Add buffered get_env variants to os2

core/os/os2/env_windows.odin

@@ -4,7 +4,7 @@ package os2
 import win32 "core:sys/windows"
 import "base:runtime"
 
-_lookup_env :: proc(key: string, allocator: runtime.Allocator) -> (value: string, found: bool) {
+_lookup_env_alloc :: proc(key: string, allocator: runtime.Allocator) -> (value: string, found: bool) {
 	if key == "" {
 		return
 	}
@@ -36,6 +36,36 @@ _lookup_env :: proc(key: string, allocator: runtime.Allocator) -> (value: string
 	return
 }
 
+// This version of `lookup_env` doesn't allocate and instead requires the user to provide a buffer.
+// Note that it is limited to environment names and values of 512 utf-16 values each
+// due to the necessary utf-8 <> utf-16 conversion.
+@(require_results)
+_lookup_env_buf :: proc(buf: []u8, key: string) -> (value: string, err: Error) {
+	key_buf: [513]u16
+	wkey := win32.utf8_to_wstring(key_buf[:], key)
+	if wkey == nil {
+		return "", .Buffer_Full
+	}
+
+	n2 := win32.GetEnvironmentVariableW(wkey, nil, 0)
+	if n2 == 0 {
+		return "", .Env_Var_Not_Found
+	}
+
+	val_buf: [513]u16
+	n2 = win32.GetEnvironmentVariableW(wkey, raw_data(val_buf[:]), u32(len(val_buf[:])))
+	if n2 == 0 {
+		return "", .Env_Var_Not_Found
+	} else if int(n2) > len(buf) {
+		return "", .Buffer_Full
+	}
+
+	value = win32.utf16_to_utf8(buf, val_buf[:n2])
+
+	return value, nil
+}
+_lookup_env :: proc{_lookup_env_alloc, _lookup_env_buf}
+
 _set_env :: proc(key, value: string) -> Error {
 	temp_allocator := TEMP_ALLOCATOR_GUARD({})
 	k := win32_utf8_to_wstring(key,   temp_allocator) or_return