[LB policy API] change metadata mutation API to handle discarded picks (

grpc#36968) Previously, metadata mutations were made by the picker directly, which meant that they would be applied even if the channel winds up discarding the pick due to the returned subchannel having been disconnected by the time the pick result is returned. This changes the API such that pickers return metadata mutations along with the pick result, so that the mutations won't get applied unless the pick result is actually used. Closes grpc#36968 COPYBARA_INTEGRATE_REVIEW=grpc#36968 from markdroth:lb_metadata_api 2765da6 PiperOrigin-RevId: 645451869
sourabhsinghs · Sep 26, 2024 · f1d602f · f1d602f
1 parent c3a1279
commit f1d602f
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Showing 40 changed files with 355 additions and 259 deletions.
diff --git a/BUILD b/BUILD
@@ -3818,6 +3818,7 @@ grpc_cc_library(
         "//src/core:iomgr_fwd",
         "//src/core:json",
         "//src/core:latch",
+        "//src/core:lb_metadata",
         "//src/core:lb_policy",
         "//src/core:lb_policy_registry",
         "//src/core:loop",

diff --git a/CMakeLists.txt b/CMakeLists.txt
diff --git a/Makefile b/Makefile
diff --git a/Package.swift b/Package.swift
diff --git a/build_autogenerated.yaml b/build_autogenerated.yaml
diff --git a/config.m4 b/config.m4
diff --git a/config.w32 b/config.w32
diff --git a/gRPC-C++.podspec b/gRPC-C++.podspec
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
diff --git a/grpc.gemspec b/grpc.gemspec
diff --git a/package.xml b/package.xml
diff --git a/src/core/BUILD b/src/core/BUILD
@@ -3456,6 +3456,7 @@ grpc_cc_library(
     hdrs = ["load_balancing/lb_policy.h"],
     external_deps = [
         "absl/base:core_headers",
+        "absl/container:inlined_vector",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",
@@ -3525,6 +3526,21 @@ grpc_cc_library(
     ],
 )
 
+grpc_cc_library(
+    name = "lb_metadata",
+    srcs = ["client_channel/lb_metadata.cc"],
+    hdrs = ["client_channel/lb_metadata.h"],
+    external_deps = [
+        "absl/log:log",
+        "absl/strings",
+        "absl/types:optional",
+    ],
+    deps = [
+        "lb_policy",
+        "metadata_batch",
+    ],
+)
+
 grpc_cc_library(
     name = "subchannel_interface",
     hdrs = ["load_balancing/subchannel_interface.h"],
@@ -5009,6 +5025,7 @@ grpc_cc_library(
         "slice_refcount",
         "status_helper",
         "time",
+        "upb_utils",
         "uuid_v4",
         "validation_errors",
         "//:backoff",
@@ -5039,14 +5056,13 @@ grpc_cc_library(
 grpc_cc_library(
     name = "upb_utils",
     hdrs = [
-        "xds/grpc/upb_utils.h",
+        "util/upb_utils.h",
     ],
     external_deps = [
         "absl/strings",
         "@com_google_protobuf//upb:base",
     ],
     language = "c++",
-    deps = ["//:gpr_platform"],
 )
 
 grpc_cc_library(