Possible bug with transaction.on_commit

Hi,

I believe that I encountered a bug or at least a serious limitation with transaction.on_commit in Django 1.9.x. Essentially, the on_commit hooks seem to be tied to the database connection instead of the transaction. This means that unrelated transactions may trigger on_commit hooks, which results in undesired execution order. Here is an example:

from django.db import transaction

from foobar.models import Model1, Model2

@transaction.atomic

def outer():

    print("START - OUTER")

    for i in range(4):

        f1(i)

    print("END - OUTER")

@transaction.atomic

def f1(i):

    model = Model1(description="test {0}".format(i))

    model.save()

    transaction.on_commit(lambda: commit_hook(model, i))

def commit_hook(model, i):

    print("START - on_commit hook")

    f2(i)

    print("STOP - on_commit hook")

@transaction.atomic

def f2(i):

    print("CALLING F2")

    model = Model2(description="test {0}".format(i))

    model.save()

Some quick explanations: 

- outer is the outermost atomic block. f1 will only create a savepoint(). This works as expected, there is only one commit (trace erased for simplicity). 

- f2 is wrapped in an "outermost" atomic block and indeed, f2 will be called four times and there will be four commits (trace erased for simplicity).

- f1 register a commit_hook. I expect that at the end of outer(), all commit hooks will be called.

The expected trace is:

START - OUTER

END - OUTER

START - on_commit hook

CALLING F2

STOP - on_commit hook

START - on_commit hook

CALLING F2

STOP - on_commit hook

START - on_commit hook

CALLING F2

STOP - on_commit hook

START - on_commit hook

CALLING F2

STOP - on_commit hook

The actual trace (f2 is triggering the on_commit hooks registered in f1/outer):

START - OUTER

END - OUTER

START - on_commit hook

CALLING F2

START - on_commit hook

CALLING F2

START - on_commit hook

CALLING F2

START - on_commit hook

CALLING F2

STOP - on_commit hook

STOP - on_commit hook

STOP - on_commit hook

STOP - on_commit hook

I wanted to know if someone encountered this issue or if I am misunderstanding on_commit before opening a ticket.

Regards,

Bart

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