mir/agent: fix indefinite wait if fail to start prompt session

For some reason, Mir still provide an FD for a prompt session even if it
fails to start. When this FD is passed to the prompt provider, it will
just hang, causing the whole trust store user chain to just wait. To fix
this, make sure the agent checks for the error before continue.

Related: ubports/ubuntu-touch#1668

src/core/trust/mir/agent.cpp

@@ -56,6 +56,11 @@ mir::PromptSessionVirtualTable::PromptSessionVirtualTable()
 {
 }
 
+std::string mir::PromptSessionVirtualTable::error_message()
+{
+    return std::string(mir_prompt_session_error_message(prompt_session));
+}
+
 int mir::PromptSessionVirtualTable::new_fd_for_prompt_provider()
 {
     static const unsigned int fd_count = 1;
@@ -241,10 +246,19 @@ core::trust::Request::Answer mir::Agent::authenticate_request_with_parameters(co
                     parameters.application.pid,
                     Agent::on_trust_session_changed_state,
                     &cb_context),
-        // Acquire a new fd for the prompt provider.
-        scope.prompt_session->new_fd_for_prompt_provider()
+        /* fd */ -1
     };
 
+    auto error = scope.prompt_session->error_message();
+    if (!error.empty()) {
+        throw std::runtime_error{
+            "Unable to create a prompt session: " + error
+        };
+    }
+
+    // Acquire a new fd for the prompt provider.
+    scope.fd = scope.prompt_session->new_fd_for_prompt_provider();
+
     // And prepare the actual execution in a child process.
     mir::PromptProviderHelper::InvocationArguments args
     {

src/core/trust/mir/agent.h

@@ -64,6 +64,9 @@ class CORE_TRUST_DLL_PUBLIC PromptSessionVirtualTable
     PromptSessionVirtualTable(MirPromptSession* prompt_session);
     virtual ~PromptSessionVirtualTable() = default;
 
+    // Retrieve a text description of the last error.
+    virtual std::string error_message();
+
     // Requests a new, pre-authenticated fd for associating prompt providers.
     // Returns the fd or throws std::runtime_error.
     virtual int new_fd_for_prompt_provider();

tests/mir_agent_test.cpp

@@ -42,6 +42,9 @@ struct MockPromptSessionVirtualTable : public core::trust::mir::PromptSessionVir
     {
     }
 
+    // Retrieve a text description of the last error.
+    MOCK_METHOD0(error_message, std::string());
+
     // Requests a new, pre-authenticated fd for associating prompt providers.
     // Returns the fd or throws std::runtime_error.
     MOCK_METHOD0(new_fd_for_prompt_provider, int());
@@ -234,6 +237,9 @@ TEST(MirAgent, creates_prompt_session_and_execs_helper_with_preauthenticated_fd)
     ON_CALL(*connection_vtable, create_prompt_session_sync(_, _, _))
             .WillByDefault(Return(prompt_session_vtable));
 
+    ON_CALL(*prompt_session_vtable, error_message())
+            .WillByDefault(Return(std::string()));
+
     ON_CALL(*prompt_session_vtable, new_fd_for_prompt_provider())
             .WillByDefault(Return(pre_authenticated_fd));
 
@@ -305,6 +311,9 @@ TEST(MirAgent, calls_into_app_info_resolver_with_app_id)
     ON_CALL(*connection_vtable, create_prompt_session_sync(_, _, _))
             .WillByDefault(Return(prompt_session_vtable));
 
+    ON_CALL(*prompt_session_vtable, error_message())
+            .WillByDefault(Return(std::string()));
+
     ON_CALL(*prompt_session_vtable, new_fd_for_prompt_provider())
             .WillByDefault(Return(pre_authenticated_fd));
 
@@ -380,6 +389,9 @@ TEST(MirAgent, sig_kills_prompt_provider_process_on_status_change)
                         a_spinning_process,
                         core::posix::StandardStream::empty)));
 
+    ON_CALL(*prompt_session_vtable, error_message())
+            .WillByDefault(Return(std::string()));
+
     ON_CALL(*prompt_session_vtable, new_fd_for_prompt_provider())
             .WillByDefault(
                 Return(
@@ -445,6 +457,65 @@ TEST(MirAgent, sig_kills_prompt_provider_process_on_status_change)
         asynchronously_stop_the_prompting_session.join();
 }
 
+TEST(MirAgent, dont_exec_provider_when_unable_to_create_a_prompt_session)
+{
+    using namespace ::testing;
+
+    const core::trust::Pid app_pid {21};
+    const std::string app_icon{"/tmp"};
+    const std::string app_name {"Does not exist"};
+    const std::string app_id {"does.not.exist.application"};
+    const core::trust::Feature feature{42};
+    const std::string app_description {"This is just an extended description %1%"};
+    const int pre_authenticated_fd {42};
+
+    auto connection_vtable = a_mocked_connection_vtable();
+    auto prompt_session_vtable = a_mocked_prompt_session_vtable();
+
+    auto prompt_provider_exec_helper = a_mocked_prompt_provider_calling_bin_false();
+    auto app_info_resolver = a_mocked_app_info_resolver();
+
+    ON_CALL(*connection_vtable, create_prompt_session_sync(_, _, _))
+            .WillByDefault(Return(prompt_session_vtable));
+
+    ON_CALL(*prompt_session_vtable, error_message())
+            .WillByDefault(Return(std::string("Fabricated error.")));
+    ON_CALL(*prompt_session_vtable, new_fd_for_prompt_provider())
+            .WillByDefault(Return(pre_authenticated_fd));
+
+    ON_CALL(*app_info_resolver, resolve(_))
+            .WillByDefault(Return(core::trust::mir::AppInfo{app_icon, app_name, app_id}));
+
+    EXPECT_CALL(*connection_vtable, create_prompt_session_sync(app_pid, _, _)).Times(1);
+    EXPECT_CALL(*prompt_session_vtable, error_message()).Times(1);
+    // We have an error. New FD shouldn't be requested, and the prompt provider
+    // shouldn't be exec'ed, otherwise the provider can hang.
+    EXPECT_CALL(*prompt_session_vtable, new_fd_for_prompt_provider()).Times(0);
+    EXPECT_CALL(*prompt_provider_exec_helper,
+                exec_prompt_provider_with_arguments(_)).Times(0);
+
+    core::trust::mir::Agent agent
+    {
+        core::trust::mir::Agent::Configuration
+        {
+            connection_vtable,
+            prompt_provider_exec_helper,
+            core::trust::mir::Agent::translator_only_accepting_exit_status_success(),
+            app_info_resolver
+        }
+    };
+
+    EXPECT_THROW(agent.authenticate_request_with_parameters(
+                  core::trust::Agent::RequestParameters
+                  {
+                     core::trust::Uid{::getuid()},
+                     app_pid,
+                     app_id,
+                     feature,
+                     app_description
+                  }), std::runtime_error);
+}
+
 TEST(TrustPrompt, aborts_for_missing_title)
 {
     // And we pass in an empty argument vector