feat: reconcile with on-disk changes before writing configuration

internal/config/configuration.go

@@ -4,6 +4,9 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"time"
+
+	"github.com/reugn/gemini-cli/gemini"
 )
 
 // Configuration contains the details of the application configuration.
@@ -14,9 +17,12 @@ type Configuration struct {
 	// Data is the application data. This data is loaded from the configuration
 	// file and is used to configure the application.
 	Data *ApplicationData
+	// lastModTime is the time the configuration file was last modified.
+	lastModTime time.Time
 }
 
 // NewConfiguration returns a new Configuration from a JSON file.
+// If the file does not exist, it is created with default values.
 func NewConfiguration(filePath string) (*Configuration, error) {
 	configuration := &Configuration{
 		filePath: filePath,
@@ -34,28 +40,108 @@ func NewConfiguration(filePath string) (*Configuration, error) {
 	defer file.Close()
 
 	decoder := json.NewDecoder(file)
-	err = decoder.Decode(configuration.Data)
-	if err != nil {
+	if err := decoder.Decode(configuration.Data); err != nil {
 		return nil, fmt.Errorf("error decoding JSON: %w", err)
 	}
 
 	return configuration, nil
 }
 
 // Flush serializes and writes the configuration to the file.
+//
+// If the file is modified since the last load/flush, the configuration is re-read
+// and merged with the on-disk data.
 func (c *Configuration) Flush() error {
+	// Reload the configuration if the file was modified since the last load/flush.
+	if err := c.reloadIfStale(); err != nil {
+		return err
+	}
+
+	// Create the file if it does not exist.
 	file, err := os.Create(c.filePath)
 	if err != nil {
 		return fmt.Errorf("error opening file: %w", err)
 	}
 	defer file.Close()
 
+	// Serialize the configuration data to the file.
 	encoder := json.NewEncoder(file)
 	encoder.SetIndent("", "  ")
-	err = encoder.Encode(c.Data)
-	if err != nil {
+	if err := encoder.Encode(c.Data); err != nil {
 		return fmt.Errorf("error encoding JSON: %w", err)
 	}
 
-	return file.Sync()
+	// Sync the file to disk.
+	if err := file.Sync(); err != nil {
+		return fmt.Errorf("error syncing file: %w", err)
+	}
+
+	// Get the file information.
+	info, err := os.Stat(c.filePath)
+	if err != nil {
+		return fmt.Errorf("error stating file: %w", err)
+	}
+
+	// Update the last modified time.
+	c.lastModTime = info.ModTime()
+
+	return nil
+}
+
+// reloadIfStale re-reads and merges the on-disk configuration if the file was
+// modified since the last load/flush.
+func (c *Configuration) reloadIfStale() error {
+	info, err := os.Stat(c.filePath)
+	if err != nil {
+		if os.IsNotExist(err) { // ignore error if file does not exist
+			return nil
+		}
+		return fmt.Errorf("error stating config file: %w", err)
+	}
+
+	// If the file was not modified since the last load/flush, do nothing.
+	if !c.lastModTime.IsZero() && !info.ModTime().After(c.lastModTime) {
+		return nil
+	}
+
+	// Re-read the configuration file.
+	file, err := os.Open(c.filePath)
+	if err != nil {
+		return fmt.Errorf("error reopening config file: %w", err)
+	}
+	defer file.Close()
+
+	onDisk := newDefaultApplicationData()
+	if err := json.NewDecoder(file).Decode(onDisk); err != nil {
+		return fmt.Errorf("error decoding updated config: %w", err)
+	}
+
+	// Merge the on-disk data into the current configuration.
+	c.mergeApplicationData(onDisk)
+
+	return nil
+}
+
+// mergeApplicationData merges on-disk data into the current configuration.
+func (c *Configuration) mergeApplicationData(onDisk *ApplicationData) {
+	if onDisk == nil || c.Data == nil {
+		return
+	}
+
+	// The CLI never modifies these fields; always overwrite with the on-disk values.
+	c.Data.SystemPrompts = onDisk.SystemPrompts
+	c.Data.SafetySettings = onDisk.SafetySettings
+	c.Data.Tools = onDisk.Tools
+
+	// Merge history records.
+	if onDisk.History != nil {
+		if c.Data.History == nil {
+			c.Data.History = make(map[string][]*gemini.SerializableContent, len(onDisk.History))
+		}
+		for label, records := range onDisk.History {
+			if _, exists := c.Data.History[label]; !exists {
+				c.Data.History[label] = records
+			}
+		}
+	}
 }