Support drop-in config files

internal/broker/config.go

@@ -3,6 +3,9 @@ package broker
 import (
 	"errors"
 	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
 	"strings"
 
 	"gopkg.in/ini.v1"
@@ -27,11 +30,39 @@ const (
 	sshSuffixesKey = "ssh_allowed_suffixes"
 )
 
+func getDropInFiles(cfgPath string) ([]any, error) {
+	// Check if a .d directory exists and return the paths to the files in it.
+	dropInDir := cfgPath + ".d"
+	files, err := os.ReadDir(dropInDir)
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	var dropInFiles []any
+	// files is empty if the directory does not exist
+	for _, file := range files {
+		if file.IsDir() {
+			continue
+		}
+		dropInFiles = append(dropInFiles, filepath.Join(dropInDir, file.Name()))
+	}
+
+	return dropInFiles, nil
+}
+
 // parseConfigFile parses the config file and returns a map with the configuration keys and values.
 func parseConfigFile(cfgPath string) (userConfig, error) {
 	cfg := userConfig{}
 
-	iniCfg, err := ini.Load(cfgPath)
+	dropInFiles, err := getDropInFiles(cfgPath)
+	if err != nil {
+		return cfg, err
+	}
+
+	iniCfg, err := ini.Load(cfgPath, dropInFiles...)
 	if err != nil {
 		return cfg, err
 	}

internal/broker/config_test.go

@@ -40,6 +40,17 @@ client_id = <CLIENT_ID
 [oidc]
 issuer = https://<ISSUER_URL>
 client_id = <CLIENT_ID>
+`,
+
+	"overwrite_lower_precedence": `
+[oidc]
+issuer = https://lower-precedence-issuer.url.com
+client_id = lower_precedence_client_id
+`,
+
+	"overwrite_higher_precedence": `
+[oidc]
+issuer = https://higher-precedence-issuer.url.com
 `,
 }
 
@@ -48,17 +59,21 @@ func TestParseConfig(t *testing.T) {
 
 	tests := map[string]struct {
 		configType string
+		dropInType string
 
 		wantErr bool
 	}{
 		"Successfully_parse_config_file":                      {},
 		"Successfully_parse_config_file_with_optional_values": {configType: "valid+optional"},
+		"Successfully_parse_config_with_drop_in_files":        {dropInType: "valid"},
 
 		"Do_not_fail_if_values_contain_a_single_template_delimiter": {configType: "singles"},
 
-		"Error_if_file_does_not_exist": {configType: "inexistent", wantErr: true},
-		"Error_if_file_is_unreadable":  {configType: "unreadable", wantErr: true},
-		"Error_if_file_is_not_updated": {configType: "template", wantErr: true},
+		"Error_if_file_does_not_exist":             {configType: "inexistent", wantErr: true},
+		"Error_if_file_is_unreadable":              {configType: "unreadable", wantErr: true},
+		"Error_if_file_is_not_updated":             {configType: "template", wantErr: true},
+		"Error_if_drop_in_directory_is_unreadable": {dropInType: "unreadable-dir", wantErr: true},
+		"Error_if_drop_in_file_is_unreadable":      {dropInType: "unreadable-file", wantErr: true},
 	}
 	for name, tc := range tests {
 		t.Run(name, func(t *testing.T) {
@@ -81,6 +96,31 @@ func TestParseConfig(t *testing.T) {
 				require.NoError(t, err, "Setup: Failed to make config file unreadable")
 			}
 
+			dropInDir := confPath + ".d"
+			if tc.dropInType != "" {
+				err = os.Mkdir(dropInDir, 0700)
+				require.NoError(t, err, "Setup: Failed to create drop-in directory")
+			}
+
+			switch tc.dropInType {
+			case "valid":
+				// Create multiple drop-in files to test that they are loaded in the correct order.
+				err = os.WriteFile(filepath.Join(dropInDir, "00-drop-in.conf"), []byte(configTypes["overwrite_lower_precedence"]), 0600)
+				require.NoError(t, err, "Setup: Failed to write drop-in file")
+				err = os.WriteFile(filepath.Join(dropInDir, "01-drop-in.conf"), []byte(configTypes["overwrite_higher_precedence"]), 0600)
+				require.NoError(t, err, "Setup: Failed to write drop-in file")
+				// Create the main config file, to test that the options which are not overwritten by the drop-in files
+				// are still present.
+				err = os.WriteFile(confPath, []byte(configTypes["valid+optional"]), 0600)
+				require.NoError(t, err, "Setup: Failed to write config file")
+			case "unreadable-dir":
+				err = os.Chmod(dropInDir, 0000)
+				require.NoError(t, err, "Setup: Failed to make drop-in directory unreadable")
+			case "unreadable-file":
+				err = os.WriteFile(filepath.Join(dropInDir, "00-drop-in.conf"), []byte(configTypes["valid"]), 0000)
+				require.NoError(t, err, "Setup: Failed to make drop-in file unreadable")
+			}
+
 			cfg, err := parseConfigFile(confPath)
 			if tc.wantErr {
 				require.Error(t, err)

internal/broker/testdata/TestParseConfig/golden/Successfully_parse_config_with_drop_in_files/config.txt

@@ -0,0 +1,5 @@
+clientID=lower_precedence_client_id
+clientSecret=
+issuerURL=https://higher-precedence-issuer.url.com
+homeBaseDir=/home
+allowedSSHSuffixes=[]