Update the webauthn_credential_id_sequence in Postgres (#19048)

* Update the webauthn_credential_id_sequence in Postgres

There is (yet) another problem with v210 in that Postgres will silently allow preset
ID insertions ... but it will not update the sequence value.

This PR simply adds a little step to the end of the v210 migration to update the
sequence number.

Users who have already migrated who find that they cannot insert new
webauthn_credentials into the DB can either run:

```bash
gitea doctor recreate-table webauthn_credential
```

or

```bash
./gitea doctor --run=check-db-consistency --fix
```

which will fix the bad sequence.

Fix #19012

Signed-off-by: Andrew Thornton <art27@cantab.net>

docs/content/doc/developers/guidelines-backend.md

@@ -42,7 +42,7 @@ To maintain understandable code and avoid circular dependencies it is important
   - `modules/setting`: Store all system configurations read from ini files and has been referenced by everywhere. But they should be used as function parameters when possible.
   - `modules/git`: Package to interactive with `Git` command line or Gogit package.
 - `public`: Compiled frontend files (javascript, images, css, etc.)
-- `routers`: Handling of server requests. As it uses other Gitea packages to serve the request, other packages (models, modules or services) shall not depend on routers.
+- `routers`: Handling of server requests. As it uses other Gitea packages to serve the request, other packages (models, modules or services) must not depend on routers.
   - `routers/api` Contains routers for `/api/v1` aims to handle RESTful API requests. 
   - `routers/install` Could only respond when system is in INSTALL mode (INSTALL_LOCK=false). 
   - `routers/private` will only be invoked by internal sub commands, especially `serv` and `hooks`. 
@@ -106,10 +106,20 @@ i.e. `servcies/user`, `models/repository`.
 Since there are some packages which use the same package name, it is possible that you find packages like `modules/user`, `models/user`, and `services/user`. When these packages are imported in one Go file, it's difficult to know which package we are using and if it's a variable name or an import name. So, we always recommend to use import aliases. To differ from package variables which are commonly in camelCase, just use **snake_case** for import aliases.
 i.e. `import user_service "code.gitea.io/gitea/services/user"`
 
+### Important Gotchas
+
+- Never write `x.Update(exemplar)` without an explicit `WHERE` clause:
+  - This will cause all rows in the table to be updated with the non-zero values of the exemplar - including IDs.
+  - You should usually write `x.ID(id).Update(exemplar)`.
+- If during a migration you are inserting into a table using `x.Insert(exemplar)` where the ID is preset:
+  - You will need to ``SET IDENTITY_INSERT `table` ON`` for the MSSQL variant (the migration will fail otherwise)
+  - However, you will also need to update the id sequence for postgres - the migration will silently pass here but later insertions will fail:
+    ``SELECT setval('table_name_id_seq', COALESCE((SELECT MAX(id)+1 FROM `table_name`), 1), false)``
+
 ### Future Tasks
 
 Currently, we are creating some refactors to do the following things:
 
 - Correct that codes which doesn't follow the rules.
 - There are too many files in `models`, so we are moving some of them into a sub package `models/xxx`.
-- Some `modules` sub packages should be moved to `services` because they depends on `models`.
+- Some `modules` sub packages should be moved to `services` because they depend on `models`.

models/migrations/v210.go

@@ -174,5 +174,11 @@ func remigrateU2FCredentials(x *xorm.Engine) error {
 		regs = regs[:0]
 	}
 
+	if x.Dialect().URI().DBType == schemas.POSTGRES {
+		if _, err := x.Exec("SELECT setval('webauthn_credential_id_seq', COALESCE((SELECT MAX(id)+1 FROM `webauthn_credential`), 1), false)"); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }