protobuf | custom protoc generator to make

How about using TaggedOutput.

MySQL stored procedures do have all the necessary programming constructs to do this, so theoretically it is possible. However, it will involve quite a bit of programming and is probably not worth doing on database side.

However, here is the rough sketch of what the stored procedures would have to do. Refer to Protobuf encoding documentation for details.

1. Create a stored procedure read_varint() that will read the first varint from a blob, and returns it as integer and the remaining blob.
2. Create a stored procedure decode_point() that repeatedly calls read_varint() to take a tag, then calls read_varint() to read field data and assigns it to variable based on tag number.
3. Create similar decode_timestamp() for the built-in google.protobuf.Timestamp datatype and call it from decode_point() for the timestamp field.
4. Create a stored procedure decode_entry() that repeatedly calls read_varint() to take a tag, and either decodes time directly or calls decode_point(), depending on the tag value.
5. Extend all these procedures to skip fields with unknown tag numbers.

Interestingly, your message types only have varint fields. So at the bare minimum, you could just call read_varint() repeatedly to get all the varints to a list. Then each field is always at a specific index. However, I don't recommend this, because it wouldn't be possible to extend it to handle future fields of different types.

As per Dominik Ficek's comment

In this case the error is because the string is invalid.
And it is invalid because the value (string) returned by CURL includes all the data from the HTTP request, for example:

The protoc generation should have produced a method called GetMetadata for both types. You can check if the incoming message implements an interface using a type assertion (see the tour of go for more details), then call that method to get the metadata.

This is what I meant with this comment

though boost::system::system_error could invite issues back

The trouble is, error categories are global singleton instances, with object identity (i.e. compared for equality by their address).

You'r ending up with multiple instances in multiple modules. The usual solution is to

• dynamically link to Boost System, so all libraries use the same copy (this does however sometimes run into initialization order issues)
• if that doesn't solve it, make sure all modules actually load the same library (version?)
• In recent boosts I think there's the option to build Boost System completely header-only. This may or may not involve new-fangled C++14 inline, I haven't checked.

If all else fails, do your own translation. There's related issues that might give you ideas:

• Is it possible to convert a boost::system::error_code to a std:error_code?
• Do note that exceptions may run into very similar issues due to RTTI not necessarily matching; exceptions need to be exported types and linked dynamically from the same module for the exception handlers to match the thrown runtime type id.

Is it normal or expected to compare only errorCode.value() against enums?

No it is not. According to some sources Boost as well as standard library promise to map generic error category to errc which is standard - so you could do that, but you still have to figure out whether that is the category, so doesn't help your scenario.

Ok I solved the issue.

I had installed protoc via snap and the stable channel had version 3.11.4

Now I upgraded to 3.14.0 and everything is working well.

You either need to disable the dependencyConvergence check in the POM, or you need to add an entry to the section of your POM which contains the version of error_prone_annotations that you want to use.

Change

google.protobuf.TimeStamp last_updated = 3;

to

google.protobuf.Timestamp last_updated = 3;