fix(stream): avoid using designated initializer

[Abhra303]

Following the comment, a warning is popping up while building the stream_family.cc file.

[19/37] Building CXX object src/server/CMakeFiles/dragonfly_lib.dir/stream_family.cc.o
/home/roy/work/dragonfly/src/server/stream_family.cc: In function ‘facade::OpResult<streamID> dfly::{anonymous}::OpAdd(const dfly::OpArgs&, std::string_view, const dfly::{anonymous}::AddOpts&, facade::CmdArgList)’:
/home/roy/work/dragonfly/src/server/stream_family.cc:543:5: warning: missing initializer for member ‘streamAddTrimArgs::id’ [-Wmissing-field-initializers]
  543 |     };
      |     ^
/home/roy/work/dragonfly/src/server/stream_family.cc:543:5: warning: missing initializer for member ‘streamAddTrimArgs::id_given’ [-Wmissing-field-initializers]
/home/roy/work/dragonfly/src/server/stream_family.cc:543:5: warning: missing initializer for member ‘streamAddTrimArgs::seq_given’ [-Wmissing-field-initializers]
[...]

It is because a designated initializer is used to initialize a streamAddTrimArgs type variable. The -Wmissing-field-initializers throws a warning if all the struct fields are not initialized while using a designated initializer.

In our case, we don't need to set other fields as they are mainly used for redis's internal xadd implementation specific fields and are not used in streamTrim function.

[royjacobson]

This is still uninitialized, you should add this -

Suggested change

	streamAddTrimArgs add_args;
	streamAddTrimArgs add_args = {0};

C++ is a bit weird sometimes ;)

[Abhra303]

Ok, but I am unsure - doesn't C++ automatically set struct field to empty values?

[royjacobson]

It's complicated... to be technical, the trivial default constructor leaves integer members uninitialized. Adding '= {0}' makes it into a zero initialization so you get all zeros instead of random stack memory.

(obligatory link to https://twitter.com/i/status/1004017362381795329)

[Abhra303]

done :-)

[dranikpg]

By the way, designated initializers are not part of the standard (until 20) 🤓, but are supported by most compilers. I actually wanted to leave a comment on this case, but checked other places and saw that we use them from time to time.