More "thread-safe" LazyString

[tkf]

This patch makes LazyString thread-safe in the sense that

julia/base/strings/lazy.jl

Lines 52 to 58 in 4b7b1fd

	## Safety properties for concurrent programs
	A lazy string itself does not introduce any concurrency problems even if it is printed in
	multiple Julia tasks. However, if `print` methods on a captured value can have a
	concurrency issue when invoked without synchronizations, printing the lazy string may cause
	an issue. Furthermore, the `print` methods on the captured values may be invoked multiple
	times.

It's probably a non-issue ATM but it's also very cheap to do this anyway. I just noticed it while writing #44935.

[vtjnash]

You appear to have the @atomic macros, so does this work?

Suggested change

	old = swapfield!(l, :str, str, :acquire_release)
	old = @atomicswap :acquire_release l.str = str

But it is not actually the function you seem to have meant to call, which seems to be:

Suggested change

	old = swapfield!(l, :str, str, :acquire_release)
	old, = @atomicreplace :acquire_release l.str nothing => str

[tkf]

Ah, yes, thanks, I needed a CAS there.

We can't use the macros for operations due to the missing length(::Array) method at this point. I juggled around the definitions just enough for @atomic field::T. Or is it better to define some array methods earlier?

[tkf]

Or is it better to define some array methods earlier?

Maybe this is simple enough: b8dd080

[vtjnash]

Should we add a serialize method also, which drops the data and just keeps the string:

serialize(s::AbstractSerializer, l::LazyStr) =
    serialize(s, _LazyStr((), string(l)))

[JeffBezanson]

Nice, thanks!!