Allow to tag PRE_COMMANDS to run them before loading plugins

Fixes #3936

README.md

@@ -1122,15 +1122,19 @@ Example in `.mega-linter.yml` config file
 ```yaml
 PRE_COMMANDS:
   - command: npm install eslint-plugin-whatever
-    cwd: "root"        # Will be run at the root of MegaLinter docker image
+    cwd: root        # Will be run at the root of MegaLinter docker image
     secured_env: true  # True by default, but if defined to false, no global variable will be hidden (for example if you need GITHUB_TOKEN)
   - command: echo "pre-test command has been called"
-    cwd: "workspace"   # Will be run at the root of the workspace (usually your repository root)
+    cwd: workspace   # Will be run at the root of the workspace (usually your repository root)
     continue_if_failed: False  # Will stop the process if command is failed (return code > 0)
   - command: pip install flake8-cognitive-complexity
     venv: flake8 # Will be run within flake8 python virtualenv. There is one virtualenv per python-based linter, with the same name
   - command: export MY_OUTPUT_VAR="my output var" && export MY_OUTPUT_VAR2="my output var2"
     output_variables: ["MY_OUTPUT_VAR","MY_OUTPUT_VAR2"] # Will collect the values of output variables and update MegaLinter own ENV context
+  - command: echo "Some command called before loading MegaLinter plugins"
+    cwd: workspace   # Will be run at the root of the workspace (usually your repository root)
+    continue_if_failed: False  # Will stop the process if command is failed (return code > 0)
+    tag: before_plugins # Tag indicating that the command will be run before loading plugins
 ```
 
 <!-- config-precommands-section-end -->

megalinter/MegaLinter.py

@@ -147,6 +147,9 @@ def __init__(self, params=None):
         self.flavor_suggestions = None
 
         # Initialize plugins
+        self.pre_commands_results = pre_post_factory.run_pre_commands(
+            self, "before_plugins"
+        )
         plugin_factory.initialize_plugins(self.request_id)
 
         # Copy node_modules in current folder if necessary
@@ -175,7 +178,7 @@ def __init__(self, params=None):
             )
 
         # Run user-defined commands
-        self.pre_commands_results = pre_post_factory.run_pre_commands(self)
+        self.pre_commands_results += pre_post_factory.run_pre_commands(self)
         self.post_commands_results = []
         # Initialize linters and gather criteria to browse files
         self.load_linters()

megalinter/descriptors/schemas/megalinter-configuration.jsonschema.json

@@ -46,6 +46,13 @@
           "title": "Secured ENV variables",
           "type": "boolean"
         },
+        "tag": {
+          "examples": [
+            "before_plugins"
+          ],
+          "title": "Tag defining at which commands entry point the command will be run",
+          "type": "string"
+        },
         "venv": {
           "examples": [
             "flake8"

megalinter/pre_post_factory.py

@@ -10,13 +10,13 @@
 
 
 # User defined commands to run before running linters
-def run_pre_commands(mega_linter):
-    return run_pre_post_commands("PRE_COMMANDS", "[Pre]", mega_linter)
+def run_pre_commands(mega_linter, tag="default"):
+    return run_pre_post_commands("PRE_COMMANDS", "[Pre]", mega_linter, tag)
 
 
 # User defined commands to run after running linters
-def run_post_commands(mega_linter):
-    return run_pre_post_commands("POST_COMMANDS", "[Post]", mega_linter)
+def run_post_commands(mega_linter, tag="default"):
+    return run_pre_post_commands("POST_COMMANDS", "[Post]", mega_linter, tag)
 
 
 # Commands to run before running all linters in a descriptor
@@ -52,9 +52,21 @@ def run_linter_post_commands(mega_linter, linter):
 
 
 # Get commands from configuration
-def run_pre_post_commands(key, log_key, mega_linter):
+def run_pre_post_commands(key, log_key, mega_linter, tag="default"):
     pre_or_post_commands = config.get_list(mega_linter.request_id, key, None)
-    return run_commands(pre_or_post_commands, log_key, mega_linter)
+    if pre_or_post_commands is None:
+        logging.debug(f"{log_key} No commands declared in user configuration")
+        return []
+    # Filter commands according to tag
+    applicable_pre_post_commands = []
+    for command in pre_or_post_commands:
+        if (
+            tag == "default"
+            and (("tag" not in command) or (command["tag"] == "default"))
+        ) or (tag != "default" and "tag" in command and command["tag"] == tag):
+            applicable_pre_post_commands += [command]
+    # Run matching commands
+    return run_commands(applicable_pre_post_commands, log_key, mega_linter)
 
 
 # Perform run of commands