[bugfix] Fix wrong URI used in dereferenceLocal (#4097)

# 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.

Previously we were passing a *followers* URI into the federatingDB `Followers` function, which expects an *account* URI. This led to a nil body when doing internal dereferences (ie., while sending out messages). I think I probably wrote this bug when I refactored stuff a while ago.

This PR fixes that up by making the dereferenceLocal function more consistent, and falling back to an http dereference if nothing found. Clarifies behavior of the federatingdb.Get function as well, by commenting the shit out of it.

Also adds tests, precious tests.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4066

## 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.
- [x] 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/4097
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/transport/dereference.go

@@ -19,37 +19,41 @@ package transport
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"net/url"
 
 	apiutil "code.superseriousbusiness.org/gotosocial/internal/api/util"
 	"code.superseriousbusiness.org/gotosocial/internal/config"
+	"code.superseriousbusiness.org/gotosocial/internal/db"
 	"code.superseriousbusiness.org/gotosocial/internal/gtserror"
-	"code.superseriousbusiness.org/gotosocial/internal/uris"
+	"code.superseriousbusiness.org/gotosocial/internal/log"
 )
 
 func (t *transport) Dereference(ctx context.Context, iri *url.URL) (*http.Response, error) {
-	// If the request is to us, we can shortcut for
-	// certain URIs rather than going through the normal
-	// request flow, thereby saving time and energy.
+	// If the request is to us, we can try to shortcut
+	// rather than going through the normal request flow.
+	//
+	// Only bail on a real error, otherwise continue
+	// to just make a normal http request to ourself.
 	if iri.Host == config.GetHost() {
-		switch {
-
-		case uris.IsFollowersPath(iri):
-			// The request is for followers of one of
-			// our accounts, which we can shortcut.
-			return t.controller.dereferenceLocalFollowers(ctx, iri)
-
-		case uris.IsUserPath(iri):
-			// The request is for one of our
-			// accounts, which we can shortcut.
-			return t.controller.dereferenceLocalUser(ctx, iri)
-
-		case uris.IsAcceptsPath(iri):
-			// The request is for an Accept on
-			// our instance, which we can shortcut.
-			return t.controller.dereferenceLocalAccept(ctx, iri)
+		rsp, err := t.controller.dereferenceLocal(ctx, iri)
+		if err != nil && !errors.Is(err, db.ErrNoEntries) {
+			// Real error.
+			err := gtserror.Newf("error trying dereferenceLocal: %w", err)
+			return nil, err
 		}
+
+		if rsp != nil {
+			// Got something!
+			//
+			// No need for
+			// further business.
+			return rsp, nil
+		}
+
+		// Blast out a cheeky warning so we can keep track of this.
+		log.Warnf(ctx, "about to perform request to self: GET %s", iri)
 	}
 
 	// Build IRI just once