operations: Implement rich errors for UserMergeBranch access checks

When `UserMergeBranch` fails during hook execution, then we will not
cause the RPC to fail but instead embed the error in the response's
`PreReceiveError` field. This has led to quite some problems: Praefect
cannot determine failing RPC calls, we need to do weird games where we
need to make sure to pass through Rails' error messages verbosely, and
most importantly the upstream caller cannot easily deduce what really
has failed without comparing returned error messages. As a result, this
pattern which we use across many of our user-facing RPCs is quite
lacking.

To fix this, we can use gRPC's rich error model. This error model allows
us to embed arbitrary proto messages into the returned error, which
grants us the ability to return structured errors to the caller. Like
this, we can embed various information into errors in a structured way
which can then be extracted by upstream callers simply by unmarshalling
those error details.

Convert UserMergeBranch to use this rich error model as a proof concept.
For now, rich errors only include details in case the access checks help
to keep this change as focussed as possible. If this new model proves
viable, we may extend this to also include additional information in
different error cases, like for example the set of conflicting files in
case a merge fails.

Changelog: changed

2 files changed, 26 insertions, 0 deletions

diff --git a/ruby/proto/gitaly/errors_pb.rb b/ruby/proto/gitaly/errors_pb.rb
new file mode 100644
index 000000000..e2c7a0ae8
--- /dev/null
+++ b/ruby/proto/gitaly/errors_pb.rb
@@ -0,0 +1,19 @@
+# Generated by the protocol buffer compiler.  DO NOT EDIT!
+# source: errors.proto
+
+require 'google/protobuf'
+
+Google::Protobuf::DescriptorPool.generated_pool.build do
+  add_file("errors.proto", :syntax => :proto3) do
+    add_message "gitaly.AccessCheckError" do
+      optional :error_message, :string, 1
+      optional :protocol, :string, 2
+      optional :user_id, :string, 3
+      optional :changes, :bytes, 4
+    end
+  end
+end
+
+module Gitaly
+  AccessCheckError = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.AccessCheckError").msgclass
+end
diff --git a/ruby/proto/gitaly/operations_pb.rb b/ruby/proto/gitaly/operations_pb.rb
index 9ee34cea9..24e6f9197 100644
--- a/ruby/proto/gitaly/operations_pb.rb
+++ b/ruby/proto/gitaly/operations_pb.rb
@@ -5,6 +5,7 @@ require 'google/protobuf'
 
 require 'lint_pb'
 require 'shared_pb'
+require 'errors_pb'
 require 'google/protobuf/timestamp_pb'
 Google::Protobuf::DescriptorPool.generated_pool.build do
   add_file("operations.proto", :syntax => :proto3) do
@@ -71,6 +72,11 @@ Google::Protobuf::DescriptorPool.generated_pool.build do
       optional :branch_update, :message, 3, "gitaly.OperationBranchUpdate"
       optional :pre_receive_error, :string, 4
     end
+    add_message "gitaly.UserMergeBranchError" do
+      oneof :error do
+        optional :access_check, :message, 1, "gitaly.AccessCheckError"
+      end
+    end
     add_message "gitaly.UserMergeToRefRequest" do
       optional :repository, :message, 1, "gitaly.Repository"
       optional :user, :message, 2, "gitaly.User"
@@ -277,6 +283,7 @@ module Gitaly
   UserCreateTagResponse = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserCreateTagResponse").msgclass
   UserMergeBranchRequest = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserMergeBranchRequest").msgclass
   UserMergeBranchResponse = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserMergeBranchResponse").msgclass
+  UserMergeBranchError = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserMergeBranchError").msgclass
   UserMergeToRefRequest = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserMergeToRefRequest").msgclass
   UserMergeToRefResponse = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.UserMergeToRefResponse").msgclass
   OperationBranchUpdate = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("gitaly.OperationBranchUpdate").msgclass