[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>

3 files changed, 62 insertions, 11 deletions

diff --git a/internal/typeutils/frontendtointernal.go b/internal/typeutils/frontendtointernal.go
index 973b20632..239527eb1 100644
--- a/internal/typeutils/frontendtointernal.go
+++ b/internal/typeutils/frontendtointernal.go
@@ -227,15 +227,15 @@ func APIInteractionPolicyToInteractionPolicy(
 	}
 
 	return &gtsmodel.InteractionPolicy{
-		CanLike: gtsmodel.PolicyRules{
+		CanLike: &gtsmodel.PolicyRules{
 			AutomaticApproval: canLikeAlways,
 			ManualApproval:    canLikeWithApproval,
 		},
-		CanReply: gtsmodel.PolicyRules{
+		CanReply: &gtsmodel.PolicyRules{
 			AutomaticApproval: canReplyAlways,
 			ManualApproval:    canReplyWithApproval,
 		},
-		CanAnnounce: gtsmodel.PolicyRules{
+		CanAnnounce: &gtsmodel.PolicyRules{
 			AutomaticApproval: canAnnounceAlways,
 			ManualApproval:    canAnnounceWithApproval,
 		},
diff --git a/internal/typeutils/internaltoas.go b/internal/typeutils/internaltoas.go
index cef48e194..4762e3c8b 100644
--- a/internal/typeutils/internaltoas.go
+++ b/internal/typeutils/internaltoas.go
@@ -1939,6 +1939,19 @@ func (c *Converter) InteractionPolicyToASInteractionPolicy(
 	policy := streams.NewGoToSocialInteractionPolicy()
 
 	/*
+		Implementation note for the below:
+		While it's possible for remote instances to set
+		sub-policies like canLike, canReply, etc to null
+		values, or omit them entirely, GtS always falls
+		back to default non-nil sub-policies when storing
+		policies created for local statuses. Therefore,
+		since we only ever serialize our *own* statuses
+		to AS format using this function, it's safe to
+		assume that the values will always be set, rather
+		than checking for nil ptrs.
+	*/
+
+	/*
 		CAN LIKE
 	*/
 
diff --git a/internal/typeutils/internaltofrontend.go b/internal/typeutils/internaltofrontend.go
index b6cb2bb37..bdb33243d 100644
--- a/internal/typeutils/internaltofrontend.go
+++ b/internal/typeutils/internaltofrontend.go
@@ -2782,7 +2782,10 @@ func (c *Converter) ThemesToAPIThemes(themes []*gtsmodel.Theme) []apimodel.Theme
 // into an apimodel interaction policy.
 //
 // Provided status can be nil to convert a
-// policy without a particular status in mind.
+// policy without a particular status in mind,
+// but ***if status is nil then sub-policies
+// CanLike, CanReply, and CanAnnounce on
+// the given policy must *not* be nil.***
 //
 // RequestingAccount can also be nil for
 // unauthorized requests (web, public api etc).
@@ -2792,19 +2795,54 @@ func (c *Converter) InteractionPolicyToAPIInteractionPolicy(
 	status *gtsmodel.Status,
 	requester *gtsmodel.Account,
 ) (*apimodel.InteractionPolicy, error) {
-	apiPolicy := &apimodel.InteractionPolicy{
-		CanFavourite: apimodel.PolicyRules{
+	apiPolicy := new(apimodel.InteractionPolicy)
+
+	// gtsmodel CanLike -> apimodel CanFavourite
+	if policy.CanLike != nil {
+		// Use the set CanLike value.
+		apiPolicy.CanFavourite = apimodel.PolicyRules{
 			AutomaticApproval: policyValsToAPIPolicyVals(policy.CanLike.AutomaticApproval),
 			ManualApproval:    policyValsToAPIPolicyVals(policy.CanLike.ManualApproval),
-		},
-		CanReply: apimodel.PolicyRules{
+		}
+	} else {
+		// Use default CanLike value for this vis.
+		pCanLike := gtsmodel.DefaultCanLikeFor(status.Visibility)
+		apiPolicy.CanFavourite = apimodel.PolicyRules{
+			AutomaticApproval: policyValsToAPIPolicyVals(pCanLike.AutomaticApproval),
+			ManualApproval:    policyValsToAPIPolicyVals(pCanLike.ManualApproval),
+		}
+	}
+
+	// gtsmodel CanReply -> apimodel CanReply
+	if policy.CanReply != nil {
+		// Use the set CanReply value.
+		apiPolicy.CanReply = apimodel.PolicyRules{
 			AutomaticApproval: policyValsToAPIPolicyVals(policy.CanReply.AutomaticApproval),
 			ManualApproval:    policyValsToAPIPolicyVals(policy.CanReply.ManualApproval),
-		},
-		CanReblog: apimodel.PolicyRules{
+		}
+	} else {
+		// Use default CanReply value for this vis.
+		pCanReply := gtsmodel.DefaultCanReplyFor(status.Visibility)
+		apiPolicy.CanReply = apimodel.PolicyRules{
+			AutomaticApproval: policyValsToAPIPolicyVals(pCanReply.AutomaticApproval),
+			ManualApproval:    policyValsToAPIPolicyVals(pCanReply.ManualApproval),
+		}
+	}
+
+	// gtsmodel CanAnnounce -> apimodel CanReblog
+	if policy.CanAnnounce != nil {
+		// Use the set CanAnnounce value.
+		apiPolicy.CanReblog = apimodel.PolicyRules{
 			AutomaticApproval: policyValsToAPIPolicyVals(policy.CanAnnounce.AutomaticApproval),
 			ManualApproval:    policyValsToAPIPolicyVals(policy.CanAnnounce.ManualApproval),
-		},
+		}
+	} else {
+		// Use default CanAnnounce value for this vis.
+		pCanAnnounce := gtsmodel.DefaultCanAnnounceFor(status.Visibility)
+		apiPolicy.CanReblog = apimodel.PolicyRules{
+			AutomaticApproval: policyValsToAPIPolicyVals(pCanAnnounce.AutomaticApproval),
+			ManualApproval:    policyValsToAPIPolicyVals(pCanAnnounce.ManualApproval),
+		}
 	}
 
 	defer func() {