[lua] Implement Sys.time, Sys.getEnv, and Timer.stamp for lua-vanilla mode

[jdonaldson]

Summary

• Sys.time() — use os.time() instead of throwing. Note: integer-second precision only (vs microsecond precision with luv). Open question: should we emit a compiler warning here?
• Sys.getEnv() — use os.getenv() instead of throwing
• Timer.stamp() — use os.clock() directly on all Lua targets (subsecond precision, avoids the Sys.time() path entirely)

These are the remaining Haxe-side changes needed for #12843 (hxcoro with -D lua-vanilla). The bit ops part was already resolved by #12599.

Open question

os.time() only returns integer seconds. Should we warn at compile time when Sys.time() is used in lua-vanilla mode, since users might expect subsecond precision? Or is it fine as-is since the docs don't guarantee precision?

Test plan

• CI passes on all targets
• Verify Timer.stamp() returns subsecond values on Lua
• Verify Sys.getEnv() works in lua-vanilla mode

[tobil4sk]

os.time() only returns integer seconds. Should we warn at compile time when Sys.time() is used in lua-vanilla mode, since users might expect subsecond precision? Or is it fine as-is since the docs don't guarantee precision?

I think we should use os.clock for both, unless we specify a behaviour that requires otherwise. That way haxe.Timer.stamp() can continue to call Sys.time().

EDIT: Lua docs say:

The os.clock function returns the number of seconds of CPU time for the program

We already use it for Sys.cpuTime(). So I guess the question is whether it is wrong for Sys.time() to give cpu time.

[jdonaldson]

I looked at what every other target does for Sys.time():

Target	Underlying call	Type	Precision
C++	QueryPerformanceCounter / mach_absolute_time / CLOCK_MONOTONIC	Monotonic	Nanosecond
JVM	System.currentTimeMillis() / 1000.0	Wall-clock (epoch)	Millisecond
Python	time.time()	Wall-clock (epoch)	Microsecond
PHP	microtime(true)	Wall-clock (epoch)	Microsecond
Neko	sys_time native	Wall-clock	Float
HL	sys_time native	Wall-clock	Float
Lua (luv)	luv.Misc.gettimeofday()	Wall-clock (epoch)	Microsecond
Lua (vanilla)	os.time()	Wall-clock (epoch)	Integer seconds

Every target returns wall-clock time (C++ is the outlier with monotonic). Using os.clock() for Sys.time() would make lua-vanilla the only target returning CPU time — which is semantically different. A sleeping/waiting program would show 0 elapsed time with os.clock(), while all other targets would show the actual wall time elapsed.

The precision is unfortunate, but vanilla Lua simply doesn't have a subsecond wall-clock API without external libraries. os.time() is the correct semantic match — it just has integer-second granularity.

For Timer.stamp() I think os.clock() is fine since it's already used for Sys.cpuTime() and the docs describe it as a monotonic timestamp for benchmarking.

[jdonaldson]

I remember not using os.time() for this reason. I think it might be ok for lua-vanilla, maybe with some sort of warning? I wasn't using it for anything critical, so I'm all ears on what the right tradeoff is here.