Use a constant empty tar header to avoid extra allocations

[segiddins]

No description provided.

[martinemde]

Nice.

[martinemde]

I approved it before double checking, here's the additional I think it needs.

[martinemde]

To really be identical you need the following. The default values are not the same as what an empty header outputs.

Suggested change

	EMPTY = new({ # :nodoc:
	mode: 0,
	name: "",
	prefix: "",
	size: 0,
	empty: true,
	}).freeze
	EMPTY = new({ # :nodoc:
	checksum: 0,
	gname: "",
	linkname: "",
	magic: "",
	mode: 0,
	name: "",
	prefix: "",
	size: 0,
	uname: "",
	version: 0,
	empty: true,
	}).freeze

Here's my irb doublecheck:

irb(main):033> EMPTY = Gem::Package::TarHeader.new({ checksum: 0, gname: "", linkname: "", magic: "", mode: 0, name: "", prefix: "", size: 0, uname: "", version: 0, empty: true,  })
=> #<Gem::Package::TarHeader:0x00000001203e60e0 @checksum=0, @devmajor=0, @devminor=0, @empty=true, @gid=0, @gname="", @linkname="", @magic="", @mode=0, @mtime=0, @name="", @prefix="", @size=0, @typeflag="0", @uid=0, @uname="", @version=0>
irb(main):034> Gem::Package::TarHeader.from(StringIO.new(Gem::Package::TarHeader::EMPTY_HEADER))
=> #<Gem::Package::TarHeader:0x00000001204cde90 @checksum=0, @devmajor=0, @devminor=0, @empty=true, @gid=0, @gname="", @linkname="", @magic="", @mode=0, @mtime=0, @name="", @prefix="", @size=0, @typeflag="0", @uid=0, @uname="", @version=0>

We should also add a test.

[simi]

🤔 wondering what's the impact of this change

[martinemde]

@simi are you thinking the tar header is expected to be mutable? That's worth considering...

[simi]

@simi are you thinking the tar header is expected to be mutable? That's worth considering...

No, I was just wondering what's memory footprint impact.

[deivid-rodriguez]

I also wonder about impact. Do we instantiate an empty tar reader once per gem, and this would reuse it for gem install invocations that install many gems?

Anyways, code changes look good to me!

[deivid-rodriguez]

This seems like a neat change regardless of the rest :)

[segiddins]

There is one empty tar header per tar file read, in my experience. each gem requires reading 2 tar files.

[deivid-rodriguez]

Should we squash into a single commit? Right now there's three commits, two of them with the same commit message, which seems a bit confusing! Other than that, seems great!