runtime: flexible layering configuration. (#7032)

To support TDS and to provide operator flexibility in how it relates to
existing runtime layering, this PR introduces support for explicit
control over layering. This eliminates the distinguished role of
subdirectory and override_subdirectory, for example, in favor of
arbitrary disk overlay layers.

This PR deprecates the existing runtime configuration in favor of the
new layered variant.

As a bonus, the PR fixes some of the runtime stats and related docs.

Risk level: Low
Testing: various unit tests (including server_test) additions.

Part of #6708.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4dafba65baaf9769723f895761268eed31af629b

envoy/config/bootstrap/v2/bootstrap.proto

@@ -110,9 +110,15 @@ message Bootstrap {
 
   reserved 10;
 
-  // Configuration for the runtime configuration provider. If not specified, a
-  // “null” provider will be used which will result in all defaults being used.
-  Runtime runtime = 11;
+  // Configuration for the runtime configuration provider (deprecated). If not
+  // specified, a “null” provider will be used which will result in all defaults
+  // being used.
+  Runtime runtime = 11 [deprecated = true];
+
+  // Configuration for the runtime configuration provider. If not
+  // specified, a “null” provider will be used which will result in all defaults
+  // being used.
+  LayeredRuntime layered_runtime = 17;
 
   // Configuration for the local administration HTTP server.
   Admin admin = 12;
@@ -200,13 +206,14 @@ message Watchdog {
   google.protobuf.Duration multikill_timeout = 4;
 }
 
-// Runtime :ref:`configuration overview <config_runtime>`.
+// Runtime :ref:`configuration overview <config_runtime>` (deprecated).
 message Runtime {
   // The implementation assumes that the file system tree is accessed via a
   // symbolic link. An atomic link swap is used when a new tree should be
   // switched to. This parameter specifies the path to the symbolic link. Envoy
   // will watch the location for changes and reload the file system tree when
-  // they happen. If this parameter is not set, there will be no disk based runtime.
+  // they happen. If this parameter is not set, there will be no disk based
+  // runtime.
   string symlink_root = 1;
 
   // Specifies the subdirectory to load within the root directory. This is
@@ -228,3 +235,51 @@ message Runtime {
   // encoding <config_runtime_proto_json>`.
   google.protobuf.Struct base = 4;
 }
+
+message RuntimeLayer {
+  // :ref:`Static runtime <config_runtime_bootstrap>` layer.
+  message StaticLayer {
+    // This follows the :ref:`runtime protobuf JSON representation encoding
+    // <config_runtime_proto_json>`.
+    google.protobuf.Struct value = 1;
+  }
+
+  // :ref:`Disk runtime <config_runtime_local_disk>` layer.
+  message DiskLayer {
+    // The implementation assumes that the file system tree is accessed via a
+    // symbolic link. An atomic link swap is used when a new tree should be
+    // switched to. This parameter specifies the path to the symbolic link.
+    // Envoy will watch the location for changes and reload the file system tree
+    // when they happen. See documentation on runtime :ref:`atomicity
+    // <config_runtime_atomicity>` for further details on how reloads are
+    // treated.
+    string symlink_root = 1;
+
+    // :ref:`Append <config_runtime_local_disk_service_cluster_subdirs>` the
+    // service cluster to the path under symlink root.
+    bool append_service_cluster = 2;
+  }
+
+  // :ref:`Admin console runtime <config_runtime_admin>` layer.
+  message AdminLayer {
+  }
+
+  // Descriptive name for the runtime layer. This is only used for the runtime
+  // :http:get:`/runtime` output.
+  string name = 1;
+
+  oneof layer_specifier {
+    // Unlike static xDS resources, this static layer is overridable by later
+    // layers in the runtime virtual filesystem.
+    google.protobuf.Struct static_layer = 2;
+    DiskLayer disk_layer = 3;
+    AdminLayer admin_layer = 4;
+  }
+}
+
+// Runtime :ref:`configuration overview <config_runtime>`.
+message LayeredRuntime {
+  // The :ref:`layers <config_runtime_layering>` of the runtime. This is ordered
+  // such that later layers in the list overlay earlier entries.
+  repeated RuntimeLayer layers = 1;
+}