Does Scenario produces incorrect JUJU_HOOK_NAME?

[Abuelodelanada]

While I was working on this PR, I discovered some behaviour in Scenario that appears to be incorrect.

When simulating events for relations whose endpoint names contain hyphens (e.g. receive-ca-cert), Scenario sets JUJU_HOOK_NAME to a mixed format with underscores in the prefix and hyphens in the suffix (e.g. receive_ca_cert-relation-changed), instead of the all-hyphens format that real Juju provides (receive-ca-cert-relation-changed).

This causes unit tests to pass against incorrect hook name values, masking real bugs in charm code that depends on JUJU_HOOK_NAME.

Step 1 — Normalisation

In the function _normalise_name...

def _normalise_name(s: str):
    """Event names, in Scenario, uniformly use underscores instead of dashes."""
    return s.replace('-', '_')

all dashes become underscores: "receive-ca-cert-relation-changed" → "receive_ca_cert_relation_changed"

Step 2 — _EventPath.__new__

Then in _EventPath.__new__

def __new__(cls, string: str):
    string = _normalise_name(string)
    instance = super().__new__(cls, string)
    instance.name = name = string.split('.')[-1]
    instance.owner_path = string.split('.')[:-1] or ['on']
    instance.suffix, instance.type = _EventPath._get_suffix_and_type(name)
    instance.prefix = string.removesuffix(instance.suffix)
    instance._is_custom = instance.suffix == ''
    return instance

For "receive_ca_cert_relation_changed":

• suffix = "_relation_changed" (matched from _RELATION_EVENTS_SUFFIX)
• prefix = "receive_ca_cert"

Step 3 — _juju_name construction

# This is the event name as Juju provides it, with dashes not underscores.
object.__setattr__(self, '_juju_name', f'{path.prefix}{path.suffix.replace("_", "-")}')

Only the suffix gets underscores converted back to hyphens:

• path.prefix = "receive_ca_cert" (underscores preserved)
• path.suffix.replace("_", "-") = "-relation-changed"
• Result: "receive_ca_cert-relation-changed" ← WRONG

Step 4 — Assignment to JUJU_HOOK_NAME

'JUJU_HOOK_NAME': '' if event._is_action_event else event._juju_name,

Expected behaviour

JUJU_HOOK_NAME should be set to "receive-ca-cert-relation-changed" (all hyphens), matching real Juju behaviour.

From Juju logs I see:

ran "receive-ca-cert-relation-created" hook (via hook dispatching script: dispatch)

how to reproduce it

from ops.testing import Context, Relation, State
from charm import MyCharm

ctx = Context(MyCharm)
rel = Relation("receive-ca-cert", remote_app_data={"certificates": "[]"})
state = State(leader=True, relations=[rel])

# During ctx.run(), JUJU_HOOK_NAME will be "receive_ca_cert-relation-changed"
# instead of the correct "receive-ca-cert-relation-changed"
ctx.run(ctx.on.relation_changed(rel), state)
event = ctx.on.relation_changed(rel)
print(event._juju_name) # receive_ca_cert-relation-changed

[tonyandrewmeyer]

I verified the real Juju behaviour against a live LXD controller on both 3/stable and 4/stable.

I created a minimal machine charm with a hyphenated peer relation endpoint receive-ca-cert and a dispatch script that logs the relevant JUJU_* environment variables on every hook:

#!/bin/bash
juju-log "ENVDUMP|HOOK_NAME=[${JUJU_HOOK_NAME}]|DISPATCH_PATH=[${JUJU_DISPATCH_PATH}]|RELATION=[${JUJU_RELATION}]|RELATION_ID=[${JUJU_RELATION_ID}]"

# metadata.yaml
name: cacharm
peers:
  receive-ca-cert:
    interface: catest

I deployed with 2 units so that -relation-created, -relation-joined, and -relation-changed all fire, then read the values back from juju debug-log.

For the receive-ca-cert relation, both versions set identical values:

Variable	Juju 3.6.23 (3/stable)	Juju 4.0.5 (4/stable)
JUJU_HOOK_NAME	receive-ca-cert-relation-changed	receive-ca-cert-relation-changed
JUJU_DISPATCH_PATH	hooks/receive-ca-cert-relation-changed	hooks/receive-ca-cert-relation-changed
JUJU_RELATION	receive-ca-cert	receive-ca-cert
JUJU_RELATION_ID	receive-ca-cert:0	receive-ca-cert:0

The same all-hyphens form applies to -relation-created and -relation-joined.

as the issue suggests, Juju uses all hyphens; there are no underscores anywhere in the hook name.

