GetEnvVariable: return std::optional<std::string> (#1017)

- `GetEnvVariable` now returns `std::optional<std::string>` so callers can distinguish an unset variable from one set to an empty value.
- Windows path uses `SetLastError(ERROR_SUCCESS)` + `ERROR_ENVVAR_NOT_FOUND` to detect "not found"; POSIX path returns `nullopt` when `getenv` returns `nullptr`.
- All call sites migrated. Most use `.value_or("")` to preserve current empty-or-unset behavior. The diagnostic helpers in `zen-test/artifactprovider-tests.cpp` now report `<unset>` vs `<empty>` distinctly.
- Added a check in the `ExpandEnvironmentVariables` test confirming `nullopt` for an unset variable; PATH/HOME lookups in that test use `REQUIRE(has_value())` so a missing var fails cleanly instead of throwing `bad_optional_access`.

1 files changed, 30 insertions, 14 deletions

diff --git a/src/zencore/filesystem.cpp b/src/zencore/filesystem.cpp
index 281cb8e2e..e8ceac5c0 100644
--- a/src/zencore/filesystem.cpp
+++ b/src/zencore/filesystem.cpp
@@ -2936,41 +2936,52 @@ GetDirectoryContent(const std::filesystem::path& RootDir,
 	Visitor.AsyncVisitDirectory(WrapperVisitor.RelativeRoot, std::move(WrapperVisitor.Content));
 }
 
-std::string
+std::optional<std::string>
 GetEnvVariable(std::string_view VariableName)
 {
 	ZEN_ASSERT(!VariableName.empty());
 #if ZEN_PLATFORM_WINDOWS
-
+	const std::string NameStr(VariableName);
 	std::vector<CHAR> EnvVariableBuffer(1023 + 1);
-	DWORD RESULT = GetEnvironmentVariableA(std::string(VariableName).c_str(), EnvVariableBuffer.data(), (DWORD)EnvVariableBuffer.size());
+	SetLastError(ERROR_SUCCESS);
+	DWORD RESULT = GetEnvironmentVariableA(NameStr.c_str(), EnvVariableBuffer.data(), (DWORD)EnvVariableBuffer.size());
 	if (RESULT == 0)
 	{
-		return "";
+		if (GetLastError() == ERROR_ENVVAR_NOT_FOUND)
+		{
+			return std::nullopt;
+		}
+		return std::string{};
 	}
 	if (RESULT <= EnvVariableBuffer.size())
 	{
 		return std::string(EnvVariableBuffer.data(), size_t(RESULT));
 	}
 	EnvVariableBuffer.resize(size_t(RESULT));
-	RESULT = GetEnvironmentVariableA(std::string(VariableName).c_str(), EnvVariableBuffer.data(), (DWORD)EnvVariableBuffer.size());
+	SetLastError(ERROR_SUCCESS);
+	RESULT = GetEnvironmentVariableA(NameStr.c_str(), EnvVariableBuffer.data(), (DWORD)EnvVariableBuffer.size());
 	if (RESULT == 0)
 	{
-		return "";
+		if (GetLastError() == ERROR_ENVVAR_NOT_FOUND)
+		{
+			return std::nullopt;
+		}
+		return std::string{};
 	}
 	if (RESULT <= EnvVariableBuffer.size())
 	{
 		return std::string(EnvVariableBuffer.data(), size_t(RESULT));
 	}
+	return std::nullopt;
 #endif
 #if ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
-	char* EnvVariable = getenv(std::string(VariableName).c_str());
+	const char* EnvVariable = getenv(std::string(VariableName).c_str());
 	if (EnvVariable)
 	{
 		return std::string(EnvVariable);
 	}
+	return std::nullopt;
 #endif
-	return "";
 }
 
 std::string
@@ -2987,7 +2998,7 @@ ExpandEnvironmentVariables(std::string_view Input)
 			if (End != std::string_view::npos && End > i + 1)
 			{
 				std::string_view VarName = Input.substr(i + 1, End - i - 1);
-				std::string		 Value	 = GetEnvVariable(VarName);
+				std::string		 Value	 = GetEnvVariable(VarName).value_or("");
 				if (!Value.empty())
 				{
 					Result += Value;
@@ -4242,12 +4253,16 @@ TEST_CASE("ExpandEnvironmentVariables")
 	// Known variable
 #	if ZEN_PLATFORM_WINDOWS
 	// PATH is always set on Windows
-	std::string PathValue = GetEnvVariable("PATH");
+	std::optional<std::string> PathOpt = GetEnvVariable("PATH");
+	REQUIRE(PathOpt.has_value());
+	const std::string& PathValue = *PathOpt;
 	CHECK(!PathValue.empty());
 	CHECK_EQ(ExpandEnvironmentVariables("%PATH%"), PathValue);
 	CHECK_EQ(ExpandEnvironmentVariables("prefix/%PATH%/suffix"), "prefix/" + PathValue + "/suffix");
 #	else
-	std::string HomeValue = GetEnvVariable("HOME");
+	std::optional<std::string> HomeOpt = GetEnvVariable("HOME");
+	REQUIRE(HomeOpt.has_value());
+	const std::string& HomeValue = *HomeOpt;
 	CHECK(!HomeValue.empty());
 	CHECK_EQ(ExpandEnvironmentVariables("%HOME%"), HomeValue);
 	CHECK_EQ(ExpandEnvironmentVariables("prefix/%HOME%/suffix"), "prefix/" + HomeValue + "/suffix");
@@ -4255,13 +4270,14 @@ TEST_CASE("ExpandEnvironmentVariables")
 
 	// Unknown variable is left unexpanded
 	CHECK_EQ(ExpandEnvironmentVariables("%ZEN_UNLIKELY_SET_VAR_12345%"), "%ZEN_UNLIKELY_SET_VAR_12345%");
+	CHECK_FALSE(GetEnvVariable("ZEN_UNLIKELY_SET_VAR_12345").has_value());
 
 	// Multiple variables
 #	if ZEN_PLATFORM_WINDOWS
-	std::string OSValue = GetEnvVariable("OS");
-	if (!OSValue.empty())
+	std::optional<std::string> OSValue = GetEnvVariable("OS");
+	if (OSValue && !OSValue->empty())
 	{
-		CHECK_EQ(ExpandEnvironmentVariables("%PATH%/%OS%"), PathValue + "/" + OSValue);
+		CHECK_EQ(ExpandEnvironmentVariables("%PATH%/%OS%"), PathValue + "/" + *OSValue);
 	}
 #	endif
 }