Teach bazelisk.py about .bazeliskrc

README.md

@@ -166,7 +166,7 @@ You can control the user agent that Bazelisk sends in all HTTP requests by setti
 
 # .bazeliskrc configuration file
 
-The Go version supports a `.bazeliskrc` file in the root directory of a workspace and the user home directory. This file allows users to set environment variables persistently.
+A `.bazeliskrc` file in the root directory of a workspace or the user home directory allows users to set environment variables persistently. (The Python implementation of Bazelisk doesn't check the user home directory yet, only the workspace directory.)
 
 Example file content:
 

bazelisk.py

@@ -67,6 +67,50 @@
 
 BAZEL_UPSTREAM = "bazelbuild"
 
+_dotfiles_dict = None
+
+
+def get_dotfiles_dict():
+    """Loads all supported dotfiles and returns a unified name=value dictionary
+    for their config settings. The dictionary is only loaded on the first call
+    to this function; subsequent calls used a cached result, so won't change.
+    """
+    global _dotfiles_dict
+    if _dotfiles_dict is not None:
+        return _dotfiles_dict
+    _dotfiles_dict = dict()
+    env_files = []
+    # Here we're only checking the workspace root. Ideally, we would also check
+    # the user's home directory to match the Go version. When making that edit,
+    # be sure to obey the correctly prioritization of workspace / home rcfiles.
+    root = find_workspace_root()
+    if root:
+        env_files.append(os.path.join(root, ".bazeliskrc"))
+    for env_file in env_files:
+        try:
+            with open(env_file, "r") as f:
+                rcdata = f.read()
+        except Exception:
+            continue
+        for line in rcdata.splitlines():
+            line = line.split("#", 1)[0].strip()
+            if not line:
+                continue
+            some_name, some_value = line.split("=", 1)
+            _dotfiles_dict[some_name] = some_value
+    return _dotfiles_dict
+
+
+def get_env_or_config(name, default=None):
+    """Reads a configuration value from the environment, but falls back to
+    reading it from .bazeliskrc in the workspace root."""
+    if name in os.environ:
+        return os.environ[name]
+    dotfiles = get_dotfiles_dict()
+    if name in dotfiles:
+        return dotfiles[name]
+    return default
+
 
 def decide_which_bazel_version_to_use():
     # Check in this order:
@@ -79,8 +123,9 @@ def decide_which_bazel_version_to_use():
     # - workspace_root/.bazelversion exists -> read contents, that version.
     # - workspace_root/WORKSPACE contains a version -> that version. (TODO)
     # - fallback: latest release
-    if "USE_BAZEL_VERSION" in os.environ:
-        return os.environ["USE_BAZEL_VERSION"]
+    use_bazel_version = get_env_or_config("USE_BAZEL_VERSION")
+    if use_bazel_version is not None:
+        return use_bazel_version
 
     workspace_root = find_workspace_root()
     if workspace_root:
@@ -229,7 +274,7 @@ def determine_bazel_filename(version):
 
     filename_suffix = determine_executable_filename_suffix()
     bazel_flavor = "bazel"
-    if os.environ.get("BAZELISK_NOJDK", "0") != "0":
+    if get_env_or_config("BAZELISK_NOJDK", "0") != "0":
         bazel_flavor = "bazel_nojdk"
     return "{}-{}-{}-{}{}".format(bazel_flavor, version, operating_system, machine, filename_suffix)
 
@@ -280,8 +325,9 @@ def determine_url(version, is_commit, bazel_filename):
     # Example: '0.19.1' -> ('0.19.1', None), '0.20.0rc1' -> ('0.20.0', 'rc1')
     (version, rc) = re.match(r"(\d*\.\d*(?:\.\d*)?)(rc\d+)?", version).groups()
 
-    if "BAZELISK_BASE_URL" in os.environ:
-        return "{}/{}/{}".format(os.environ["BAZELISK_BASE_URL"], version, bazel_filename)
+    bazelisk_base_url = get_env_or_config("BAZELISK_BASE_URL")
+    if bazelisk_base_url is not None:
+        return "{}/{}/{}".format(bazelisk_base_url, version, bazel_filename)
     else:
         return "https://releases.bazel.build/{}/{}/{}".format(
             version, rc if rc else "release", bazel_filename
@@ -345,7 +391,7 @@ def download_bazel_into_directory(version, is_commit, directory):
 def download(url, destination_path):
     sys.stderr.write("Downloading {}...\n".format(url))
     request = Request(url)
-    if "BAZELISK_BASE_URL" in os.environ:
+    if get_env_or_config("BAZELISK_BASE_URL") is not None:
         parts = urlparse(url)
         creds = None
         try:
@@ -360,7 +406,7 @@ def download(url, destination_path):
 
 
 def get_bazelisk_directory():
-    bazelisk_home = os.environ.get("BAZELISK_HOME")
+    bazelisk_home = get_env_or_config("BAZELISK_HOME")
     if bazelisk_home is not None:
         return bazelisk_home
 

bazelisk_test.sh

@@ -508,6 +508,14 @@ echo "# test_bazel_version_from_environment"
 test_bazel_version_from_environment
 echo
 
+echo "# test_bazel_version_prefer_environment_to_bazeliskrc"
+test_bazel_version_prefer_environment_to_bazeliskrc
+echo
+
+echo "# test_bazel_version_from_workspace_bazeliskrc"
+test_bazel_version_from_workspace_bazeliskrc
+echo
+
 echo "# test_bazel_version_from_file"
 test_bazel_version_from_file
 echo
@@ -545,14 +553,6 @@ if [[ $BAZELISK_VERSION == "GO" ]]; then
   test_bazel_version_from_base_url
   echo
 
-  echo "# test_bazel_version_prefer_environment_to_bazeliskrc"
-  test_bazel_version_prefer_environment_to_bazeliskrc
-  echo
-
-  echo "# test_bazel_version_from_workspace_bazeliskrc"
-  test_bazel_version_from_workspace_bazeliskrc
-  echo
-
   echo "# test_bazel_version_from_user_home_bazeliskrc"
   test_bazel_version_from_user_home_bazeliskrc
   echo