Implement compliance key lifecycle on main service, add message retention and rate limits

backend/services/shared/message_retention.py

@@ -0,0 +1,106 @@
+"""
+Single MESSAGE_RETENTION_DAYS policy (required env, no in-code default).
+
+- Positive float: age-based cleanup after that many days (same cutoff for compliance MEK,
+  soft-deleted DM keys, and edit-history rows). Value may be an arithmetic expression.
+- 0: retain forever (no time-based cleanup; compliance MEK is still stored when a public key is configured).
+- -1: do not store compliance-wrapped MEK; no time-based cleanup (same as 0 for expiry).
+"""
+
+from __future__ import annotations
+
+import ast
+import math
+import os
+from dataclasses import dataclass
+from datetime import timedelta
+MESSAGE_RETENTION_DAYS = "MESSAGE_RETENTION_DAYS"
+
+_state: MessageRetentionState | None = None
+
+
+@dataclass(frozen=True)
+class MessageRetentionState:
+    """Parsed MESSAGE_RETENTION_DAYS (days, after evaluating optional expression)."""
+
+    days: float
+
+    def never_store_compliance_mek(self) -> bool:
+        return self.days == -1.0
+
+    def cleanup_enabled(self) -> bool:
+        return self.days > 0.0
+
+    def retention_timedelta(self) -> timedelta:
+        return timedelta(days=self.days)
+
+
+def _eval_numeric(node: ast.AST) -> float:
+    if isinstance(node, ast.Constant):
+        if isinstance(node.value, bool):
+            raise ValueError("MESSAGE_RETENTION_DAYS expression must be numeric")
+        if isinstance(node.value, (int, float)):
+            return float(node.value)
+        raise ValueError("MESSAGE_RETENTION_DAYS expression must be numeric")
+    if isinstance(node, ast.UnaryOp) and isinstance(node.op, ast.USub):
+        return -_eval_numeric(node.operand)
+    if isinstance(node, ast.UnaryOp) and isinstance(node.op, ast.UAdd):
+        return _eval_numeric(node.operand)
+    if isinstance(node, ast.BinOp):
+        left = _eval_numeric(node.left)
+        right = _eval_numeric(node.right)
+        if isinstance(node.op, ast.Add):
+            return left + right
+        if isinstance(node.op, ast.Sub):
+            return left - right
+        if isinstance(node.op, ast.Mult):
+            return left * right
+        if isinstance(node.op, ast.Div):
+            return left / right
+        if isinstance(node.op, ast.FloorDiv):
+            return left // right
+        if isinstance(node.op, ast.Mod):
+            return left % right
+        if isinstance(node.op, ast.Pow):
+            return left ** right
+        raise ValueError("Unsupported operator in MESSAGE_RETENTION_DAYS")
+    if isinstance(node, ast.Num):  # py<3.8 compatibility
+        return float(node.n)
+    raise ValueError("Unsupported syntax in MESSAGE_RETENTION_DAYS (only numbers and + - * / // % **)")
+
+
+def eval_message_retention_expression(raw: str) -> float:
+    s = raw.strip()
+    if not s:
+        raise ValueError("MESSAGE_RETENTION_DAYS must not be empty")
+    tree = ast.parse(s, mode="eval")
+    if not isinstance(tree, ast.Expression):
+        raise ValueError("Invalid MESSAGE_RETENTION_DAYS expression")
+    value = _eval_numeric(tree.body)
+    if math.isnan(value) or math.isinf(value):
+        raise ValueError("MESSAGE_RETENTION_DAYS must be finite")
+    if value < 0 and value != -1.0:
+        raise ValueError("MESSAGE_RETENTION_DAYS must be >= 0, or exactly -1")
+    return value
+
+
+def load_message_retention_from_env() -> MessageRetentionState:
+    raw = os.getenv(MESSAGE_RETENTION_DAYS)
+    if raw is None or not str(raw).strip():
+        raise ValueError(
+            "MESSAGE_RETENTION_DAYS environment variable must be set "
+            "(float days; expressions like 1/24/60*5 allowed; 0 = retain forever; -1 = do not store compliance MEK)"
+        )
+    return MessageRetentionState(days=eval_message_retention_expression(str(raw)))
+
+
+def get_message_retention() -> MessageRetentionState:
+    global _state
+    if _state is None:
+        _state = load_message_retention_from_env()
+    return _state
+
+
+def reset_message_retention_cache_for_tests() -> None:
+    global _state
+    _state = None