Add development guidelines for PushRules

Adds business logic locations, entrypoints, and flow diagram for Push
Rules feature.

doc/development/backend/create_source_code_be/index.md

@@ -29,6 +29,10 @@ Source Code Management shares ownership of Code Owners with the Code Review grou
 
 - [Approval Rules](../../merge_request_concepts/approval_rules.md)
 
+### Push Rules
+
+- [Push Rules development guidelines](../../push_rules/index.md)
+
 ### Protected Branches
 
 Details about Protected Branches models can be found in the [Code Owners](../../code_owners/index.md#related-models) technical reference page.

doc/development/push_rules/index.md

+---
+stage: Create
+group: Source Code
+info: Any user with at least the Maintainer role can merge updates to this content. For details, see https://docs.gitlab.com/ee/development/development_processes.html#development-guidelines-review.
+---
+
+# Push Rules development guidelines
+
+This document was created to help contributors understand the code design of
+[Push Rules](../../user/project/repository/push_rules.md). You should read this
+document before making changes to the code for this feature.
+
+This document is intentionally limited to an overview of how the code is
+designed, as code can change often. To understand how a specific part of the
+feature works, view the code and the specs. The details here explain how the
+major components of the Push Rules feature work.
+
+NOTE:
+This document should be updated when parts of the codebase referenced in this
+document are updated, removed, or new parts are added.
+
+## Business logic
+
+The business logic is contained in two main places. The `PushRule` model stores
+the settings for a rule and then we have checks that use those settings to
+change the push behavior.
+
+- `PushRule`: the main model used to store the configuration of each push rule.
+  - Defined in `ee/app/models/push_rule.rb`.
+- `EE::Gitlab::Checks::DiffCheck`: Diff check prevents filenames matching the
+  push rule's `file_name_regex` and also files with names matching known secret
+  files, for example `id_rsa`.
+  - Defined in `ee/lib/ee/gitlab/checks/diff_check.rb`.
+- `EE::Gitlab::Checks::PushRuleCheck`: Executes various push rule checks.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rule_check.rb`.
+- `EE::Gitlab::Checks::PushRules::BranchCheck`: Executes push rule checks
+  related to branch rules.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rules/branch_check.rb`.
+- `EE::Gitlab::Checks::PushRules::CommitCheck`: Executes push rule checks
+  related to commit rules.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rules/commit_check.rb`.
+- `EE::Gitlab::Checks::PushRules::FileSizeCheck`: Executes push rule checks
+  related to file size rules.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rules/file_size_check.rb`.
+- `EE::Gitlab::Checks::PushRules::SecretsCheck`: Executes push rule checks
+  related to secrets rules.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rules/secrets_check.rb`.
+- `EE::Gitlab::Checks::PushRules::TagCheck`: Executes push rule checks
+  related to tag rules.
+  - Defined in `ee/lib/ee/gitlab/checks/push_rules/tag_check.rb`.
+
+## Entrypoints
+
+The following controllers and APIs are all entrypoints into the push rules logic:
+
+- `Admin::PushRulesController`: This controller is used to manage the global push rule.
+- `Group::PushRulesController`: This controller is used to manage the group-level push rule.
+- `Project::PushRulesController`: This controller is used to manage the project-level push rule.
+- `Api::Internal::Base`: This `/internal/allowed` endpoint is called when pushing to GitLab over SSH to
+  ensure the user is allowed to push. The `/internal/allowed` endpoint performs a
+  `Gitlab::Checks::DiffCheck`. In EE, this includes push rules checks.
+  - Defined in `lib/api/internal/base.rb`.
+- `Repositories::GitHttpController`: When changes are pushed to GitLab over HTTP, the controller performs an access
+  check to ensure the user is allowed to push. The checks perform a
+  `Gitlab::Checks::DiffCheck`. In EE, this includes push rules checks.
+  - Defined in `app/controllers/repositories/git_http_controller.rb`.
+
+## Flow
+
+These flowcharts should help explain the flow from the controllers down to the
+models for different features.
+
+### Git push over SSH
+
+```mermaid
+graph TD
+  Repositories::GitHttpController --> Gitlab::GitAccess
+  Api::Internal::Base --> Gitlab::GitAccess
+  Gitlab::GitAccess --> Gitlab::Checks::ChangesAccess
+  Gitlab::Checks::ChangesAccess --> Gitlab::Checks::SingleChangeAccess
+  Gitlab::Checks::ChangesAccess --> EE::Gitlab::Checks::PushRuleCheck
+  Gitlab::Checks::SingleChangeAccess --> Gitlab::Checks::DiffCheck
+  EE::Gitlab::Checks::PushRuleCheck -->|Only if pushing to a tag| EE::Gitlab::Checks::PushRules::TagCheck
+  EE::Gitlab::Checks::PushRuleCheck -->|Only if pushing to a branch| EE::Gitlab::Checks::PushRules::BranchCheck
+  EE::Gitlab::Checks::PushRuleCheck --> EE::Gitlab::Checks::PushRules::FileSizeCheck
+  EE::Gitlab::Checks::PushRuleCheck --> EE::Gitlab::Checks::PushRules::SecretsCheck
+  EE::Gitlab::Checks::PushRuleCheck --> EE::Gitlab::Checks::PushRules::SecretsCheck
+```
+
+NOTE:
+The `PushRuleCheck` only triggers checks in parallel if the
+`parallel_push_checks` feature flag is enabled. Otherwise tag or branch check
+runs first, then file size, then secrets.