Add helper for building consistent descriptor pool.

tools/BUILD

@@ -121,3 +121,32 @@ cc_test(
         "@com_google_protobuf//:protobuf",
     ],
 )
+
+cc_library(
+    name = "descriptor_pool_builder",
+    srcs = ["descriptor_pool_builder.cc"],
+    hdrs = ["descriptor_pool_builder.h"],
+    deps = [
+        "//common:minimal_descriptor_database",
+        "//internal:status_macros",
+        "@com_google_absl//absl/base:nullability",
+        "@com_google_absl//absl/container:flat_hash_set",
+        "@com_google_absl//absl/status",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/types:span",
+        "@com_google_protobuf//:protobuf",
+    ],
+)
+
+cc_test(
+    name = "descriptor_pool_builder_test",
+    srcs = ["descriptor_pool_builder_test.cc"],
+    deps = [
+        ":descriptor_pool_builder",
+        "//internal:testing",
+        "@com_google_absl//absl/status",
+        "@com_google_absl//absl/status:status_matchers",
+        "@com_google_cel_spec//proto/cel/expr/conformance/proto2:test_all_types_cc_proto",
+        "@com_google_protobuf//:protobuf",
+    ],
+)

tools/descriptor_pool_builder.cc

@@ -0,0 +1,111 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "tools/descriptor_pool_builder.h"
+
+#include <memory>
+#include <vector>
+
+#include "google/protobuf/descriptor.pb.h"
+#include "absl/base/nullability.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/status/status.h"
+#include "absl/strings/str_cat.h"
+#include "absl/types/span.h"
+#include "common/minimal_descriptor_database.h"
+#include "internal/status_macros.h"
+#include "google/protobuf/descriptor.h"
+
+namespace cel {
+
+namespace {
+
+absl::Status FindDeps(
+    std::vector<const google::protobuf::FileDescriptor*>& to_resolve,
+    absl::flat_hash_set<const google::protobuf::FileDescriptor*>& resolved,
+    DescriptorPoolBuilder& builder) {
+  while (!to_resolve.empty()) {
+    const auto* file = to_resolve.back();
+    to_resolve.pop_back();
+    if (resolved.contains(file)) {
+      continue;
+    }
+    google::protobuf::FileDescriptorProto file_proto;
+    file->CopyTo(&file_proto);
+    // Note: order doesn't matter here as long as all the cross references are
+    // correct in the final database.
+    CEL_RETURN_IF_ERROR(builder.AddFileDescriptor(file_proto));
+    resolved.insert(file);
+    for (int i = 0; i < file->dependency_count(); ++i) {
+      to_resolve.push_back(file->dependency(i));
+    }
+  }
+  return absl::OkStatus();
+}
+
+}  // namespace
+
+DescriptorPoolBuilder::StateHolder::StateHolder(
+    google::protobuf::DescriptorDatabase* base)
+    : base(base), merged(base, &extensions), pool(&merged) {}
+
+DescriptorPoolBuilder::DescriptorPoolBuilder()
+    : state_(std::make_shared<DescriptorPoolBuilder::StateHolder>(
+          cel::GetMinimalDescriptorDatabase())) {}
+
+std::shared_ptr<const google::protobuf::DescriptorPool>
+DescriptorPoolBuilder::Build() && {
+  auto alias =
+      std::shared_ptr<const google::protobuf::DescriptorPool>(state_, &state_->pool);
+  state_.reset();
+  return alias;
+}
+
+absl::Status DescriptorPoolBuilder::AddTransitiveDescriptorSet(
+    absl::Nonnull<const google::protobuf::Descriptor*> desc) {
+  absl::flat_hash_set<const google::protobuf::FileDescriptor*> resolved;
+  std::vector<const google::protobuf::FileDescriptor*> to_resolve{desc->file()};
+  return FindDeps(to_resolve, resolved, *this);
+}
+
+absl::Status DescriptorPoolBuilder::AddTransitiveDescriptorSet(
+    absl::Span<absl::Nonnull<const google::protobuf::Descriptor*>> descs) {
+  absl::flat_hash_set<const google::protobuf::FileDescriptor*> resolved;
+  std::vector<absl::Nonnull<const google::protobuf::FileDescriptor*>> to_resolve;
+  to_resolve.reserve(descs.size());
+  for (const google::protobuf::Descriptor* desc : descs) {
+    to_resolve.push_back(desc->file());
+  }
+
+  return FindDeps(to_resolve, resolved, *this);
+}
+
+absl::Status DescriptorPoolBuilder::AddFileDescriptor(
+    const google::protobuf::FileDescriptorProto& file) {
+  if (!state_->extensions.Add(file)) {
+    return absl::InvalidArgumentError(
+        absl::StrCat("proto descriptor conflict: ", file.name()));
+  }
+  return absl::OkStatus();
+}
+
+absl::Status DescriptorPoolBuilder::AddFileDescriptorSet(
+    const google::protobuf::FileDescriptorSet& file) {
+  for (const auto& file : file.file()) {
+    CEL_RETURN_IF_ERROR(AddFileDescriptor(file));
+  }
+  return absl::OkStatus();
+}
+
+}  // namespace cel

