feat(campaign): govern attachment reuse

src/govoplan_campaign/backend/attachments/reuse.py

@@ -0,0 +1,184 @@
+from __future__ import annotations
+
+import hashlib
+from collections import defaultdict
+from dataclasses import dataclass
+from pathlib import Path
+
+from govoplan_campaign.backend.campaign.models import (
+    AttachmentReuseAction,
+    AttachmentReuseAllowance,
+    AttachmentReusePolicy,
+)
+from govoplan_campaign.backend.messages.models import MessageDraft, MessageIssue
+
+
+@dataclass(frozen=True, slots=True)
+class _AttachmentUse:
+    message: MessageDraft
+    message_key: str
+    recipient_key: tuple[str, ...]
+    source_identity: str
+    file_name: str
+
+
+@dataclass(slots=True)
+class AttachmentReuseEvaluation:
+    report: dict[str, object]
+    issues_by_entry_index: dict[int, list[MessageIssue]]
+
+
+def evaluate_attachment_reuse(
+    messages: list[MessageDraft],
+    *,
+    policy: AttachmentReusePolicy,
+) -> AttachmentReuseEvaluation:
+    """Evaluate repeated resolved-file use without exposing source paths.
+
+    A use is one resolved file occurrence in one attachment rule. The same
+    source file can therefore be detected both across built messages and when
+    two rules add it to one message. Allowed findings remain in the build
+    protocol; policy violations additionally become recipient-level issues.
+    """
+
+    uses_by_source: dict[str, list[_AttachmentUse]] = defaultdict(list)
+    for message in messages:
+        if not message.active:
+            continue
+        message_key = str(message.entry_id or message.entry_index)
+        recipient_key = _recipient_key(message, fallback=message_key)
+        for attachment in message.attachments:
+            for match in attachment.matches:
+                source_identity = _source_identity(match)
+                uses_by_source[source_identity].append(
+                    _AttachmentUse(
+                        message=message,
+                        message_key=message_key,
+                        recipient_key=recipient_key,
+                        source_identity=source_identity,
+                        file_name=Path(match).name,
+                    )
+                )
+
+    findings: list[dict[str, object]] = []
+    issues_by_entry_index: dict[int, list[MessageIssue]] = defaultdict(list)
+    affected_entry_indexes: set[int] = set()
+    allowed_count = 0
+    violation_count = 0
+
+    for source_identity, uses in sorted(uses_by_source.items()):
+        if len(uses) < 2:
+            continue
+        fingerprint = hashlib.sha256(source_identity.encode("utf-8")).hexdigest()
+        message_keys = {item.message_key for item in uses}
+        recipient_keys = {item.recipient_key for item in uses}
+        allowed, explanation = _is_allowed(
+            policy,
+            message_count=len(message_keys),
+            recipient_count=len(recipient_keys),
+        )
+        disposition = "allowed" if allowed else policy.action.value
+        finding = {
+            "file_fingerprint": fingerprint,
+            "file_name": uses[0].file_name,
+            "use_count": len(uses),
+            "message_count": len(message_keys),
+            "recipient_count": len(recipient_keys),
+            "disposition": disposition,
+            "explanation": explanation,
+        }
+        findings.append(finding)
+        if allowed:
+            allowed_count += 1
+            continue
+
+        violation_count += 1
+        behavior = _issue_behavior(policy.action)
+        severity = (
+            "error" if policy.action == AttachmentReuseAction.BLOCK else "warning"
+        )
+        for use in _unique_message_uses(uses):
+            affected_entry_indexes.add(use.message.entry_index)
+            issues_by_entry_index[use.message.entry_index].append(
+                MessageIssue(
+                    severity=severity,
+                    code="duplicate_attachment_reuse",
+                    message=(
+                        f"Attachment {use.file_name!r} is reused {len(uses)} times "
+                        f"across {len(message_keys)} built message(s); the configured "
+                        f"policy requires {disposition}."
+                    ),
+                    behavior=behavior,
+                    source="attachments:reuse_policy",
+                    details={
+                        **finding,
+                        "policy": policy.model_dump(mode="json"),
+                    },
+                )
+            )
+
+    return AttachmentReuseEvaluation(
+        report={
+            "contract_version": "1",
+            "policy": policy.model_dump(mode="json"),
+            "duplicate_file_count": len(findings),
+            "allowed_file_count": allowed_count,
+            "violation_file_count": violation_count,
+            "affected_message_count": len(affected_entry_indexes),
+            "findings": findings,
+        },
+        issues_by_entry_index=dict(issues_by_entry_index),
+    )
+
+
+def _source_identity(value: str) -> str:
+    return str(Path(value).resolve(strict=False))
+
+
+def _recipient_key(message: MessageDraft, *, fallback: str) -> tuple[str, ...]:
+    addresses = message.to or message.bcc or message.cc
+    normalized = sorted(
+        {
+            item.email.strip().casefold()
+            for item in addresses
+            if item.email and item.email.strip()
+        }
+    )
+    return tuple(normalized) if normalized else (f"entry:{fallback}",)
+
+
+def _is_allowed(
+    policy: AttachmentReusePolicy,
+    *,
+    message_count: int,
+    recipient_count: int,
+) -> tuple[bool, str]:
+    if policy.action == AttachmentReuseAction.ALLOW:
+        return True, "The campaign policy explicitly allows attachment reuse."
+    if (
+        policy.allow_within == AttachmentReuseAllowance.SAME_MESSAGE
+        and message_count == 1
+    ):
+        return True, "Reuse is confined to one built message as allowed by policy."
+    if (
+        policy.allow_within == AttachmentReuseAllowance.SAME_RECIPIENT
+        and recipient_count == 1
+    ):
+        return True, "Reuse is confined to one recipient as allowed by policy."
+    return False, (
+        "Reuse crosses the configured allowance and is handled by the "
+        f"{policy.action.value} policy."
+    )
+
+
+def _issue_behavior(action: AttachmentReuseAction) -> str:
+    if action == AttachmentReuseAction.REVIEW:
+        return "ask"
+    return action.value
+
+
+def _unique_message_uses(uses: list[_AttachmentUse]) -> list[_AttachmentUse]:
+    unique: dict[int, _AttachmentUse] = {}
+    for use in uses:
+        unique.setdefault(use.message.entry_index, use)
+    return list(unique.values())