fix: fix segmentation fault due to the use of an invalid grpc server_ object.

BuildAndStart() can fail for any number of reasons (e.g. when passed a
port reserved for privileged use or a non-existent local address). We
were not checking for this and would try to use the server
unconditionally, causing a segmentation fault (e.g. when emulator was
called by a non-root user as: ./emulator -p 100).

This adds a fix so that we can check that we have a valid server
object before trying to use it.

Also added a unit test for the failing path.

---------

Signed-off-by: Brian Gitonga Marete <bgmarete@gmail.com>

Author: marete

google/cloud/bigtable/emulator/emulator.cc

@@ -33,8 +33,17 @@ int main(int argc, char* argv[]) {
       absl::StrCat("Usage: %s -h <host> -p <port>", argv[0]));
   absl::ParseCommandLine(argc, argv);
 
-  auto server = google::cloud::bigtable::emulator::CreateDefaultEmulatorServer(
-      absl::GetFlag(FLAGS_host), absl::GetFlag(FLAGS_port));
+  auto maybe_server =
+      google::cloud::bigtable::emulator::CreateDefaultEmulatorServer(
+          absl::GetFlag(FLAGS_host), absl::GetFlag(FLAGS_port));
+  if (!maybe_server) {
+    std::cerr << "CreateDefaultEmulatorServer() failed. See logs for "
+                 "possible reason"
+              << std::endl;
+    return 1;
+  }
+
+  auto& server = maybe_server.value();
 
   std::cout << "Server running on port " << server->bound_port() << "\n";
   server->Wait();

google/cloud/bigtable/emulator/server.cc

@@ -333,6 +333,7 @@ class DefaultEmulatorServer : public EmulatorServer {
   int bound_port() override { return bound_port_; }
   void Shutdown() override { server_->Shutdown(); }
   void Wait() override { server_->Wait(); }
+  bool HasValidServer() { return static_cast<bool>(server_); }
 
  private:
   int bound_port_;
@@ -343,9 +344,17 @@ class DefaultEmulatorServer : public EmulatorServer {
   std::unique_ptr<grpc::Server> server_;
 };
 
-std::unique_ptr<EmulatorServer> CreateDefaultEmulatorServer(
+StatusOr<std::unique_ptr<EmulatorServer>> CreateDefaultEmulatorServer(
     std::string const& host, std::uint16_t port) {
-  return std::unique_ptr<EmulatorServer>(new DefaultEmulatorServer(host, port));
+  auto* default_emulator_server = new DefaultEmulatorServer(host, port);
+  if (!default_emulator_server->HasValidServer()) {
+    return UnknownError("An unknown error occurred when starting server",
+                        GCP_ERROR_INFO()
+                            .WithMetadata("host", host)
+                            .WithMetadata("port", absl::StrCat("%d", port)));
+  }
+
+  return std::unique_ptr<EmulatorServer>(default_emulator_server);
 }
 
 }  // namespace emulator

google/cloud/bigtable/emulator/server.h

@@ -15,6 +15,7 @@
 #ifndef GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_BIGTABLE_EMULATOR_SERVER_H
 #define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_BIGTABLE_EMULATOR_SERVER_H
 
+#include "google/cloud/status_or.h"
 #include <cstdint>
 #include <memory>
 #include <string>
@@ -36,7 +37,7 @@ class EmulatorServer {
   virtual void Wait() = 0;
 };
 
-std::unique_ptr<EmulatorServer> CreateDefaultEmulatorServer(
+StatusOr<std::unique_ptr<EmulatorServer>> CreateDefaultEmulatorServer(
     std::string const& host, std::uint16_t port);
 
 }  // namespace emulator

google/cloud/bigtable/emulator/server_test.cc

@@ -13,13 +13,15 @@
 // limitations under the License.
 
 #include "google/cloud/bigtable/emulator/server.h"
+#include "google/cloud/testing_util/status_matchers.h"
 #include <google/bigtable/admin/v2/bigtable_table_admin.grpc.pb.h>
 #include <google/bigtable/admin/v2/bigtable_table_admin.pb.h>
 #include <google/bigtable/admin/v2/table.pb.h>
 #include <google/bigtable/v2/bigtable.grpc.pb.h>
 #include <google/bigtable/v2/bigtable.pb.h>
 #include <gmock/gmock.h>
 #include <grpcpp/grpcpp.h>
+#include <gtest/gtest.h>
 
 namespace google {
 namespace cloud {
@@ -33,7 +35,9 @@ class ServerTest : public ::testing::Test {
   grpc::ClientContext ctx_;
 
   void SetUp() override {
-    server_ = CreateDefaultEmulatorServer("127.0.0.1", 0);
+    auto maybe_server = CreateDefaultEmulatorServer("127.0.0.1", 0);
+    ASSERT_STATUS_OK(maybe_server);
+    server_ = std::move(maybe_server.value());
     channel_ = grpc::CreateChannel(
         "localhost:" + std::to_string(server_->bound_port()),
         grpc::InsecureChannelCredentials());
@@ -215,6 +219,12 @@ TEST_F(ServerTest, TableAdminUpdateTable) {
   EXPECT_NE(status.error_code(), grpc::StatusCode::UNIMPLEMENTED);
 }
 
+// Test that the failure path for server creation does not crash.
+TEST(ServerCreationTest, TestServerCreationFailurePath) {
+  auto maybe_server = CreateDefaultEmulatorServer("invalid_host_address", 0);
+  ASSERT_EQ(false, maybe_server.ok());
+}
+
 }  // namespace emulator
 }  // namespace bigtable
 }  // namespace cloud