tools/descriptor_pool_builder.h

@@ -0,0 +1,93 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef THIRD_PARTY_CEL_CPP_TOOLS_DESCRIPTOR_POOL_BUILDER_H_
+#define THIRD_PARTY_CEL_CPP_TOOLS_DESCRIPTOR_POOL_BUILDER_H_
+
+#include <memory>
+#include <utility>
+
+#include "google/protobuf/descriptor.pb.h"
+#include "absl/base/nullability.h"
+#include "absl/status/status.h"
+#include "absl/types/span.h"
+#include "google/protobuf/descriptor.h"
+#include "google/protobuf/descriptor_database.h"
+
+namespace cel {
+
+// A helper class for building a descriptor pool from a set proto file
+// descriptors. Manages lifetime for the descriptor databases backing
+// the pool.
+//
+// Client must ensure that types are not added multiple times.
+//
+// Note: in the constructed pool, the definitions for the required types for
+// CEL will shadow any added to the builder. Clients should not modify types
+// from the google.protobuf package in general, but if they do the behavior of
+// the constructed descriptor pool will be inconsistent.
+class DescriptorPoolBuilder {
+ public:
+  DescriptorPoolBuilder();
+
+  DescriptorPoolBuilder& operator=(const DescriptorPoolBuilder&) = delete;
+  DescriptorPoolBuilder(const DescriptorPoolBuilder&) = delete;
+  DescriptorPoolBuilder& operator=(const DescriptorPoolBuilder&&) = delete;
+  DescriptorPoolBuilder(DescriptorPoolBuilder&&) = delete;
+
+  ~DescriptorPoolBuilder() = default;
+
+  // Returns a shared pointer to the new descriptor pool that manages the
+  // underlying descriptor databases backing the pool.
+  //
+  // Consumes the builder instance. It is unsafe to make any further changes
+  // to the descriptor databases after accessing the pool.
+  std::shared_ptr<const google::protobuf::DescriptorPool> Build() &&;
+
+  // Utility for adding the transitive dependencies of a message with a linked
+  // descriptor.
+  absl::Status AddTransitiveDescriptorSet(
+      absl::Nonnull<const google::protobuf::Descriptor*> desc);
+
+  absl::Status AddTransitiveDescriptorSet(
+      absl::Span<absl::Nonnull<const google::protobuf::Descriptor*>>);
+
+  // Adds a file descriptor set to the pool. Client must ensure that all
+  // dependencies are satisfied and that files are not added multiple times.
+  absl::Status AddFileDescriptorSet(const google::protobuf::FileDescriptorSet& files);
+
+  // Adds a single proto file descriptor set to the pool. Client must ensure
+  // that all dependencies are satisfied and that files are not added multiple
+  // times.
+  absl::Status AddFileDescriptor(const google::protobuf::FileDescriptorProto& file);
+
+ private:
+  struct StateHolder {
+    explicit StateHolder(google::protobuf::DescriptorDatabase* base);
+
+    google::protobuf::DescriptorDatabase* base;
+    google::protobuf::SimpleDescriptorDatabase extensions;
+    google::protobuf::MergedDescriptorDatabase merged;
+    google::protobuf::DescriptorPool pool;
+  };
+
+  explicit DescriptorPoolBuilder(std::shared_ptr<StateHolder> state)
+      : state_(std::move(state)) {}
+
+  std::shared_ptr<StateHolder> state_;
+};
+
+}  // namespace cel
+
+#endif  // THIRD_PARTY_CEL_CPP_TOOLS_DESCRIPTOR_POOL_BUILDER_H_