config: apply environment overrides before unmarshal

pkg/config/config.go

@@ -359,12 +359,13 @@ func (c *Config) Load(ctx context.Context) error {
 		}
 	}
 
+	c.mergeEnvVars()
+
 	err := c.FileConfig.Unmarshal(c)
 	if err != nil {
 		return err
 	}
 
-	c.mergeEnvVars()
 	return c.expandOSPathFlagValues()
 }
 

pkg/config/environment_test.go

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package config
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/pflag"
+)
+
+func TestLoadEnvironmentOverridesConfig(t *testing.T) {
+	t.Setenv("GNMIC_CLUSTER_NAME", "collector-cluster")
+	t.Setenv("GNMIC_CLUSTERING_LOCKER_ADDRESS", "consul.example:8500")
+
+	configFile := filepath.Join(t.TempDir(), "gnmic.yaml")
+	if err := os.WriteFile(
+		configFile,
+		[]byte(`clustering:
+  locker:
+    type: consul
+    address: file.example:8500
+`),
+		0o600,
+	); err != nil {
+		t.Fatalf("os.WriteFile() failed: %v", err)
+	}
+
+	cfg := New()
+	cfg.CfgFile = configFile
+
+	// Global flags are bound before Config.Load in the actual application.
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flags.String("cluster-name", "default-cluster", "")
+	if err := cfg.FileConfig.BindPFlag(
+		"cluster-name",
+		flags.Lookup("cluster-name"),
+	); err != nil {
+		t.Fatalf("BindPFlag() failed: %v", err)
+	}
+
+	if err := cfg.Load(context.Background()); err != nil {
+		t.Fatalf("Config.Load() failed: %v", err)
+	}
+
+	if got, want := cfg.ClusterName, "collector-cluster"; got != want {
+		t.Errorf("ClusterName = %q, want %q", got, want)
+	}
+
+	if cfg.Clustering == nil {
+		t.Fatal("Clustering is nil, want configuration populated from file and environment")
+	}
+
+	if got, want := cfg.Clustering.Locker["type"], "consul"; got != want {
+		t.Errorf("Clustering.Locker[type] = %v, want %q", got, want)
+	}
+
+	if got, want := cfg.Clustering.Locker["address"], "consul.example:8500"; got != want {
+		t.Errorf("Clustering.Locker[address] = %v, want %q", got, want)
+	}
+}