Memoize cwstring when used for env lookup / modification on Windows (#51371)

~This is just me proposing a suggestion from @KristofferC in
https://discourse.julialang.org/t/debug-has-massive-performance-impact/103974/22,
it's all his code / idea, not mine.~

This PR makes it so that on windows, we pre-allocate an `IdDict` and
every time someone looks up environment variables (motivating example
here is `@debug` statements), we store the result of
`cwstring(::String)` in that `IdDict` so that it doesn't need to be
re-computed for future uses.

The idea behind this is that people have observed that [using `@debug`
is significantly more costly on Windows than other
platforms](https://discourse.julialang.org/t/debug-has-massive-performance-impact/103974),
even though we have documented in that manual that it should be a really
cheap operation. @KristofferC suggests this is due to the fact that
[checking environment variables in Windows is more
costly](https://discourse.julialang.org/t/debug-has-massive-performance-impact/103974/18).

~The idea here is that we preallocate a `Cwstring` on Windows that just
holds the text `"JULIA_DEBUG"`, so that if `access_env(f,
"JULIA_DEBUG")` gets called, we don't need to create a new `Cwstring`
and then throw it away each time.~

---------

Co-authored-by: Ian Butterworth <i.r.butterworth@gmail.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: 3 people

base/env.jl

@@ -3,12 +3,29 @@
 if Sys.iswindows()
     const ERROR_ENVVAR_NOT_FOUND = UInt32(203)
 
+    const env_dict = IdDict{String, Vector{Cwchar_t}}()
+    const env_lock = ReentrantLock()
+
+    function memoized_env_lookup(str::AbstractString)
+        # Windows environment variables have a different format from Linux / MacOS, and previously
+        # incurred allocations because we had to convert a String to a Vector{Cwchar_t} each time
+        # an environment variable was looked up. This function memoizes that lookup process, storing
+        # the String => Vector{Cwchar_t} pairs in env_dict
+        var = get(env_dict, str, nothing)
+        if isnothing(var)
+            var = @lock env_lock begin
+                env_dict[str] = cwstring(str)
+            end
+        end
+        var
+    end
+
     _getenvlen(var::Vector{UInt16}) = ccall(:GetEnvironmentVariableW,stdcall,UInt32,(Ptr{UInt16},Ptr{UInt16},UInt32),var,C_NULL,0)
     _hasenv(s::Vector{UInt16}) = _getenvlen(s) != 0 || Libc.GetLastError() != ERROR_ENVVAR_NOT_FOUND
-    _hasenv(s::AbstractString) = _hasenv(cwstring(s))
+    _hasenv(s::AbstractString) = _hasenv(memoized_env_lookup(s))
 
     function access_env(onError::Function, str::AbstractString)
-        var = cwstring(str)
+        var = memoized_env_lookup(str)
         len = _getenvlen(var)
         if len == 0
             return Libc.GetLastError() != ERROR_ENVVAR_NOT_FOUND ? "" : onError(str)
@@ -21,7 +38,7 @@ if Sys.iswindows()
     end
 
     function _setenv(svar::AbstractString, sval::AbstractString, overwrite::Bool=true)
-        var = cwstring(svar)
+        var = memoized_env_lookup(svar)
         val = cwstring(sval)
         if overwrite || !_hasenv(var)
             ret = ccall(:SetEnvironmentVariableW,stdcall,Int32,(Ptr{UInt16},Ptr{UInt16}),var,val)
@@ -30,7 +47,7 @@ if Sys.iswindows()
     end
 
     function _unsetenv(svar::AbstractString)
-        var = cwstring(svar)
+        var = memoized_env_lookup(svar)
         ret = ccall(:SetEnvironmentVariableW,stdcall,Int32,(Ptr{UInt16},Ptr{UInt16}),var,C_NULL)
         windowserror(:setenv, ret == 0 && Libc.GetLastError() != ERROR_ENVVAR_NOT_FOUND)
     end