canonicalize Windows environment variables to uppercase

NEWS.md

@@ -29,7 +29,6 @@ New library functions
 Standard library changes
 ------------------------
 
-
 #### LinearAlgebra
 
 * Added keyword arguments `rtol`, `atol` to `pinv` and `nullspace` ([#29998]).
@@ -42,6 +41,10 @@ Standard library changes
 
 * Fixed `repr` such that it displays `DateTime` as it would be entered in Julia ([#30200]).
 
+#### Miscellaneous
+
+* Since environment variables on Windows are case-insensitive, `ENV` now converts its keys
+  to uppercase for display, iteration, and copying ([#30593]).
 
 External dependencies
 ---------------------

base/env.jl

@@ -70,6 +70,10 @@ struct EnvDict <: AbstractDict{String,String}; end
 
 Reference to the singleton `EnvDict`, providing a dictionary interface to system environment
 variables.
+
+(On Windows, system environment variables are case-insensitive, and `ENV` correspondingly converts
+all keys to uppercase for display, iteration, and copying.  Portable code should not rely on the
+existence of lower-case environment variables or on the ability to distinguish variables by case.)
 """
 const ENV = EnvDict()
 
@@ -85,6 +89,16 @@ push!(::EnvDict, kv::Pair{<:AbstractString}) = setindex!(ENV, kv.second, kv.firs
 
 if Sys.iswindows()
     GESW() = (pos = ccall(:GetEnvironmentStringsW,stdcall,Ptr{UInt16},()); (pos,pos))
+    function winuppercase(s::AbstractString)
+        isempty(s) && return s
+        LOCALE_INVARIANT = 0x0000007f
+        LCMAP_UPPERCASE  = 0x00000200
+        ws = transcode(UInt16, String(s))
+        result = ccall(:LCMapStringW, stdcall, Cint, (UInt32, UInt32, Ptr{UInt16}, Cint, Ptr{UInt16}, Cint),
+                       LOCALE_INVARIANT, LCMAP_UPPERCASE, ws, length(ws), ws, length(ws))
+        systemerror(:LCMapStringW, iszero(result))
+        return transcode(String, ws)
+    end
     function iterate(hash::EnvDict, block::Tuple{Ptr{UInt16},Ptr{UInt16}} = GESW())
         if unsafe_load(block[1]) == 0
             ccall(:FreeEnvironmentStringsW, stdcall, Int32, (Ptr{UInt16},), block[2])
@@ -100,7 +114,7 @@ if Sys.iswindows()
         if m === nothing
             error("malformed environment entry: $env")
         end
-        return (Pair{String,String}(m.captures[1], m.captures[2]), (pos+(len+1)*2, blk))
+        return (Pair{String,String}(winuppercase(m.captures[1]), m.captures[2]), (pos+(len+1)*2, blk))
     end
 else # !windows
     function iterate(::EnvDict, i=0)

test/env.jl

@@ -36,7 +36,7 @@ end
 end
 @testset "#17956" begin
     @test length(ENV) > 1
-    k1, k2 = "__test__", "__test1__"
+    k1, k2 = "__TEST__", "__TEST1__"
     withenv(k1=>k1, k2=>k2) do
         b_k1, b_k2 = false, false
         for (k, v) in ENV
@@ -81,3 +81,24 @@ end
     @test ENV["testing_envdict"] == "tested"
     delete!(ENV, "testing_envdict")
 end
+
+if Sys.iswindows()
+    @testset "windows case-insensitivity" begin
+        for k in ("testing_envdict", "testing_envdict_\u00ee")
+            K = uppercase(k)
+            v = "tested $k"
+            ENV[k] = v
+            @test haskey(ENV, K)
+            @test ENV[K] == v
+            @test K in keys(ENV)
+            @test K in collect(keys(ENV))
+            @test k ∉ collect(keys(ENV))
+            env = copy(ENV)
+            @test haskey(env, K)
+            @test env[K] == v
+            @test !haskey(env, k)
+            delete!(ENV, k)
+            @test !haskey(ENV, K)
+        end
+    end
+end