[chore] Update interactionPolicy sub-policy parsing in line with documented defaults (#4229)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Brings our parsing of unset sub-policies in line with the defaults documented here: https://docs.gotosocial.org/en/v0.19.1/federation/interaction_policy/#defaults-per-sub-policy

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4146

Part of https://codeberg.org/superseriousbusiness/gotosocial/issues/4026

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4229
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

2 files changed, 114 insertions, 26 deletions

diff --git a/internal/ap/extract.go b/internal/ap/extract.go
index 344e0fdb3..8dbb903a3 100644
--- a/internal/ap/extract.go
+++ b/internal/ap/extract.go
@@ -1113,6 +1113,9 @@ func ExtractVisibility(addressable Addressable, actorFollowersURI string) (gtsmo
 // Will be nil (default policy) for Statusables that have no policy
 // set on them, or have a null policy. In such a case, the caller
 // should assume the default policy for the status's visibility level.
+//
+// Sub-policies of the returned policy, eg., CanLike, CanReply, may
+// each be nil if they were not set on the interaction policy.
 func ExtractInteractionPolicy(
 	statusable Statusable,
 	owner *gtsmodel.Account,
@@ -1139,6 +1142,8 @@ func ExtractInteractionPolicy(
 		return nil
 	}
 
+	// There's a policy key/value
+	// set, extract sub-policies.
 	return &gtsmodel.InteractionPolicy{
 		CanLike:     extractCanLike(policy.GetGoToSocialCanLike(), owner),
 		CanReply:    extractCanReply(policy.GetGoToSocialCanReply(), owner),
@@ -1146,67 +1151,82 @@ func ExtractInteractionPolicy(
 	}
 }
 
+// Returns either a parsed CanLike sub-policy, or nil
+// if canLike is not set, ie., if this post is from an
+// instance that doesn't know / care about canLike.
 func extractCanLike(
 	prop vocab.GoToSocialCanLikeProperty,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
+) *gtsmodel.PolicyRules {
 	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	propIter := prop.At(0)
 	if !propIter.IsGoToSocialCanLike() {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
-	return extractPolicyRules(propIter.Get(), owner)
+	withRules := propIter.Get()
+	if withRules == nil {
+		return nil
+	}
+
+	return extractPolicyRules(withRules, owner)
 }
 
+// Returns either a parsed CanReply sub-policy, or nil
+// if canReply is not set, ie., if this post is from an
+// instance that doesn't know / care about canReply.
 func extractCanReply(
 	prop vocab.GoToSocialCanReplyProperty,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
+) *gtsmodel.PolicyRules {
 	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	propIter := prop.At(0)
 	if !propIter.IsGoToSocialCanReply() {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
-	return extractPolicyRules(propIter.Get(), owner)
+	withRules := propIter.Get()
+	if withRules == nil {
+		return nil
+	}
+
+	return extractPolicyRules(withRules, owner)
 }
 
+// Returns either a parsed CanAnnounce sub-policy, or nil
+// if canAnnounce is not set, ie., if this post is from an
+// instance that doesn't know / care about canAnnounce.
 func extractCanAnnounce(
 	prop vocab.GoToSocialCanAnnounceProperty,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
+) *gtsmodel.PolicyRules {
 	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	propIter := prop.At(0)
 	if !propIter.IsGoToSocialCanAnnounce() {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	withRules := propIter.Get()
 	if withRules == nil {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
-	return extractPolicyRules(propIter.Get(), owner)
+	return extractPolicyRules(withRules, owner)
 }
 
 func extractPolicyRules(
 	withRules WithPolicyRules,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
-	if withRules == nil {
-		return gtsmodel.PolicyRules{}
-	}
-
+) *gtsmodel.PolicyRules {
 	// Check for `automaticApproval` and
 	// `manualApproval` properties first.
 	var (
@@ -1216,7 +1236,7 @@ func extractPolicyRules(
 	if (automaticApproval != nil && automaticApproval.Len() != 0) ||
 		(manualApproval != nil && manualApproval.Len() != 0) {
 		// At least one is set, use these props.
-		return gtsmodel.PolicyRules{
+		return &gtsmodel.PolicyRules{
 			AutomaticApproval: extractPolicyValues(automaticApproval, owner),
 			ManualApproval:    extractPolicyValues(manualApproval, owner),
 		}
@@ -1226,7 +1246,7 @@ func extractPolicyRules(
 	// and `withApproval` properties.
 	//
 	// TODO: Remove this in GtS v0.21.0.
-	return gtsmodel.PolicyRules{
+	return &gtsmodel.PolicyRules{
 		AutomaticApproval: extractPolicyValues(withRules.GetGoToSocialAlways(), owner),
 		ManualApproval:    extractPolicyValues(withRules.GetGoToSocialApprovalRequired(), owner),
 	}
diff --git a/internal/ap/extractpolicy_test.go b/internal/ap/extractpolicy_test.go
index 24b198b29..85221dcef 100644
--- a/internal/ap/extractpolicy_test.go
+++ b/internal/ap/extractpolicy_test.go
@@ -102,13 +102,13 @@ func (suite *ExtractPolicyTestSuite) TestExtractPolicy() {
 	)
 
