[DPE-2838] Secret labels (#270)

* Secret labels

* Missing arg

* Pop the right databag

* Copy over safe secret get

Author: dragomirp

lib/charms/data_platform_libs/v0/data_secrets.py

@@ -0,0 +1,143 @@
+"""Secrets related helper classes/functions."""
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+from typing import Dict, Literal, Optional
+
+from ops import Secret, SecretInfo
+from ops.charm import CharmBase
+from ops.model import SecretNotFoundError
+
+# The unique Charmhub library identifier, never change it
+LIBID = "d77fb3d01aba41ed88e837d0beab6be5"
+
+# Increment this major API version when introducing breaking changes
+LIBAPI = 0
+
+# Increment this PATCH version before using `charmcraft publish-lib` or reset
+# to 0 if you are raising the major API version
+LIBPATCH = 1
+
+
+APP_SCOPE = "app"
+UNIT_SCOPE = "unit"
+Scopes = Literal["app", "unit"]
+
+
+class DataSecretsError(Exception):
+    """A secret that we want to create already exists."""
+
+
+class SecretAlreadyExistsError(DataSecretsError):
+    """A secret that we want to create already exists."""
+
+
+def generate_secret_label(charm: CharmBase, scope: Scopes) -> str:
+    """Generate unique group_mappings for secrets within a relation context.
+
+    Defined as a standalone function, as the choice on secret labels definition belongs to the
+    Application Logic. To be kept separate from classes below, which are simply to provide a
+    (smart) abstraction layer above Juju Secrets.
+    """
+    members = [charm.app.name, scope]
+    return f"{'.'.join(members)}"
+
+
+# Secret cache
+
+
+class CachedSecret:
+    """Abstraction layer above direct Juju access with caching.
+
+    The data structure is precisely re-using/simulating Juju Secrets behavior, while
+    also making sure not to fetch a secret multiple times within the same event scope.
+    """
+
+    def __init__(self, charm: CharmBase, label: str, secret_uri: Optional[str] = None):
+        self._secret_meta = None
+        self._secret_content = {}
+        self._secret_uri = secret_uri
+        self.label = label
+        self.charm = charm
+
+    def add_secret(self, content: Dict[str, str], scope: Scopes) -> Secret:
+        """Create a new secret."""
+        if self._secret_uri:
+            raise SecretAlreadyExistsError(
+                "Secret is already defined with uri %s", self._secret_uri
+            )
+
+        if scope == APP_SCOPE:
+            secret = self.charm.app.add_secret(content, label=self.label)
+        else:
+            secret = self.charm.unit.add_secret(content, label=self.label)
+        self._secret_uri = secret.id
+        self._secret_meta = secret
+        return self._secret_meta
+
+    @property
+    def meta(self) -> Optional[Secret]:
+        """Getting cached secret meta-information."""
+        if self._secret_meta:
+            return self._secret_meta
+
+        if not (self._secret_uri or self.label):
+            return
+
+        try:
+            self._secret_meta = self.charm.model.get_secret(label=self.label)
+        except SecretNotFoundError:
+            if self._secret_uri:
+                self._secret_meta = self.charm.model.get_secret(
+                    id=self._secret_uri, label=self.label
+                )
+        return self._secret_meta
+
+    def get_content(self) -> Dict[str, str]:
+        """Getting cached secret content."""
+        if not self._secret_content:
+            if self.meta:
+                self._secret_content = self.meta.get_content()
+        return self._secret_content
+
+    def set_content(self, content: Dict[str, str]) -> None:
+        """Setting cached secret content."""
+        if self.meta:
+            self.meta.set_content(content)
+            self._secret_content = content
+
+    def get_info(self) -> Optional[SecretInfo]:
+        """Wrapper function for get the corresponding call on the Secret object if any."""
+        if self.meta:
+            return self.meta.get_info()
+
+
+class SecretCache:
+    """A data structure storing CachedSecret objects."""
+
+    def __init__(self, charm):
+        self.charm = charm
+        self._secrets: Dict[str, CachedSecret] = {}
+
+    def get(self, label: str, uri: Optional[str] = None) -> Optional[CachedSecret]:
+        """Getting a secret from Juju Secret store or cache."""
+        if not self._secrets.get(label):
+            secret = CachedSecret(self.charm, label, uri)
+
+            # Checking if the secret exists, otherwise we don't register it in the cache
+            if secret.meta:
+                self._secrets[label] = secret
+        return self._secrets.get(label)
+
+    def add(self, label: str, content: Dict[str, str], scope: Scopes) -> CachedSecret:
+        """Adding a secret to Juju Secret."""
+        if self._secrets.get(label):
+            raise SecretAlreadyExistsError(f"Secret {label} already exists")
+
+        secret = CachedSecret(self.charm, label)
+        secret.add_secret(content, scope)
+        self._secrets[label] = secret
+        return self._secrets[label]
+
+
+# END: Secret cache