Allow greater customization of environment variables by plugins.

internal/compile.bzl

@@ -41,12 +41,14 @@ proto_compile_attrs = {
     ),
 }
 
-def proto_compile_impl(ctx):
+def proto_compile_impl(ctx, base_env = {}):
     """
     Common implementation function for lang_*_compile rules.
 
     Args:
         ctx: The Bazel rule execution context object.
+        base_env: Default environment to use for protoc if
+            "use_built_in_shell_environment" = False for a given plugin.
 
     Returns:
         Providers:
@@ -63,9 +65,9 @@ def proto_compile_impl(ctx):
     extra_protoc_files = ctx.files.extra_protoc_files
 
     # Execute with extracted attrs
-    return proto_compile(ctx, options, extra_protoc_args, extra_protoc_files)
+    return proto_compile(ctx, options, extra_protoc_args, extra_protoc_files, base_env)
 
-def proto_compile(ctx, options, extra_protoc_args, extra_protoc_files):
+def proto_compile(ctx, options, extra_protoc_args, extra_protoc_files, base_env):
     """
     Common implementation function for lang_*_compile rules.
 
@@ -74,6 +76,8 @@ def proto_compile(ctx, options, extra_protoc_args, extra_protoc_files):
         options: The mutable options dict.
         extra_protoc_args: The mutable extra_protoc_args list.
         extra_protoc_files: The mutable extra_protoc_files list.
+        base_env: Default environment to use for protoc if
+            "use_built_in_shell_environment" = False for a given plugin.
 
     Returns:
         Providers:
@@ -327,7 +331,7 @@ def proto_compile(ctx, options, extra_protoc_args, extra_protoc_files):
             command = "echo '\n##### SANDBOX BEFORE RUNNING PROTOC' && find . -type l && " + command
 
         if verbose > 3:
-            command = "env && " + command
+            command = "echo Printing environment used to invoke protoc.... && env && " + command
             for f in cmd_inputs:
                 print("INPUT:", f.path)  # buildifier: disable=print
             for f in protos:
@@ -337,14 +341,23 @@ def proto_compile(ctx, options, extra_protoc_args, extra_protoc_files):
             for f in plugin_outputs:
                 print("EXPECTED OUTPUT:", f.path)  # buildifier: disable=print
 
-        # Run protoc
+        env = {}
+        if (len(env) > 0) and plugin.use_built_in_shell_environment:
+            fail("env and use_built_in_shell_environment are mutually exclusive; " +
+                 " both set for plugin {}".format(plugin.name))
+
+        if not plugin.use_built_in_shell_environment:
+            env = dict(base_env, **env)
+
+        # Run protoc (https://bazel.build/rules/lib/actions#run_shell)
         ctx.actions.run_shell(
             mnemonic = mnemonic,
             command = command,
             arguments = [args],
             inputs = cmd_inputs,
             tools = tools,
             outputs = plugin_protoc_outputs,
+            env = env,
             use_default_shell_env = plugin.use_built_in_shell_environment,
             input_manifests = cmd_input_manifests,
             progress_message = "Compiling protoc outputs for {} plugin on target {}".format(

internal/plugin.bzl

@@ -15,6 +15,7 @@ def _proto_plugin_impl(ctx):
             tool = ctx.attr.tool,
             tool_executable = ctx.executable.tool,
             use_built_in_shell_environment = ctx.attr.use_built_in_shell_environment,
+            env = ctx.attr.env,
             protoc_plugin_name = ctx.attr.protoc_plugin_name,
             exclusions = ctx.attr.exclusions,
             data = ctx.files.data,
@@ -53,6 +54,10 @@ proto_plugin = rule(
             doc = "Whether the tool should use the built in shell environment or not",
             default = True,
         ),
+        "env": attr.string_dict(
+            doc = "Sets the dictionary of environment variables to use when invoking protoc. Must be None if use_built_in_shell_environment is true.",
+            default = {},
+        ),
         "protoc_plugin_name": attr.string(
             doc = "The name used for the plugin binary on the protoc command line. Useful for targeting built-in plugins. Uses plugin name when not set",
         ),

internal/providers.bzl

@@ -13,6 +13,7 @@ ProtoPluginInfo = provider(
         "tool": "The plugin binary. If absent, it is assumed the plugin is built-in to protoc itself and plugin_name will be used if available, otherwise the plugin name",
         "tool_executable": "The plugin binary executable",
         "use_built_in_shell_environment": "Whether the tool should use the built in shell environment or not",
+        "env": "Sets the dictionary of environment variables to use when invoking protoc. Must be None if use_built_in_shell_environment is true.",
         "protoc_plugin_name": "The name used for the plugin binary on the protoc command line. Useful for targeting built-in plugins. Uses plugin name when not set",
         "exclusions": "Exclusion filters to apply when generating outputs with this plugin. Used to prevent generating files that are included in the protobuf library, for example. Can exclude either by proto name prefix or by proto folder prefix",
         "data": "Additional files required for running the plugin",