configurable scan parallelism based on hdd or sdd

backend/library/config.go

@@ -7,11 +7,39 @@ import (
 	"os"
 )
 
-var errNotDirectory = errors.New("path is not a directory")
+var (
+	errNotDirectory           = errors.New("path is not a directory")
+	errUnknownScanConcurrency = errors.New("unknown scan concurrency mode")
+)
+
+// ScanConcurrency controls how many parallel workers the scanner
+// uses for metadata extraction.  The choice directly affects I/O
+// throughput on spinning disks vs SSDs.
+type ScanConcurrency string
+
+// Valid ScanConcurrency modes.
+const (
+	// ScanConcurrencyAuto detects whether the library resides on
+	// a rotational disk and chooses workers accordingly.
+	ScanConcurrencyAuto ScanConcurrency = "auto"
+
+	// ScanConcurrencySSD uses runtime.NumCPU() workers, maximising
+	// throughput on solid-state storage.
+	ScanConcurrencySSD ScanConcurrency = "ssd"
+
+	// ScanConcurrencyHDD uses a small number of workers to limit
+	// I/O contention on spinning disks.
+	ScanConcurrencyHDD ScanConcurrency = "hdd"
+)
+
+// DefaultScanConcurrency is the mode used when no value is
+// configured.
+const DefaultScanConcurrency = ScanConcurrencyAuto
 
 // Config holds Library config data.
 type Config struct {
-	DirectoryPath Directory `form:"Directory" schema:"directory,required"`
+	DirectoryPath   Directory       `toml:"DirectoryPath"`
+	ScanConcurrency ScanConcurrency `toml:"ScanConcurrency"`
 }
 
 // Directory represents a filesystem path to a music directory.
@@ -23,24 +51,54 @@ func NewConfig(dir string) (*Config, error) {
 		DirectoryPath: Directory(dir),
 	}
 	if err := config.Validate(); err != nil {
-		return nil, fmt.Errorf("validation error for new library config: %w", err)
+		return nil, fmt.Errorf(
+			"validation error for new library config: %w",
+			err,
+		)
 	}
 
 	return config, nil
 }
 
-// Validate checks that the configured directory exists.
+// ApplyDefaults fills zero-value fields with sensible defaults.
+func (c *Config) ApplyDefaults() {
+	if c.ScanConcurrency == "" {
+		c.ScanConcurrency = DefaultScanConcurrency
+	}
+}
+
+// Validate checks that the configured directory exists and that
+// the scan concurrency mode is recognised.
 func (c *Config) Validate() error {
+	c.ApplyDefaults()
+
 	if len(c.DirectoryPath) != 0 {
 		dirInfo, err := os.Stat(string(c.DirectoryPath))
 		if err != nil {
-			return fmt.Errorf("problem getting info on library dir (%s): %w", c.DirectoryPath, err)
+			return fmt.Errorf(
+				"problem getting info on library dir (%s): %w",
+				c.DirectoryPath, err,
+			)
 		}
 
 		if !dirInfo.IsDir() {
-			return fmt.Errorf("%s: %w", c.DirectoryPath, errNotDirectory)
+			return fmt.Errorf(
+				"%s: %w", c.DirectoryPath, errNotDirectory,
+			)
 		}
 	}
 
+	switch c.ScanConcurrency {
+	case ScanConcurrencyAuto,
+		ScanConcurrencySSD,
+		ScanConcurrencyHDD:
+		// Valid.
+	default:
+		return fmt.Errorf(
+			"%w: %q", errUnknownScanConcurrency,
+			c.ScanConcurrency,
+		)
+	}
+
 	return nil
 }