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, 20 insertions, 20 deletions

diff --git a/src/zenutil/testartifactprovider.cpp b/src/zenutil/testartifactprovider.cpp
index 666a1758d..45b29d497 100644
--- a/src/zenutil/testartifactprovider.cpp
+++ b/src/zenutil/testartifactprovider.cpp
@@ -321,35 +321,35 @@ namespace {
 #if ZEN_PLATFORM_MAC
 		return true;
 #else
-		std::string Disabled = GetEnvVariable("AWS_EC2_METADATA_DISABLED");
+		std::string Disabled = GetEnvVariable("AWS_EC2_METADATA_DISABLED").value_or("");
 		return Disabled == "true" || Disabled == "TRUE" || Disabled == "1";
 #endif
 	}
 
 	void ApplyAwsEnvDefaults(S3ClientOptions& Client)
 	{
-		if (std::string Region = GetEnvVariable("AWS_DEFAULT_REGION"); !Region.empty())
+		if (std::optional<std::string> Region = GetEnvVariable("AWS_DEFAULT_REGION"); Region && !Region->empty())
 		{
-			Client.Region = std::move(Region);
+			Client.Region = std::move(*Region);
 		}
-		else if (std::string FallbackRegion = GetEnvVariable("AWS_REGION"); !FallbackRegion.empty())
+		else if (std::optional<std::string> FallbackRegion = GetEnvVariable("AWS_REGION"); FallbackRegion && !FallbackRegion->empty())
 		{
-			Client.Region = std::move(FallbackRegion);
+			Client.Region = std::move(*FallbackRegion);
 		}
 
 		if (Client.Endpoint.empty())
 		{
-			Client.Endpoint = GetEnvVariable("AWS_ENDPOINT_URL");
+			Client.Endpoint = GetEnvVariable("AWS_ENDPOINT_URL").value_or("");
 		}
 
 		if (Client.Credentials.AccessKeyId.empty() && !Client.CredentialProvider)
 		{
-			std::string AccessKeyId = GetEnvVariable("AWS_ACCESS_KEY_ID");
-			if (!AccessKeyId.empty())
+			std::optional<std::string> AccessKeyId = GetEnvVariable("AWS_ACCESS_KEY_ID");
+			if (AccessKeyId && !AccessKeyId->empty())
 			{
-				Client.Credentials.AccessKeyId	   = std::move(AccessKeyId);
-				Client.Credentials.SecretAccessKey = GetEnvVariable("AWS_SECRET_ACCESS_KEY");
-				Client.Credentials.SessionToken	   = GetEnvVariable("AWS_SESSION_TOKEN");
+				Client.Credentials.AccessKeyId	   = std::move(*AccessKeyId);
+				Client.Credentials.SecretAccessKey = GetEnvVariable("AWS_SECRET_ACCESS_KEY").value_or("");
+				Client.Credentials.SessionToken	   = GetEnvVariable("AWS_SESSION_TOKEN").value_or("");
 			}
 			else if (!IsImdsDisabled())
 			{
@@ -376,11 +376,11 @@ GetDefaultLocalTestArtifactPath()
 bool
 S3TestArtifactsAvailable()
 {
-	if (GetEnvVariable(kTestArtifactsS3EnvVar).empty())
+	if (GetEnvVariable(kTestArtifactsS3EnvVar).value_or("").empty())
 	{
 		return false;
 	}
-	if (!GetEnvVariable("AWS_ACCESS_KEY_ID").empty() || !GetEnvVariable("AWS_SESSION_TOKEN").empty())
+	if (!GetEnvVariable("AWS_ACCESS_KEY_ID").value_or("").empty() || !GetEnvVariable("AWS_SESSION_TOKEN").value_or("").empty())
 	{
 		return true;
 	}
@@ -390,7 +390,7 @@ S3TestArtifactsAvailable()
 bool
 TestArtifactsAvailable()
 {
-	if (!GetEnvVariable(kTestArtifactsPathEnvVar).empty())
+	if (!GetEnvVariable(kTestArtifactsPathEnvVar).value_or("").empty())
 	{
 		return true;
 	}
@@ -402,10 +402,10 @@ CreateTestArtifactProvider(TestArtifactProviderOptions Options)
 {
 	if (Options.CacheDir.empty())
 	{
-		std::string EnvValue = GetEnvVariable(kTestArtifactsPathEnvVar);
-		if (!EnvValue.empty())
+		std::optional<std::string> EnvValue = GetEnvVariable(kTestArtifactsPathEnvVar);
+		if (EnvValue && !EnvValue->empty())
 		{
-			Options.CacheDir = std::filesystem::path(EnvValue);
+			Options.CacheDir = std::filesystem::path(*EnvValue);
 		}
 	}
 	if (Options.CacheDir.empty())
@@ -419,10 +419,10 @@ CreateTestArtifactProvider(TestArtifactProviderOptions Options)
 
 	if (Options.S3Client.BucketName.empty() || Options.S3KeyPrefix.empty())
 	{
-		std::string EnvValue = GetEnvVariable(kTestArtifactsS3EnvVar);
-		if (!EnvValue.empty())
+		std::optional<std::string> EnvValue = GetEnvVariable(kTestArtifactsS3EnvVar);
+		if (EnvValue && !EnvValue->empty())
 		{
-			ApplyS3UrlToOptions(EnvValue, Options.S3Client, Options.S3KeyPrefix);
+			ApplyS3UrlToOptions(*EnvValue, Options.S3Client, Options.S3KeyPrefix);
 		}
 	}