Teach bazelisk.py about .bazeliskrc

Update README to list new capabilities; the Python implementation
comes a bit closer to the Go implementation.

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.
 
 Example file content:
 
@@ -179,21 +179,21 @@ BAZELISK_GITHUB_TOKEN=abc
 The following variables can be set:
 
 - `BAZELISK_BASE_URL`
-- `BAZELISK_CLEAN`
-- `BAZELISK_GITHUB_TOKEN`
+- `BAZELISK_CLEAN` (Go version only)
+- `BAZELISK_GITHUB_TOKEN` (Go version only)
 - `BAZELISK_HOME`
-- `BAZELISK_INCOMPATIBLE_FLAGS`
-- `BAZELISK_SHUTDOWN`
-- `BAZELISK_SKIP_WRAPPER`
-- `BAZELISK_USER_AGENT`
-- `BAZELISK_VERIFY_SHA256`
+- `BAZELISK_INCOMPATIBLE_FLAGS` (Go version only)
+- `BAZELISK_SHUTDOWN` (Go version only)
+- `BAZELISK_SKIP_WRAPPER` (Go version only)
+- `BAZELISK_USER_AGENT` (Go version only)
+- `BAZELISK_VERIFY_SHA256` (Go version only)
 - `USE_BAZEL_VERSION`
 
 Configuration variables are evaluated with precedence order. The preferred values are derived in order from highest to lowest precedence as follows:
 
 * Variables defined in the environment
 * Variables defined in the workspace root `.bazeliskrc`
-* Variables defined in the user home `.bazeliskrc`
+* Variables defined in the user home `.bazeliskrc` (Go version only)
 
 ## Requirements
 

bazelisk.py

@@ -68,6 +68,31 @@
 BAZEL_UPSTREAM = "bazelbuild"
 
 
+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]
+    env_files = []
+    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:
+                for line in f.readlines():
+                    line = line.split("#", maxsplit=1)[0].strip()
+                    if not line:
+                        continue
+                    some_name, some_value = line.split("=", maxsplit=1)
+                    if some_name == name:
+                        return some_value
+        except Exception:
+            pass
+    return default
+
+
 def decide_which_bazel_version_to_use():
     # Check in this order:
     # - env var "USE_BAZEL_VERSION" is set to a specific version.
@@ -79,8 +104,9 @@ 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:
@@ -226,7 +252,7 @@ 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)
 
@@ -277,8 +303,9 @@ 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
@@ -342,7 +369,7 @@ 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:
@@ -357,7 +384,7 @@ 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

@@ -479,6 +479,14 @@ 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
@@ -512,14 +520,6 @@ 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