug.zip
• fork.ml
• proc.ml