Merge ~graylog-charmers/charm-graylog:core-resource into ~graylog-charmers/charm-graylog:master

[Kevin W Monroe]

lp:1828063 add a core.snap resource to facilitate offline installs

The core snap is required by the graylog snap. To facilitate offline deployment, users need the ability to attach a core.snap resource in addition to the graylog.snap resource.

A downside of this is that future graylog charm releases will need to have a zero-byte core.snap resource attached, as in:

rm /tmp/core.snap && touch /tmp/core.snap
charm push /path/to/graylog-charm --resource core=/tmp/core.snap
charm release <graylog-X> --resource core-0 --resource graylog-Y

For normal deployments, layer-snap will recognize that core.snap is a 0-byte resource and will fall back to installing from the snap store. For offline deployments, users will need to attach the desired core.snap, as in:

juju deploy /path/to/graylog-charm \
  --resource core=/path/to/real/core.snap \
  --resource graylog=/path/to/real/graylog.snap

Also in offline deployments, we need to ensure the coren.snap is installed prior to graylog.snap, hence this is dependent on:

https://github.com/stub42/layer-snap/pull/22.

[🤖 Canonical IS Merge Bot (canonical-is-mergebot)]

This merge proposal is being monitored by mergebot. Change the status to Approved to merge.

[George Kraft (cynerva)]

LGTM once https://github.com/stub42/layer-snap/pull/22 is merged.

[Kevin W Monroe (kwmonroe)]

We did something similar to this MP for prometheus; during that review, stub requested a helper for developers to know how to use these snap resources:

https://code.launchpad.net/~prometheus-charmers/prometheus2-charm/+git/prometheus2-charm/+merge/367206

I added something similar here:

$ make charmrelease
---------------------------------------------------------------------
graylog supports core and graylog snap resources.

Attached resources will be processed during charm deployment. If a
zero-byte resource is found, the charm will install the relevant snap
from the store. This provides flexibility in choosing which snaps will
be installed by the charm.

For example, release a charm that uses the core snap from the store
(0-byte resource attached) along with a specific graylog snap:

cd /tmp
rm empty-core.snap && touch empty-core.snap
snap download --channel=stable graylog
charm push /Users/kwmonroe/charms/builds/graylog --resource core=/tmp/empty-core.snap --resource graylog=/tmp/graylog_*.snap
charm release <revision> --resource core-<m> --resource graylog-<n>

If the resources you want already exist in the charm store, omit the
declarations during 'charm push' and specify resource revisions
during 'charm release':

charm push /Users/kwmonroe/charms/builds/graylog
charm release <revision> --resource core-0 --resource graylog-3
---------------------------------------------------------------------

[George Kraft (cynerva)]

review: Approve

[Stuart Bishop (stub)]

Code changes are fine. I think the text to 'charmrelease' will confuse less experienced people tasked with publishing the charm to edge or stable channels and can be reworked, reworded from their perspective (as a newly employed CRE thrown in at the deep end, I want to publish a new graylog charm containing the fix I just landed).

review: Approve

[Stuart Bishop (stub)]

review: Approve (canonical-is-reviewers)

[🤖 Canonical IS Merge Bot (canonical-is-mergebot)]

Change successfully merged at revision f233dad8109e352ae4fb923a1deb9e5d47336b75

diff --git a/Makefile b/Makefile
index e8472b6..9e5089e 100644
--- a/Makefile
+++ b/Makefile
@@ -41,6 +41,31 @@ unittest: sysdeps lint
 charmbuild:
 	charm build --output-dir $(BUILDDEST) --report
 
+.PHONY: charmrelease
+charmrelease:
+	@echo ---------------------------------------------------------------------
+	@echo graylog supports 'core' and 'graylog' snap resources.
+	@echo
+	@echo Attached resources will be processed during charm deployment. If a
+	@echo zero-byte resource is detected, the charm will install the relevant
+	@echo snap from the snap store.
+	@echo
+	@echo Charm changes do not always require new resources. Developers often
+	@echo release new charm revisions using existing resources in the store:
+	@echo
+	@echo charm push $(BUILTCHARMDIR)
+	@echo charm release \<revision\> --resource core-0 --resource graylog-3
+	@echo
+	@echo You could also release a charm that uses the existing empty core snap
+	@echo resource along with a specific graylog snap:
+	@echo
+	@echo cd /tmp
+	@echo snap download --channel=stable graylog
+	@echo charm push $(BUILTCHARMDIR) --resource graylog=/tmp/graylog_*.snap
+	@echo charm release \<revision\> --resource core-0 --resource graylog-\<n\>
+	@echo
+	@echo ---------------------------------------------------------------------
+
 .PHONY: test
 test: check-jujumodel testdeps
 	@echo "Running functional tests (including lint and unit tests)..."
diff --git a/layer.yaml b/layer.yaml
index c7c211f..3d0f751 100644
--- a/layer.yaml
+++ b/layer.yaml
@@ -12,5 +12,7 @@ options:
     packages:
       - python3-requests
   snap:
+    core:
+      channel: stable
     graylog:
       channel: stable
diff --git a/metadata.yaml b/metadata.yaml
index 10426ed..ee0dbda 100644
--- a/metadata.yaml
+++ b/metadata.yaml
@@ -24,6 +24,10 @@ requires:
   mongodb:
     interface: mongodb
 resources:
+  core:
+    type: file
+    filename: core.snap
+    description: Snap package of core
   graylog:
     type: file
     filename: graylog.snap