 	expectedPolicy := &gtsmodel.InteractionPolicy{
-		CanLike: gtsmodel.PolicyRules{
+		CanLike: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValuePublic,
 			},
 			ManualApproval: gtsmodel.PolicyValues{},
 		},
-		CanReply: gtsmodel.PolicyRules{
+		CanReply: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValueAuthor,
 				gtsmodel.PolicyValueFollowers,
@@ -119,7 +119,75 @@ func (suite *ExtractPolicyTestSuite) TestExtractPolicy() {
 				gtsmodel.PolicyValuePublic,
 			},
 		},
-		CanAnnounce: gtsmodel.PolicyRules{
+		CanAnnounce: &gtsmodel.PolicyRules{
+			AutomaticApproval: gtsmodel.PolicyValues{
+				gtsmodel.PolicyValueAuthor,
+			},
+			ManualApproval: gtsmodel.PolicyValues{
+				gtsmodel.PolicyValuePublic,
+			},
+		},
+	}
+	suite.EqualValues(expectedPolicy, policy)
+}
+
+func (suite *ExtractPolicyTestSuite) TestExtractPolicyUnsetProps() {
+	rawNote := `{
+  "@context": [
+    "https://gotosocial.org/ns",
+    "https://www.w3.org/ns/activitystreams"
+  ],
+  "content": "hey @f0x and @dumpsterqueer",
+  "contentMap": {
+    "en": "hey @f0x and @dumpsterqueer",
+    "fr": "bonjour @f0x et @dumpsterqueer"
+  },
+  "interactionPolicy": {
+    "canAnnounce": {
+      "automaticApproval": [
+        "http://localhost:8080/users/the_mighty_zork"
+      ],
+      "manualApproval": [
+        "https://www.w3.org/ns/activitystreams#Public"
+      ]
+    }
+  },
+  "tag": [
+    {
+      "href": "https://gts.superseriousbusiness.org/users/dumpsterqueer",
+      "name": "@dumpsterqueer@superseriousbusiness.org",
+      "type": "Mention"
+    },
+    {
+      "href": "https://gts.superseriousbusiness.org/users/f0x",
+      "name": "@f0x@superseriousbusiness.org",
+      "type": "Mention"
+    }
+  ],
+  "type": "Note"
+}`
+
+	statusable, err := ap.ResolveStatusable(
+		suite.T().Context(),
+		io.NopCloser(
+			bytes.NewBufferString(rawNote),
+		),
+	)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	policy := ap.ExtractInteractionPolicy(
+		statusable,
+		// Zork didn't actually create
+		// this status but nevermind.
+		suite.testAccounts["local_account_1"],
+	)
+
+	expectedPolicy := &gtsmodel.InteractionPolicy{
+		CanLike:  nil,
+		CanReply: nil,
+		CanAnnounce: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValueAuthor,
 			},
@@ -202,13 +270,13 @@ func (suite *ExtractPolicyTestSuite) TestExtractPolicyDeprecated() {
 	)
 
 	expectedPolicy := &gtsmodel.InteractionPolicy{
-		CanLike: gtsmodel.PolicyRules{
+		CanLike: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValuePublic,
 			},
 			ManualApproval: gtsmodel.PolicyValues{},
 		},
-		CanReply: gtsmodel.PolicyRules{
+		CanReply: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValueAuthor,
 				gtsmodel.PolicyValueFollowers,
@@ -219,7 +287,7 @@ func (suite *ExtractPolicyTestSuite) TestExtractPolicyDeprecated() {
 				gtsmodel.PolicyValuePublic,
 			},
 		},
-		CanAnnounce: gtsmodel.PolicyRules{
+		CanAnnounce: &gtsmodel.PolicyRules{
 			AutomaticApproval: gtsmodel.PolicyValues{
 				gtsmodel.PolicyValueAuthor,
 			},