Raw log lines (3.6.23, identical shape on 4.0.5):

ENVDUMP|HOOK_NAME=[receive-ca-cert-relation-created]|DISPATCH_PATH=[hooks/receive-ca-cert-relation-created]|RELATION=[receive-ca-cert]|RELATION_ID=[receive-ca-cert:0]
ENVDUMP|HOOK_NAME=[receive-ca-cert-relation-joined]|DISPATCH_PATH=[hooks/receive-ca-cert-relation-joined]|RELATION=[receive-ca-cert]|RELATION_ID=[receive-ca-cert:0]
ENVDUMP|HOOK_NAME=[receive-ca-cert-relation-changed]|DISPATCH_PATH=[hooks/receive-ca-cert-relation-changed]|RELATION=[receive-ca-cert]|RELATION_ID=[receive-ca-cert:0]

What Scenario produces:

testing/src/scenario/state.py:2253 builds _juju_name as:

object.__setattr__(self, '_juju_name', f'{path.prefix}{path.suffix.replace("_", "-")}')

Tracing ctx.on.relation_changed(Relation("receive-ca-cert")):

1. context.py:288 builds the event path string "receive-ca-cert_relation_changed".
2. _normalise_name (state.py:369) replaces - → _, giving "receive_ca_cert_relation_changed".
3. _EventPath splits it into prefix="receive_ca_cert" and suffix="_relation_changed".
4. Only the suffix gets _ → - restored, producing receive_ca_cert-relation-changed.

_runtime.py:76-77 then sets both JUJU_HOOK_NAME and JUJU_DISPATCH_PATH to that malformed value. (JUJU_RELATION at _runtime.py:109 is built from relation.endpoint directly, so it is correct — only the hook-name and dispatch-path are wrong.)

Variable	Real Juju (3.6 & 4.0)	Scenario
JUJU_HOOK_NAME	receive-ca-cert-relation-changed	receive_ca_cert-relation-changed
JUJU_DISPATCH_PATH	hooks/receive-ca-cert-relation-changed	hooks/receive_ca_cert-relation-changed
JUJU_RELATION	receive-ca-cert	receive-ca-cert

[james-garner-canonical]

Thanks for this @tonyandrewmeyer! Would it be easy for you to repeat the experiment with an endpoint name containing underscores (assuming charmcraft or juju don't reject that)?

[tonyandrewmeyer]

Would it be easy for you to repeat the experiment with an endpoint name containing underscores (assuming charmcraft or juju don't reject that)?

I used the same minimal machine charm as before, but with the peer endpoint declared using underscores:

# metadata.yaml
name: uscharm
peers:
  receive_ca_cert:        # underscores, not dashes
    interface: catest

Same two unit deploy, 4.0.5 and 3.6.23.

For the receive_ca_cert relation, both versions set identical values:

Variable	Juju 3.6.23 (3/stable)	Juju 4.0.5 (4/stable)
JUJU_HOOK_NAME	receive_ca_cert-relation-changed	receive_ca_cert-relation-changed
JUJU_DISPATCH_PATH	hooks/receive_ca_cert-relation-changed	hooks/receive_ca_cert-relation-changed
JUJU_RELATION	receive_ca_cert	receive_ca_cert
JUJU_RELATION_ID	receive_ca_cert:0	receive_ca_cert:1

The same shape applies to -relation-created and -relation-joined.

Juju keeps the endpoint name exactly as declared (underscores preserved) and uses dashes only in the event-type suffix. This produces the "mixed" form receive_ca_cert-relation-changed.

Raw log lines (3.6.23; 4.0.5 is identical):

ENVDUMP|HOOK_NAME=[receive_ca_cert-relation-created]|DISPATCH_PATH=[hooks/receive_ca_cert-relation-created]|RELATION=[receive_ca_cert]|RELATION_ID=[receive_ca_cert:0]
ENVDUMP|HOOK_NAME=[receive_ca_cert-relation-joined]|DISPATCH_PATH=[hooks/receive_ca_cert-relation-joined]|RELATION=[receive_ca_cert]|RELATION_ID=[receive_ca_cert:0]
ENVDUMP|HOOK_NAME=[receive_ca_cert-relation-changed]|DISPATCH_PATH=[hooks/receive_ca_cert-relation-changed]|RELATION=[receive_ca_cert]|RELATION_ID=[receive_ca_cert:0]

So the rule is: the entity name (the prefix) is preserved verbatim — dashes or underscores — and only the event-type suffix is hyphenated. Juju does not normalise the prefix at all.