Add custom module loading

docs/development.md

@@ -39,6 +39,82 @@ Selecting a single module also runs its transitive dependencies. If a dependency
 is unavailable or the dependency graph contains a cycle, the command logs a
 warning and does not run any modules.
 
+## Custom modules
+
+Module-running `check-*` commands can load custom modules from Python files that
+are not installed as part of MVT. Load one file with:
+
+```bash
+mvt-ios check-backup --load-module ./example_module.py --output ./out ./backup
+```
+
+You can also load a folder. MVT loads non-hidden top-level `*.py` files in
+sorted order and skips `__init__.py`:
+
+```bash
+mvt-ios check-fs --load-module ./custom_modules ./filesystem-dump
+```
+
+Set `MVT_CUSTOM_MODULES` to load a folder for every module-running command. This
+folder is loaded before any `--load-module` path:
+
+```bash
+MVT_CUSTOM_MODULES=./custom_modules mvt-android check-bugreport ./bugreport.zip
+```
+
+Custom modules are normal `MVTModule` subclasses:
+
+```python
+from mvt.common.module import MVTModule
+
+
+class ExampleCustomModule(MVTModule):
+    supported_commands = (("ios", "check-backup"), ("ios", "check-fs"))
+    slug = "example_custom_module"
+
+    def run(self):
+        self.results = [{"message": "custom module ran"}]
+
+    def check_indicators(self):
+        pass
+
+    def serialize(self, result):
+        return None
+```
+
+Use `supported_commands` to restrict a module to specific platform/command
+pairs. Missing or empty `supported_commands` means the module is available to
+all commands, which keeps older modules compatible. Supported pairs are:
+
+```python
+("ios", "check-backup")
+("ios", "check-fs")
+("ios", "check-iocs")
+("android", "check-backup")
+("android", "check-bugreport")
+("android", "check-androidqf")
+("android", "check-intrusion-logs")
+("android", "check-iocs")
+```
+
+Custom modules can depend on existing MVT module classes. Dependencies are
+resolved with the same ordering logic as built-in modules, and custom modules
+are appended after built-ins before ordering:
+
+```python
+from mvt.common.module import MVTModule
+from mvt.ios.modules.backup.manifest import Manifest
+
+
+class DependentCustomModule(MVTModule):
+    supported_commands = (("ios", "check-backup"),)
+    dependencies = (Manifest,)
+
+    def run(self):
+        manifest_results = self.get_dependency_results(Manifest)
+        self.results = [{"manifest_entries": len(manifest_results)}]
+```
+
 ## Profiling
 
 Some MVT modules extract and process significant amounts of data during the analysis process or while checking results against known indicators. Care must be