Merge ~ines-almeida/launchpad-layers:add-pkgupload-interface into launchpad-layers:main

[Ines Almeida]

Create interface for txpkgupload charm

This interface has 2 main goals:
 - Set the `<interface>.available` and `<interface>.configuration.available` flags within the charms
 - Send configuration data from the upload processor to the txpkgupload using `set_config()` in `provides.py`, and fetching it from the txpkgupload charm directly once the `<interface>.configuration.available` flag is on.

This interface should be used between txpkgupload and the ppa/ftpmaster upload processors charms

Currently the ppa/ftpmaster upload processors are the providers of the interface, and the txpkgupload is the one that requires it

[Ines Almeida (ines-almeida)]

Added some of my own thoughts and questions about the code

[Colin Watson (cjwatson)]

review: Approve

[Ines Almeida (ines-almeida)]

Since we are on it, I'll wait for the txpkgupload charm MP to be ready for approval before merging this one, in case we want to make any changes. I'll leave it open for review in the meantime

[Ines Almeida (ines-almeida)]

I addressed your comments Guruprasad. Good thing this didn't merge automatically when I hit approve :)

[Guruprasad (lgp171188)]

Thanks Inês!

diff --git a/upload-queue-processor/interface.yaml b/upload-queue-processor/interface.yaml
new file mode 100644
index 0000000..2442dd8
--- /dev/null
+++ b/upload-queue-processor/interface.yaml
@@ -0,0 +1,3 @@
+name: upload-queue-processor
+summary: Package Uploader interface
+version: 1
diff --git a/upload-queue-processor/provides.py b/upload-queue-processor/provides.py
new file mode 100644
index 0000000..1c0c05b
--- /dev/null
+++ b/upload-queue-processor/provides.py
@@ -0,0 +1,37 @@
+# Copyright 2023 Canonical Ltd.  This software is licensed under the
+# GNU Affero General Public License version 3 (see the file LICENSE).
+
+from charmhelpers.core import hookenv
+from charms.reactive import Endpoint, clear_flag, set_flag, when
+
+
+class UploadProcessorProvides(Endpoint):
+    @when("endpoint.{endpoint_name}.joined")
+    def handle_joined_unit(self):
+        set_flag(self.expand_name("{endpoint_name}.available"))
+
+    @when("endpoint.{endpoint_name}.changed")
+    def handle_changed_unit(self):
+        set_flag(self.expand_name("{endpoint_name}.available"))
+        clear_flag(self.expand_name("changed"))
+
+    @when("endpoint.{endpoint_name}.departed")
+    def handle_departed_unit(self):
+        clear_flag(self.expand_name("{endpoint_name}.available"))
+        self.all_departed_units.clear()
+        clear_flag(self.expand_name("departed"))
+
+    def set_config(self, fsroot: str):
+        """Called from the provider of the interface to configure txpkgupload.
+        This will trigger a 'changed' flag in the `requires` side of the
+        relation.
+
+        :param fsroot: directory for the uploads to live within the filesystem
+        """
+        hookenv.log("Publishing configuration data txpkgupload interface")
+        for relation in self.relations:
+            relation.to_publish.update(
+                {
+                    "fsroot": fsroot,
+                }
+            )
diff --git a/upload-queue-processor/requires.py b/upload-queue-processor/requires.py
new file mode 100644
index 0000000..3f3df36
--- /dev/null
+++ b/upload-queue-processor/requires.py
@@ -0,0 +1,51 @@
+# Copyright 2023 Canonical Ltd.  This software is licensed under the
+# GNU Affero General Public License version 3 (see the file LICENSE).
+
+from charmhelpers.core import hookenv
+from charms.reactive import Endpoint, clear_flag, set_flag, when
+
+
+class UploadProcessorRequires(Endpoint):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fsroot = None
+
+    @when("endpoint.{endpoint_name}.joined")
+    def handle_joined_unit(self):
+        # We don't want to make it available before it's configured
+        # It is configured, when data is added from the provider side,
+        # triggering the 'changed' endpoint
+        hookenv.log("Txpkgupload interface available.")
+
+    @when("endpoint.{endpoint_name}.changed")
+    def handle_changed_unit(self):
+        clear_flag(self.expand_name("{endpoint_name}.configured"))
+
+        # We expect exactly one relation between a txpkgupload and a queue
+        # processor (where the txpkgupload a subordinate to the latter)
+        if len(self.relations) != 1:
+            endpoint_name = self.expand_name("{endpoint_name}")
+            clear_flag(self.expand_name("changed"))
+            raise Exception(
+                f"Expected exactly 1 relation in {endpoint_name} but found "
+                f"{len(self.relations)} instead."
+            )
+
+        # Use `fsroot` values received from the queue processor
+        # If none were received, use the ones received previously
+        received_data = self.relations[0].units.received
+        self.fsroot = received_data.get("fsroot", self.fsroot)
+
+        if self.fsroot:
+            hookenv.log("Configuration from queue processor received")
+            set_flag(self.expand_name("{endpoint_name}.configured"))
+        else:
+            hookenv.log("Waiting for configuration from queue processor")
+
+        clear_flag(self.expand_name("changed"))
+
+    @when("endpoint.{endpoint_name}.departed")
+    def handle_departed_unit(self):
+        clear_flag(self.expand_name("{endpoint_name}.configured"))
+        self.all_departed_units.clear()
+        clear_flag(self.expand_name("departed"))