refactor: static sentinel errors for #55's config validation paths (err113)

internal/config/config.go

@@ -62,7 +62,11 @@ var (
 	errTooFewConnections = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
 	errMustBeSetTogether = errors.New("must be set together")
-	errValueNull         = errors.New(
+	errMustNotBeNegative = errors.New("must not be negative")
+	errOverflowsInt64    = errors.New("overflows a 64-bit integer")
+	errNegativeBlockSize = errors.New(
+		"statfs reported negative block size")
+	errValueNull = errors.New(
 		"value is null; omit the key entirely to use the default")
 	errValuesNull = errors.New(
 		"value is null; omit a key entirely to use its default")
@@ -371,8 +375,8 @@ func (c *Config) validate() error {
 	// Zero is valid (it disables the disk cache); only negative
 	// values are rejected. No floor applies to explicit values.
 	if c.CacheMaxBytes < 0 {
-		return fmt.Errorf("config key %q: value %d must not be negative",
-			"cache_max_bytes", c.CacheMaxBytes)
+		return fmt.Errorf("config key %q: value %d %w",
+			keyCacheMaxBytes, c.CacheMaxBytes, errMustNotBeNegative)
 	}
 
 	for _, host := range c.AllowlistHosts {
@@ -619,27 +623,29 @@ func getInt64(sc *smartconfig.Config, key string, defaultVal int64) (int64, erro
 		return val, nil
 	case uint64:
 		if val > math.MaxInt64 {
-			return 0, fmt.Errorf("config key %q: value %d overflows a 64-bit integer",
-				key, val)
+			return 0, fmt.Errorf("config key %q: value %d %w",
+				key, val, errOverflowsInt64)
 		}
 
 		return int64(val), nil
 	case float64:
 		if val != math.Trunc(val) {
-			return 0, fmt.Errorf("config key %q: value %v is not an integer", key, val)
+			return 0, fmt.Errorf("config key %q: value %v is %w",
+				key, val, errNotAnInteger)
 		}
 
 		return int64(val), nil
 	case string:
 		parsed, err := strconv.ParseInt(strings.TrimSpace(val), 10, 64)
 		if err != nil {
-			return 0, fmt.Errorf("config key %q: value %q is not an integer", key, val)
+			return 0, fmt.Errorf("config key %q: value %q is %w",
+				key, val, errNotAnInteger)
 		}
 
 		return parsed, nil
 	default:
-		return 0, fmt.Errorf("config key %q: value %v (%T) is not an integer",
-			key, raw, raw)
+		return 0, fmt.Errorf("config key %q: value %v (%T) is %w",
+			key, raw, raw, errNotAnInteger)
 	}
 }