Use the same empty string literal

[ashmaroli]

What problem is this PR intended to solve?

Avoidable extra memory allocation

While profiling a project to optimize memory usage, the following came to my notice:

Allocated String Report
-----------------------------------
    154375  ""
     88132  nokogiri-1.10.7/lib/nokogiri/xml/node.rb:767

which is:

nokogiri/lib/nokogiri/xml/node.rb

Line 767 in e6b3229

native_write_to(io, encoding, indent_text * indent_times, config.options)

The explanation is that 'a string of any length' * 0 == "".

String#* generates a new string even if string literals are frozen via the pragma.
Therefore, if the above line was executed (with indent_times == 0) a 100 times, there would be an allocation of 100 empty strings (irrespective of indent_text) .

This PR replaces this deterministic allocation with a single frozen string literal.
(Hope the native method doesn't mutate it).

Have you included adequate test coverage?

No change in behavior.

Does this change affect the C or the Java implementations?

N/A

[codeclimate]

Code Climate has analyzed commit 2e15d37 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 0.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 46.2%.

View more on Code Climate.

[flavorjones]

@ashmaroli Thanks for submitting this! The failing test is related to coverage diffs, and that's related to the config issue documented in #1965. This will be in v1.11.0!

[ashmaroli]

@flavorjones Glad to know that this will be included in v1.11.0. Thank you for informing :)

[ashmaroli]

Strange.. this didn't have an affect at all.. ☹️

Allocated String Report
-----------------------------------
    154390  ""
     88132  nokogiri-1.10.7/lib/nokogiri/xml/node.rb:767

nokogiri/lib/nokogiri/xml/node.rb

Line 767 in e6b3229

native_write_to(io, encoding, indent_text * indent_times, config.options)

---

Allocated String Report
-----------------------------------
    154393  ""
     88132  nokogiri-1.11.0.rc1/lib/nokogiri/xml/node.rb:780

nokogiri/lib/nokogiri/xml/node.rb

Line 780 in 8ce75d1

native_write_to(io, encoding, indentation, config.options)

[flavorjones]

@ashmaroli are you saying you didn't test it both ways before submitting the PR? or are you saying the change didn't make it into 1.11.0? i'm a little confused by your response.

[flavorjones]

OK - Please explain to me how you're profiling so I can either revert or fix.

[flavorjones]

I've opened a new issue, #1986, to investigate. @ashmaroli I'd love your help there.