config: apply environment overrides before unmarshal

pkg/config/config.go

@@ -361,12 +361,13 @@ 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/config_test.go

@@ -766,15 +766,10 @@ func TestValidateSetInput(t *testing.T) {
 }
 
 func TestEnvironmentHelpers(t *testing.T) {
-	t.Setenv("GNMIC_API_SERVER_ADDRESS", ":9999")
 	t.Setenv("GNMIC_OUTPUTS_FILE_TYPE", "file")
 	t.Setenv("OTHER_VAR", "ignored")
 
 	got := envToMap()
-	apiServer := got["api"].(map[string]any)["server"].(map[string]any)
-	if apiServer["address"] != ":9999" {
-		t.Fatalf("envToMap api-server address = %#v", apiServer)
-	}
 	outputs := got["outputs"].(map[string]any)["file"].(map[string]any)
 	if outputs["type"] != "file" {
 		t.Fatalf("envToMap output type = %#v", outputs)
@@ -1316,11 +1311,11 @@ func TestExpandOSPathFlagValuesAndMergeEnv(t *testing.T) {
 		t.Fatalf("tls-ca = %q, want %q", got, file)
 	}
 
-	t.Setenv("GNMIC_FORMAT", "event")
+	t.Setenv("GNMIC_OUTPUTS_FILE_TYPE", "file")
 	c = New()
 	c.mergeEnvVars()
-	if got := c.FileConfig.GetString("format"); got != "event" {
-		t.Fatalf("mergeEnvVars format=%q", got)
+	if got := c.FileConfig.GetString("outputs/file/type"); got != "file" {
+		t.Fatalf("mergeEnvVars outputs/file/type=%q", got)
 	}
 }
 

pkg/config/environment.go

@@ -28,7 +28,14 @@ func envToMap() map[string]any {
 		}
 		pair[0] = strings.ToLower(strings.TrimPrefix(pair[0], envPrefix+"_"))
 		items := strings.Split(pair[0], "_")
-		mergeMap(m, items, pair[1])
+
+		// Only free-form configuration sections need underscore-separated
+		// environment variables materialized as nested maps. Known struct
+		// fields are handled by Viper's AutomaticEnv and key replacer.
+		switch items[0] {
+		case "clustering", "outputs", "inputs", "processors", "loader", "actions":
+			mergeMap(m, items, pair[1])
+		}
 	}
 	return m
 }

pkg/config/environment_test.go

@@ -0,0 +1,153 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package config
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/spf13/pflag"
+)
+
+func TestEnvToMapSplitsOnlyFreeFormSections(t *testing.T) {
+	sections := []string{
+		"clustering",
+		"outputs",
+		"inputs",
+		"processors",
+		"loader",
+		"actions",
+	}
+
+	for _, section := range sections {
+		t.Setenv(
+			"GNMIC_"+strings.ToUpper(section)+"_TEST_VALUE",
+			section,
+		)
+	}
+
+	// These are known struct-backed fields and must be handled by Viper,
+	// not interpreted as nested maps by envToMap.
+	t.Setenv("GNMIC_CLUSTER_NAME", "collector-cluster")
+	t.Setenv("GNMIC_LOG_FILE", "/tmp/gnmic-env.log")
+
+	got := envToMap()
+
+	for _, section := range sections {
+		level1, ok := got[section].(map[string]any)
+		if !ok {
+			t.Fatalf(
+				"envToMap()[%q] = %T, want map[string]any",
+				section,
+				got[section],
+			)
+		}
+
+		level2, ok := level1["test"].(map[string]any)
+		if !ok {
+			t.Fatalf(
+				"envToMap()[%q][test] = %T, want map[string]any",
+				section,
+				level1["test"],
+			)
+		}
+
+		if value, want := level2["value"], section; value != want {
+			t.Errorf(
+				"envToMap()[%q][test][value] = %v, want %q",
+				section,
+				value,
+				want,
+			)
+		}
+	}
+
+	if _, ok := got["cluster"]; ok {
+		t.Errorf(
+			"envToMap() contains cluster = %v; known cluster-name field should be excluded",
+			got["cluster"],
+		)
+	}
+
+	if _, ok := got["log"]; ok {
+		t.Errorf(
+			"envToMap() contains log = %v; known log-file field should be excluded",
+			got["log"],
+		)
+	}
+}
+
+func TestLoadEnvironmentOverridesConfig(t *testing.T) {
+	t.Setenv("GNMIC_CLUSTER_NAME", "collector-cluster")
+	t.Setenv("GNMIC_LOG_FILE", "/tmp/gnmic-env.log")
+	t.Setenv(
+		"GNMIC_CLUSTERING_LOCKER_ADDRESS",
+		"consul.example:8500",
+	)
+
+	configFile := filepath.Join(t.TempDir(), "gnmic.yaml")
+	if err := os.WriteFile(
+		configFile,
+		[]byte(`log-file: /tmp/gnmic-file.log
+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", "")
+	flags.String("log-file", "", "")
+
+	for _, name := range []string{"cluster-name", "log-file"} {
+		if err := cfg.FileConfig.BindPFlag(
+			name,
+			flags.Lookup(name),
+		); err != nil {
+			t.Fatalf("BindPFlag(%q) failed: %v", name, 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 got, want := cfg.LogFile, "/tmp/gnmic-env.log"; got != want {
+		t.Errorf("LogFile = %q, want %q", got, want)
+	}
+
+	if cfg.Clustering == nil {
+		t.Fatal("Clustering is nil, want configuration populated")
+	}
+
+	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,
+		)
+	}
+}