[bugfix] Fix nil ptr exception on creating user without running server first (#4094)

# 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.

closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4049 by removing the silly defer statement. Adds a test too, and a warning in the docs.

## 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/4094
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

2 files changed, 36 insertions, 11 deletions

diff --git a/internal/db/bundb/admin.go b/internal/db/bundb/admin.go
index 9f5ccad01..0666cf8a8 100644
--- a/internal/db/bundb/admin.go
+++ b/internal/db/bundb/admin.go
@@ -165,15 +165,11 @@ func (a *adminDB) NewSignup(ctx context.Context, newSignup gtsmodel.NewSignup) (
 		return nil, err
 	}
 
-	defer func() {
-		// Pin account to (new)
-		// user before returning.
-		user.Account = account
-	}()
-
+	// If there's already a
+	// user for this account,
+	// just pin acct + return.
 	if user != nil {
-		// Already had a user for this
-		// account, just return that.
+		user.Account = account
 		return user, nil
 	}
 
@@ -194,13 +190,21 @@ func (a *adminDB) NewSignup(ctx context.Context, newSignup gtsmodel.NewSignup) (
 	}
 
 	// If no app ID was set,
-	// use the instance app ID.
+	// get the instance app
+	// and use its ID.
 	if newSignup.AppID == "" {
 		instanceApp, err := a.state.DB.GetInstanceApplication(ctx)
-		if err != nil {
+		if err != nil && !errors.Is(err, db.ErrNoEntries) {
 			err := gtserror.Newf("db error getting instance app: %w", err)
 			return nil, err
 		}
+
+		if instanceApp == nil {
+			const errText = "instance application not yet created, run the server at least once *before* creating users"
+			err := gtserror.New(errText)
+			return nil, err
+		}
+
 		newSignup.AppID = instanceApp.ID
 	}
 
diff --git a/internal/db/bundb/admin_test.go b/internal/db/bundb/admin_test.go
index 0ac3cc073..247431e87 100644
--- a/internal/db/bundb/admin_test.go
+++ b/internal/db/bundb/admin_test.go
@@ -21,7 +21,7 @@ import (
 	"context"
 	"testing"
 
-	gtsmodel "code.superseriousbusiness.org/gotosocial/internal/db/bundb/migrations/20211113114307_init"
+	"code.superseriousbusiness.org/gotosocial/internal/gtsmodel"
 	"code.superseriousbusiness.org/gotosocial/testrig"
 	"github.com/stretchr/testify/suite"
 )
@@ -92,6 +92,27 @@ func (suite *AdminTestSuite) TestCreateInstanceAccount() {
 	suite.NotNil(acct)
 }
 
+func (suite *AdminTestSuite) TestNewSignupWithNoInstanceApp() {
+	ctx := context.Background()
+
+	// Delete the instance app.
+	if err := suite.state.DB.DeleteApplicationByID(
+		ctx,
+		suite.testApplications["instance_application"].ID,
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Try to create a new signup with no provided app ID,
+	// it should fail as it can't fetch the instance app.
+	_, err := suite.state.DB.NewSignup(ctx, gtsmodel.NewSignup{
+		Username: "whatever",
+		Email:    "whatever@wherever.org",
+		Password: "really_good_password",
+	})
+	suite.EqualError(err, "NewSignup: instance application not yet created, run the server at least once *before* creating users")
+}
+
 func TestAdminTestSuite(t *testing.T) {
 	suite.Run(t, new(AdminTestSuite))
 }