[DPE-10417] feat: migrate shared constants to single kernel (#147)

* feat: migrate shared constants to single kernel

Consolidate the cross-cutting constants the charms still define locally
(database name/port, metrics ports, TLS file names, override maps,
mapping labels, tracing and pgBackRest paths) into the library so both
substrates read one source instead of drifting copies. The
secrets/scopes/password-key constants are intentionally omitted — they
were already migrated by the start/install refactor.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

* feat: include backup user in SYSTEM_USERS

Both the VM and K8s charms must treat the backup user as a managed
system user (password rotation and internal access-group grants). The
VM charm previously compensated by appending BACKUP_USER at individual
call sites while K8s already listed it locally. Promoting it into the
shared SYSTEM_USERS makes the library the single source of truth and
lets both charms drop their divergent local definitions.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

* feat: add ALL_CLIENT_RELATIONS shared constant

The set of client-facing relations was defined locally in the VM charm.
Hoisting it into the shared library gives charms a single definition to
consume and keeps the client-relation set consistent across substrates
as it grows.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

* feat: migrate operational charm constants to the library

Backup ID formats, async-replication relation names, pgBackRest log/error
settings, the tracing protocol and the watcher relation constants were
defined locally in the charms. Hoisting them into the shared library
removes per-charm duplication (the replication relation names were defined
in both charms) and lets the charms consume a single definition.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

---------

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

Author: marceloneppel

single_kernel_postgresql/config/literals.py

@@ -21,7 +21,7 @@
 REWIND_USER = "rewind"
 SNAP_USER = "_daemon_"
 USER = "operator"
-SYSTEM_USERS = [MONITORING_USER, REPLICATION_USER, REWIND_USER, USER]
+SYSTEM_USERS = [BACKUP_USER, MONITORING_USER, REPLICATION_USER, REWIND_USER, USER]
 
 # Paths
 ## VM Paths
@@ -100,3 +100,64 @@
 CONTAINER_NAME = "postgresql"
 
 API_REQUEST_TIMEOUT = 5
+
+
+# --- Shared constants migrated from the charms ---
+
+# Database
+DATABASE = "database"
+DATABASE_DEFAULT_NAME = "postgres"
+DATABASE_PORT = "5432"
+
+# Client relations
+ALL_CLIENT_RELATIONS = [DATABASE]
+
+# Async replication relations
+REPLICATION_CONSUMER_RELATION = "replication"
+REPLICATION_OFFER_RELATION = "replication-offer"
+
+# TLS files
+TLS_KEY_FILE = "key.pem"
+TLS_CA_FILE = "ca.pem"
+TLS_CERT_FILE = "cert.pem"
+
+# Metrics ports (kept as str to match the K8s charm; VM adapts on flip)
+METRICS_PORT = "9187"
+PGBACKREST_METRICS_PORT = "9854"
+
+# Secret/database mapping labels
+USERNAME_MAPPING_LABEL = "custom-usernames"
+DATABASE_MAPPING_LABEL = "prefix-databases"
+
+# Overrides
+BACKUP_TYPE_OVERRIDES = {"full": "full", "differential": "diff", "incremental": "incr"}
+PLUGIN_OVERRIDES = {"audit": "pgaudit", "uuid_ossp": '"uuid-ossp"'}
+
+# SPI extension modules
+SPI_MODULE = ["refint", "autoinc", "insert_username", "moddatetime"]
+
+# Tracing
+TRACING_RELATION_NAME = "tracing"
+TRACING_PROTOCOL = "otlp_http"
+
+# Patroni
+PATRONI_CLUSTER_STATUS_ENDPOINT = "cluster"
+
+# pgBackRest
+PGBACKREST_LOGROTATE_FILE = "/etc/logrotate.d/pgbackrest.logrotate"
+# pgBackRest emits all error/warning output on stderr (the default is stdout) so error
+# extraction stays consistent. Reference: https://pgbackrest.org/configuration.html#section-log
+PGBACKREST_LOG_LEVEL_STDERR = "--log-level-stderr=warn"
+# Archive timeout: unable to archive WAL files within the configured timeout period.
+PGBACKREST_ARCHIVE_TIMEOUT_ERROR_CODE = 82
+
+# Backup ID formats
+BACKUP_ID_FORMAT = "%Y-%m-%dT%H:%M:%SZ"
+PGBACKREST_BACKUP_ID_FORMAT = "%Y%m%d-%H%M%S"
+
+# Watcher relation (VM)
+WATCHER_OFFER_RELATION = "watcher-offer"
+WATCHER_RELATION = "watcher"
+WATCHER_USER = "watcher"
+WATCHER_PASSWORD_KEY = "watcher-password"  # noqa: S105
+WATCHER_SECRET_LABEL = "watcher-secret"  # noqa: S105

tests/unit/test_literals.py

@@ -0,0 +1,34 @@
+# Copyright 2026 Canonical Ltd.
+# See LICENSE file for licensing details.
+from single_kernel_postgresql.config import literals
+
+
+def test_shared_string_constants():
+    assert literals.DATABASE == "database"
+    assert literals.DATABASE_DEFAULT_NAME == "postgres"
+    assert literals.DATABASE_PORT == "5432"
+    assert literals.PATRONI_CLUSTER_STATUS_ENDPOINT == "cluster"
+    assert literals.TLS_KEY_FILE == "key.pem"
+    assert literals.TLS_CA_FILE == "ca.pem"
+    assert literals.TLS_CERT_FILE == "cert.pem"
+    assert literals.USERNAME_MAPPING_LABEL == "custom-usernames"
+    assert literals.DATABASE_MAPPING_LABEL == "prefix-databases"
+    assert literals.TRACING_RELATION_NAME == "tracing"
+    assert literals.PGBACKREST_LOGROTATE_FILE == "/etc/logrotate.d/pgbackrest.logrotate"
+
+
+def test_shared_collection_constants():
+    assert literals.BACKUP_TYPE_OVERRIDES == {
+        "full": "full",
+        "differential": "diff",
+        "incremental": "incr",
+    }
+    assert literals.PLUGIN_OVERRIDES == {"audit": "pgaudit", "uuid_ossp": '"uuid-ossp"'}
+    assert literals.SPI_MODULE == ["refint", "autoinc", "insert_username", "moddatetime"]
+
+
+def test_metrics_ports_are_str():
+    assert literals.METRICS_PORT == "9187"
+    assert literals.PGBACKREST_METRICS_PORT == "9854"
+    assert isinstance(literals.METRICS_PORT, str)
+    assert isinstance(literals.PGBACKREST_METRICS_PORT, str)