feat: Accept config values with !env_file in yaml

README.md

@@ -55,7 +55,20 @@ metadata:
       secret: !env_var DISCOGS_API_KEY
 ```
 
-The application will fail to start if a referenced environment variable is not set.
+Secrets can also be loaded from a file instead — handy for Docker/Kubernetes
+secrets, which are mounted as files. Use the `!env_file` tag: the named
+environment variable holds the *path* to the file, and the file's contents
+become the value:
+
+```yaml
+metadata:
+  providers:
+    acoustid:
+      secret: !env_file ACOUSTID_CLIENT_KEY_FILE # e.g. ACOUSTID_CLIENT_KEY_FILE=/run/secrets/acoustid_client_key
+```
+
+The application will fail to start if a referenced environment variable is not
+set, or (for `!env_file`) if the file it points to can't be read or is empty.
 
 ### Build and/or run it  
 

config.example.yaml

@@ -1,10 +1,15 @@
 # Soulsolid Configuration Example
-# 
+#
 # Environment variables can be referenced using the !env_var tag:
 #   token: !env_var TELEGRAM_BOT_TOKEN
 #
-# Missing environment variables will cause startup errors.
-# To simplify setup, secrets are commented out in config.example.yaml so you can copy it to config.yaml without requiring unused variables. 
+# Secrets can also be loaded from a file (Docker/Kubernetes secrets convention)
+# using the !env_file tag: the named environment variable holds a *path*, and
+# the file's contents become the value:
+#   token: !env_file TELEGRAM_BOT_TOKEN_FILE   # TELEGRAM_BOT_TOKEN_FILE=/run/secrets/telegram_token
+#
+# Missing environment variables (or unreadable/empty files) will cause startup errors.
+# To simplify setup, secrets are commented out in config.example.yaml so you can copy it to config.yaml without requiring unused variables.
 # If no config is found, Soulsolid generates a default one automatically.
 
 libraryPath: ./music
@@ -59,11 +64,13 @@ metadata:
   acoustid:
     enabled: true
     # secret: !env_var ACOUSTID_CLIENT_KEY # You can login with Musicbrainz -> https://acoustid.org/new-application
+    # secret: !env_file ACOUSTID_CLIENT_KEY_FILE # Or load it from a file instead, e.g. a Docker secret
     deezer:
       enabled: true
     discogs:
       enabled: true
       # secret: !env_var DISCOGS_API_KEY # You can get it here -> https://www.discogs.com/settings/developers
+      # secret: !env_file DISCOGS_API_KEY_FILE # Or load it from a file instead, e.g. a Docker secret
     musicbrainz:
       enabled: true
 lyrics:

docs/features/config.md

@@ -66,6 +66,26 @@ metadata:
       secret: !env_var DISCOGS_SECRET
 ```
 
+A scalar tagged `!env_file` is resolved the same way, except the named environment
+variable is expected to hold a *path* rather than the secret itself — the file at
+that path is read and its (trimmed) contents become the value. This follows the
+Docker/Kubernetes secrets convention of mounting a secret as a file and pointing to
+it with a `_FILE`-suffixed variable, so the secret value never has to touch the
+process environment or shell history:
+
+```yaml
+telegram:
+  token: !env_file TELEGRAM_TOKEN_FILE   # TELEGRAM_TOKEN_FILE=/run/secrets/telegram_token
+metadata:
+  providers:
+    acoustid:
+      secret: !env_file ACOUSTID_CLIENT_KEY_FILE
+```
+
+The app **fails to start** if the referenced variable is unset, the file can't be
+read, or the file is empty. `!env_var` and `!env_file` can be mixed freely across
+different fields.
+
 After expansion the struct is validated with `go-playground/validator`; `libraryPath`,
 `downloadPath`, and `database.path` are required.
 

src/features/config/manager.go

@@ -49,6 +49,40 @@ func (m *Manager) processEnvVarNodes(node *yaml.Node, path []string) error {
 		return nil
 	}
 
