feat: selectively generate libraries (#3290)

In this PR:
- Allow selectively generate libraries by adding `--library-names` to
CLI.

We have a request of partially releasing google-cloud-java
(b/331628538), which requires only generating a selective list of
libraries.

This change allows a user-defined list of libraries to be generated even
if the corresponding library configs are the same.

library_generation/cli/entry_point.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 import os
 import sys
-
+from typing import Optional
 import click as click
 from library_generation.generate_pr_description import generate_pr_descriptions
 from library_generation.generate_repo import generate_from_yaml
@@ -51,6 +51,17 @@ def main(ctx):
     metadata about library generation.
     """,
 )
+@click.option(
+    "--library-names",
+    type=str,
+    default=None,
+    show_default=True,
+    help="""
+    A list of library names that will be generated, separated by comma.
+    The library name of a library is the value of library_name or api_shortname,
+    if library_name is not specified, in the generation configuration.
+    """,
+)
 @click.option(
     "--repository-path",
     type=str,
@@ -77,6 +88,7 @@ def main(ctx):
 def generate(
     baseline_generation_config_path: str,
     current_generation_config_path: str,
+    library_names: Optional[str],
     repository_path: str,
     api_definitions_path: str,
 ):
@@ -86,24 +98,32 @@ def generate(
     history.
 
     If baseline generation config is not specified but current generation
-    config is specified, generate all libraries based on current generation
-    config without commit history.
+    config is specified, generate all libraries if `library_names` is not
+    specified, based on current generation config without commit history.
 
     If current generation config is not specified but baseline generation
     config is specified, raise FileNotFoundError because current generation
     config should be the source of truth of library generation.
 
     If both baseline generation config and current generation config are not
     specified, generate all libraries based on the default generation config,
-    which is generation_config.yaml in the current working directory. Raise
-    FileNotFoundError if the default config does not exist.
+    which is generation_config.yaml in the current working directory.
+
+    If `library_names` is specified, only libraries whose name can be found in
+    the current generation config or default generation config, if current
+    generation config is not specified, will be generated. Changed libraries
+    will be ignored even if baseline and current generation config are
+    specified.
+
+    Raise FileNotFoundError if the default config does not exist.
 
     The commit history, if generated, will be available in
     repository_path/pr_description.txt.
     """
     __generate_repo_and_pr_description_impl(
         baseline_generation_config_path=baseline_generation_config_path,
         current_generation_config_path=current_generation_config_path,
+        library_names=library_names,
         repository_path=repository_path,
         api_definitions_path=api_definitions_path,
     )
@@ -112,6 +132,7 @@ def generate(
 def __generate_repo_and_pr_description_impl(
     baseline_generation_config_path: str,
     current_generation_config_path: str,
+    library_names: Optional[str],
     repository_path: str,
     api_definitions_path: str,
 ):
@@ -146,30 +167,39 @@ def __generate_repo_and_pr_description_impl(
     current_generation_config_path = os.path.abspath(current_generation_config_path)
     repository_path = os.path.abspath(repository_path)
     api_definitions_path = os.path.abspath(api_definitions_path)
+    include_library_names = _parse_library_name_from(library_names)
+
     if not baseline_generation_config_path:
-        # Execute full generation based on current_generation_config if
+        # Execute selective generation based on current_generation_config if
         # baseline_generation_config is not specified.
         # Do not generate pull request description.
         generate_from_yaml(
             config=from_yaml(current_generation_config_path),
             repository_path=repository_path,
             api_definitions_path=api_definitions_path,
+            target_library_names=include_library_names,
         )
         return
 
-    # Compare two generation configs and only generate changed libraries.
+    # Compare two generation configs to get changed libraries.
     # Generate pull request description.
     baseline_generation_config_path = os.path.abspath(baseline_generation_config_path)
     config_change = compare_config(
         baseline_config=from_yaml(baseline_generation_config_path),
         current_config=from_yaml(current_generation_config_path),
     )
-    # pass None if we want to fully generate the repository.
-    target_library_names = (
+    # Pass None if we want to fully generate the repository.
+    changed_library_names = (
         config_change.get_changed_libraries()
         if not _needs_full_repo_generation(config_change=config_change)
         else None
     )
+    # Include library names takes preference if specified.
+    target_library_names = (
+        include_library_names
+        if include_library_names is not None
+        else changed_library_names
+    )
     generate_from_yaml(
         config=config_change.current_config,
         repository_path=repository_path,
@@ -191,6 +221,12 @@ def _needs_full_repo_generation(config_change: ConfigChange) -> bool:
     return not current_config.is_monorepo() or current_config.contains_common_protos()
 
 
+def _parse_library_name_from(includes: str) -> Optional[list[str]]:
+    if includes is None:
+        return None
+    return [library_name.strip() for library_name in includes.split(",")]
+
+
 @main.command()
 @click.option(
     "--generation-config-path",

library_generation/generate_repo.py

@@ -12,9 +12,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import os
 import shutil
-
+from typing import Optional
 import library_generation.utils.utilities as util
 from library_generation.generate_composed_library import generate_composed_library
 from library_generation.model.generation_config import GenerationConfig
@@ -26,7 +25,7 @@ def generate_from_yaml(
     config: GenerationConfig,
     repository_path: str,
     api_definitions_path: str,
-    target_library_names: list[str] = None,
+    target_library_names: Optional[list[str]],
 ) -> None:
     """
     Based on the generation config, generates libraries via