decompose python parsers more + add tests around plural fields

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

syft/pkg/cataloger/python/parse_wheel_egg_metadata.go

@@ -93,15 +93,7 @@ func extractRFC5322Fields(locationReader file.LocationReadCloser) (map[string]an
 				key = strings.ReplaceAll(strings.TrimSpace(line[0:i]), "-", "")
 				val := getFieldType(key, strings.TrimSpace(line[i+1:]))
 
-				if strSlice, ok := val.([]string); ok {
-					if fields[key] == nil {
-						fields[key] = strSlice
-					} else {
-						fields[key] = append(fields[key].([]string), strSlice...)
-					}
-				} else {
-					fields[key] = val
-				}
+				fields[key] = handleSingleOrMultiField(fields[key], val)
 			} else {
 				log.Warnf("cannot parse field from path: %q from line: %q", locationReader.Path(), line)
 			}
@@ -110,6 +102,25 @@ func extractRFC5322Fields(locationReader file.LocationReadCloser) (map[string]an
 	return fields, nil
 }
 
+func handleSingleOrMultiField(existingValue, val any) any {
+	strSlice, ok := val.([]string)
+	if !ok {
+		return val
+	}
+	if existingValue == nil {
+		return strSlice
+	}
+
+	switch existingValueTy := existingValue.(type) {
+	case []string:
+		return append(existingValueTy, strSlice...)
+	case string:
+		return append([]string{existingValueTy}, strSlice...)
+	}
+
+	return append([]string{fmt.Sprintf("%s", existingValue)}, strSlice...)
+}
+
 func getFieldType(key, in string) any {
 	if plural, ok := pluralFields[key]; ok && plural {
 		return []string{in}

syft/pkg/cataloger/python/parse_wheel_egg_metadata_test.go

@@ -1,11 +1,15 @@
 package python
 
 import (
+	"io"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/go-test/deep"
 	"github.com/google/go-cmp/cmp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"github.com/anchore/syft/internal/cmptest"
 	"github.com/anchore/syft/syft/file"
@@ -180,3 +184,59 @@ func TestParseWheelEggMetadataInvalid(t *testing.T) {
 		})
 	}
 }
+
+func Test_extractRFC5322Fields(t *testing.T) {
+
+	tests := []struct {
+		name    string
+		input   string
+		want    map[string]any
+		wantErr require.ErrorAssertionFunc
+	}{
+		{
+			name: "with valid plural fields",
+			input: `
+Name: mxnet
+Version: 1.8.0
+Requires-Dist: numpy (>=1.16.6)
+Requires-Dist: requests (>=2.22.0)
+ProvidesExtra: cryptoutils ; extra == 'secure'
+ProvidesExtra: socks ; extra == 'secure'
+`,
+			want: map[string]any{
+				"Name":          "mxnet",
+				"Version":       "1.8.0",
+				"RequiresDist":  []string{"numpy (>=1.16.6)", "requests (>=2.22.0)"},
+				"ProvidesExtra": []string{"cryptoutils ; extra == 'secure'", "socks ; extra == 'secure'"},
+			},
+		},
+		{
+			name: "with invalid plural fields (overwrite)",
+			input: `
+Name: mxnet
+Version: 1.8.0
+Version: 1.9.0
+`,
+			want: map[string]any{
+				"Name":    "mxnet",
+				"Version": "1.9.0",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.wantErr == nil {
+				tt.wantErr = require.NoError
+			}
+
+			reader := file.NewLocationReadCloser(
+				file.NewLocation("/made/up"),
+				io.NopCloser(strings.NewReader(tt.input)),
+			)
+
+			got, err := extractRFC5322Fields(reader)
+			tt.wantErr(t, err)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}