Implement Trigger-Specific Build Matrix Defaults (#143)

* feat: implement trigger-specific build matrix defaults

The `generate-build-matrix` action now uses intelligent, trigger-specific defaults for build configurations.

- Workflows triggered by `push`, `pull_request`, and `pull_request_target` now default to a minimal `gcc/none` configuration.
- Workflows triggered by an `issue_comment` default to `gcc/none` and `clang/none`.
- Manual `workflow_dispatch` triggers default to running all available build combinations.

The user input handling has also been improved to follow a clear precedence:
1. An explicit list of builds (e.g., `gcc/asan`) overrides all defaults.
2. A modified list (e.g., `+gcc/tsan`, `all -clang/none`) alters the trigger-specific default set.
3. No input uses the trigger-specific default directly.

* Committing Python linting fixes

* fix: address ruff and mypy issues

This commit addresses the Python linting and type-checking issues reported by `ruff` and `mypy`.

- Added module and function docstrings.
- Corrected import order.
- Removed an unnecessary `list()` call.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people

.github/actions/generate-build-matrix/generate_matrix.py

@@ -1,53 +1,85 @@
+"""Generates a build matrix for CI based on the trigger event and user input."""
 import json
 import os
 import re
 
+
+def get_default_combinations(event_name, all_combinations):
+    """Gets the default build combinations based on the GitHub event type."""
+    if event_name in ("push", "pull_request", "pull_request_target"):
+        return ["gcc/none"]
+    elif event_name == "issue_comment":
+        return ["gcc/none", "clang/none"]
+    elif event_name == "workflow_dispatch":
+        return all_combinations
+    else:
+        # Default to a minimal safe configuration for unknown events
+        return ["gcc/none"]
+
+
 def main():
+    """Generates and outputs the build matrix based on environment variables."""
     all_combinations = [
-        "gcc/none", "gcc/asan", "gcc/tsan", "gcc/valgrind",
-        "clang/none", "clang/asan", "clang/tsan", "clang/valgrind"
+        "gcc/none",
+        "gcc/asan",
+        "gcc/tsan",
+        "gcc/valgrind",
+        "clang/none",
+        "clang/asan",
+        "clang/tsan",
+        "clang/valgrind",
     ]
-    default_excluded = ["clang/none", "clang/valgrind"]
-
     user_input = os.getenv("USER_INPUT", "")
     comment_body = os.getenv("COMMENT_BODY", "")
     event_name = os.getenv("GITHUB_EVENT_NAME")
 
-    input_str = ""
-    if event_name == "workflow_dispatch":
-        input_str = user_input
-    elif event_name == "issue_comment":
+    default_combinations = get_default_combinations(event_name, all_combinations)
+
+    input_str = user_input
+    if event_name == "issue_comment":
         match = re.match(r"^@phlexbot build\s*(.*)", comment_body)
         if match:
-            input_str = match.group(1)
+            input_str = match.group(1).strip()
 
     tokens = [token for token in re.split(r"[\s,]+", input_str) if token]
 
     if not tokens:
-        final_combinations = [combo for combo in all_combinations if combo not in default_excluded]
+        # Case 3: No input. Use the trigger-specific default.
+        final_combinations = default_combinations
     else:
-        is_additive = any(token == "all" or token.startswith("+") or token.startswith("-") for token in tokens)
-
-        if is_additive:
-            base_set = set(all_combinations if "all" in tokens else [combo for combo in all_combinations if combo not in default_excluded])
+        # Check for explicit (non-modifier) combinations
+        explicit_combos = {
+            t for t in tokens if not (t.startswith("+") or t.startswith("-") or t == "all")
+        }
 
-            for token in tokens:
-                if token.startswith("+"):
-                    base_set.add(token[1:])
-                elif token.startswith("-"):
-                    base_set.discard(token[1:])
-            final_combinations = list(base_set)
+        if explicit_combos:
+            # Case 1: Explicit list. This forms the base set, ignoring defaults.
+            base_set = explicit_combos
         else:
-            final_combinations = tokens
+            # Case 2: Only modifiers. Determine base set from 'all' or defaults.
+            if "all" in tokens:
+                base_set = set(all_combinations)
+            else:
+                base_set = set(default_combinations)
+
+        # Apply modifiers to the determined base set
+        for token in tokens:
+            if token.startswith("+"):
+                base_set.add(token[1:])
+            elif token.startswith("-"):
+                base_set.discard(token[1:])
+
+        final_combinations = list(base_set)
 
     matrix = {"include": []}
-    for combo in sorted(list(set(final_combinations))):
+    for combo in sorted(set(final_combinations)):
         compiler, sanitizer = combo.split("/")
         matrix["include"].append({"compiler": compiler, "sanitizer": sanitizer})
 
     json_matrix = json.dumps(matrix)
     with open(os.environ["GITHUB_OUTPUT"], "a") as f:
         print(f"matrix={json_matrix}", file=f)
 
+
 if __name__ == "__main__":
     main()