bazel: Add @envoy_repo target with VERSION AND PATH

bazel/repositories.bzl

@@ -52,6 +52,54 @@ _default_envoy_build_config = repository_rule(
     },
 )
 
+def _envoy_repo_impl(repository_ctx):
+    """This provides information about the Envoy repository
+
+    You can access the current version and path to the repository in .bzl/BUILD
+    files as follows:
+
+    ```starlark
+    load("@envoy_repo//:version.bzl", "VERSION")
+    ```
+
+    `VERSION` can be used to derive version-specific rules and can be passed
+    to the rules.
+
+    The `VERSION` and also the local `PATH` to the repo can be accessed in
+    python libraries/binaries. By adding `@envoy_repo` to `deps` they become
+    importable through the `envoy_repo` namespace.
+
+    As the `PATH` is local to the machine, it is generally only useful for
+    jobs that will run locally.
+
+    This can be useful for example, for tooling that needs to check the
+    repository, or to run bazel queries that cannot be run within the
+    constraints of a `genquery`.
+
+    """
+    repo_path = repository_ctx.path(repository_ctx.attr.envoy_root).dirname
+    version = repository_ctx.read(repo_path.get_child("VERSION")).strip()
+    repository_ctx.file("version.bzl", "VERSION = '%s'" % version)
+    repository_ctx.file("__init__.py", "PATH = '%s'\nVERSION = '%s'" % (repo_path, version))
+    repository_ctx.file("WORKSPACE", "")
+    repository_ctx.file("BUILD", """
+load("@rules_python//python:defs.bzl", "py_library")
+
+py_library(name = "envoy_repo", srcs = ["__init__.py"], visibility = ["//visibility:public"])
+
+""")
+
+_envoy_repo = repository_rule(
+    implementation = _envoy_repo_impl,
+    attrs = {
+        "envoy_root": attr.label(default = "@envoy//:BUILD"),
+    },
+)
+
+def envoy_repo():
+    if "envoy_repo" not in native.existing_rules().keys():
+        _envoy_repo(name = "envoy_repo")
+
 # Python dependencies.
 def _python_deps():
     # TODO(htuch): convert these to pip3_import.
@@ -100,6 +148,9 @@ def _rust_deps():
     external_http_archive("rules_rust")
 
 def envoy_dependencies(skip_targets = []):
+    # Add a binding for repository variables.
+    envoy_repo()
+
     # Setup Envoy developer tools.
     envoy_dev_binding()