+	// Check if this node has a !env_file tag: the named environment variable
+	// holds a *path* to a file (Docker/Kubernetes secrets convention, e.g.
+	// ACOUSTID_CLIENT_KEY_FILE=/run/secrets/acoustid_client_key), and the
+	// file's contents become the value.
+	if node.Tag == "!env_file" {
+		if node.Kind != yaml.ScalarNode {
+			return fmt.Errorf("!env_file tag can only be used with scalar values (line %d)", node.Line)
+		}
+
+		envVarName := strings.TrimSpace(node.Value)
+		if envVarName == "" {
+			return fmt.Errorf("!env_file tag requires environment variable name (line %d)", node.Line)
+		}
+
+		filePath := os.Getenv(envVarName)
+		if filePath == "" {
+			return fmt.Errorf("environment variable %s is not set or empty (referenced at line %d)", envVarName, node.Line)
+		}
+
+		content, err := os.ReadFile(filePath)
+		if err != nil {
+			return fmt.Errorf("failed to read file %q referenced by %s (line %d): %w", filePath, envVarName, node.Line, err)
+		}
+
+		value := strings.TrimSpace(string(content))
+		if value == "" {
+			return fmt.Errorf("file %q referenced by %s is empty (line %d)", filePath, envVarName, node.Line)
+		}
+
+		node.Tag = ""
+		node.Value = value
+		return nil
+	}
+
 	// Recursively process child nodes
 	// Start with DocumentNode content if present
 	if node.Kind == yaml.DocumentNode && len(node.Content) > 0 {

src/features/config/manager_test.go

@@ -0,0 +1,75 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadConfig_EnvVarAndEnvFileTags(t *testing.T) {
+	t.Setenv("SOULSOLID_TEST_TOKEN", "token-from-env")
+
+	secretFile := filepath.Join(t.TempDir(), "acoustid_secret")
+	if err := os.WriteFile(secretFile, []byte("secret-from-file\n"), 0o600); err != nil {
+		t.Fatalf("failed to write secret file: %v", err)
+	}
+	t.Setenv("SOULSOLID_TEST_SECRET_FILE", secretFile)
+
+	configYAML := `
+libraryPath: ./music
+downloadPath: ./downloads
+database:
+  path: ./library.db
+telegram:
+  token: !env_var SOULSOLID_TEST_TOKEN
+metadata:
+  providers:
+    acoustid:
+      enabled: true
+      secret: !env_file SOULSOLID_TEST_SECRET_FILE
+`
+	configPath := filepath.Join(t.TempDir(), "config.yaml")
+	if err := os.WriteFile(configPath, []byte(configYAML), 0o600); err != nil {
+		t.Fatalf("failed to write config file: %v", err)
+	}
+
+	m := &Manager{}
+	cfg, err := m.loadConfig(configPath)
+	if err != nil {
+		t.Fatalf("loadConfig returned error: %v", err)
+	}
+
+	if cfg.Telegram.Token != "token-from-env" {
+		t.Errorf("expected telegram token %q, got %q", "token-from-env", cfg.Telegram.Token)
+	}
+
+	acoustid, ok := cfg.Metadata.Providers["acoustid"]
+	if !ok {
+		t.Fatalf("expected acoustid provider to be present")
+	}
+	if acoustid.Secret == nil || *acoustid.Secret != "secret-from-file" {
+		t.Errorf("expected acoustid secret %q, got %v", "secret-from-file", acoustid.Secret)
+	}
+}
+
+func TestLoadConfig_EnvFileMissingFile(t *testing.T) {
+	t.Setenv("SOULSOLID_TEST_MISSING_FILE", filepath.Join(t.TempDir(), "does-not-exist"))
+
+	configYAML := `
+libraryPath: ./music
+downloadPath: ./downloads
+database:
+  path: ./library.db
+telegram:
+  token: !env_file SOULSOLID_TEST_MISSING_FILE
+`
+	configPath := filepath.Join(t.TempDir(), "config.yaml")
+	if err := os.WriteFile(configPath, []byte(configYAML), 0o600); err != nil {
+		t.Fatalf("failed to write config file: %v", err)
+	}
+
+	m := &Manager{}
+	if _, err := m.loadConfig(configPath); err == nil {
+		t.Fatal("expected error when the file referenced by !env_file does not exist")
+	}
+}