don't directly expose blackboards to filters

BUILD

@@ -2043,6 +2043,7 @@ grpc_cc_library(
         "//src/core:arena_promise",
         "//src/core:atomic_utils",
         "//src/core:bitset",
+        "//src/core:blackboard",
         "//src/core:call_destination",
         "//src/core:call_filters",
         "//src/core:call_final_info",

CMakeLists.txt

@@ -5277,6 +5277,7 @@ add_library(grpc_authorization_provider
   src/core/ext/upb-gen/src/proto/grpc/gcp/altscontext.upb_minitable.c
   src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.c
   src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.c
+  src/core/filter/blackboard.cc
   src/core/handshaker/endpoint_info/endpoint_info_handshaker.cc
   src/core/handshaker/handshaker.cc
   src/core/handshaker/handshaker_registry.cc
@@ -9066,6 +9067,7 @@ add_executable(call_utils_test
   src/core/ext/upb-gen/src/proto/grpc/gcp/altscontext.upb_minitable.c
   src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.c
   src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.c
+  src/core/filter/blackboard.cc
   src/core/handshaker/handshaker_registry.cc
   src/core/handshaker/proxy_mapper_registry.cc
   src/core/lib/address_utils/parse_address.cc

build_autogenerated.yaml

@@ -4459,6 +4459,7 @@ libs:
   - src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.h
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb.h
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.h
+  - src/core/filter/blackboard.h
   - src/core/handshaker/endpoint_info/endpoint_info_handshaker.h
   - src/core/handshaker/handshaker.h
   - src/core/handshaker/handshaker_factory.h
@@ -4795,6 +4796,7 @@ libs:
   - src/core/ext/upb-gen/src/proto/grpc/gcp/altscontext.upb_minitable.c
   - src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.c
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.c
+  - src/core/filter/blackboard.cc
   - src/core/handshaker/endpoint_info/endpoint_info_handshaker.cc
   - src/core/handshaker/handshaker.cc
   - src/core/handshaker/handshaker_registry.cc
@@ -6779,6 +6781,7 @@ targets:
   - src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.h
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb.h
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.h
+  - src/core/filter/blackboard.h
   - src/core/handshaker/handshaker_factory.h
   - src/core/handshaker/handshaker_registry.h
   - src/core/handshaker/proxy_mapper.h
@@ -7084,6 +7087,7 @@ targets:
   - src/core/ext/upb-gen/src/proto/grpc/gcp/altscontext.upb_minitable.c
   - src/core/ext/upb-gen/src/proto/grpc/gcp/handshaker.upb_minitable.c
   - src/core/ext/upb-gen/src/proto/grpc/gcp/transport_security_common.upb_minitable.c
+  - src/core/filter/blackboard.cc
   - src/core/handshaker/handshaker_registry.cc
   - src/core/handshaker/proxy_mapper_registry.cc
   - src/core/lib/address_utils/parse_address.cc

src/core/ext/filters/gcp_authentication/gcp_authentication_filter.cc

@@ -176,19 +176,13 @@ GcpAuthenticationFilter::Create(const ChannelArgs& args,
         "gcp_auth: xds config not found in channel args");
   }
   // Get existing cache or create new one.
-  RefCountedPtr<CallCredentialsCache> cache;
-  if (filter_args.old_blackboard() != nullptr) {
-    cache = filter_args.old_blackboard()->Get<CallCredentialsCache>(
-        filter_config->filter_instance_name);
-    cache->SetMaxSize(filter_config->cache_size);
-  }
-  if (cache == nullptr) {
-    cache = MakeRefCounted<CallCredentialsCache>(filter_config->cache_size);
-  }
-  if (filter_args.new_blackboard() != nullptr) {
-    filter_args.new_blackboard()->Set(filter_config->filter_instance_name,
-                                      cache);
-  }
+  auto cache = filter_args.GetOrCreateState<CallCredentialsCache>(
+      filter_config->filter_instance_name, [&]() {
+        return MakeRefCounted<CallCredentialsCache>(filter_config->cache_size);
+      });
+  // Make sure size is updated, in case we're reusing a pre-existing
+  // cache but it has the wrong size.
+  cache->SetMaxSize(filter_config->cache_size);
   // Instantiate filter.
   return std::unique_ptr<GcpAuthenticationFilter>(new GcpAuthenticationFilter(
       filter_config, std::move(xds_config), std::move(cache)));

src/core/lib/channel/promise_based_filter.h

@@ -42,6 +42,7 @@
 #include <grpc/grpc.h>
 #include <grpc/support/port_platform.h>
 
+#include "src/core/filter/blackboard.h"
 #include "src/core/lib/channel/call_finalization.h"
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/channel/channel_fwd.h"
@@ -121,8 +122,20 @@ class ChannelFilter {
           [](const V3Based& v3) { return v3.instance_id; });
     }
 
-    const Blackboard* old_blackboard() const { return old_blackboard_; }
-    Blackboard* new_blackboard() const { return new_blackboard_; }
+    // If a filter state object of type T exists for key from a previous
+    // filter stack, retains it for the new filter stack we're constructing.
+    // Otherwise, invokes create_func() to create a new filter state
+    // object for the new filter stack.  Returns the new filter state object.
+    template <typename T>
+    RefCountedPtr<T> GetOrCreateState(
+        const std::string& key,
+        absl::FunctionRef<RefCountedPtr<T>()> create_func) {
+      RefCountedPtr<T> state;
+      if (old_blackboard_ != nullptr) state = old_blackboard_->Get<T>(key);
+      if (state == nullptr) state = create_func();
+      if (new_blackboard_ != nullptr) new_blackboard_->Set(key, state);
+      return state;
+    }
 
    private:
     friend class ChannelFilter;