mime: fix ExtensionsByType bug when there are duplicates

Also, sort them so the results aren't random.

Thanks to @junedev for the bug report & repro.

Fixes #36524

Change-Id: Ic9197ebeceddfb3d0aee895d8fc12ce4d205b164
Reviewed-on: https://go-review.googlesource.com/c/go/+/214680
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz

src/mime/type.go

@@ -7,6 +7,7 @@ package mime
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 	"sync"
 )
@@ -49,7 +50,7 @@ func setMimeTypes(lowerExt, mixExt map[string]string) {
 			panic(err)
 		}
 		var exts []string
-		if ei, ok := extensions.Load(k); ok {
+		if ei, ok := extensions.Load(justType); ok {
 			exts = ei.([]string)
 		}
 		extensions.Store(justType, append(exts, k))
@@ -151,7 +152,9 @@ func ExtensionsByType(typ string) ([]string, error) {
 	if !ok {
 		return nil, nil
 	}
-	return append([]string{}, s.([]string)...), nil
+	ret := append([]string(nil), s.([]string)...)
+	sort.Strings(ret)
+	return ret, nil
 }
 
 // AddExtensionType sets the MIME type associated with

src/mime/type_test.go

@@ -188,3 +188,30 @@ func BenchmarkExtensionsByType(b *testing.B) {
 		})
 	}
 }
+
+func TestExtensionsByType2(t *testing.T) {
+	cleanup := setMimeInit(func() {
+		clearMimeTypes()
+		// Initialize built-in types like in type.go before osInitMime.
+		setMimeTypes(builtinTypesLower, builtinTypesLower)
+	})
+	defer cleanup()
+
+	tests := []struct {
+		typ  string
+		want []string
+	}{
+		{typ: "image/jpeg", want: []string{".jpeg", ".jpg"}},
+	}
+
+	for _, tt := range tests {
+		got, err := ExtensionsByType(tt.typ)
+		if err != nil {
+			t.Errorf("ExtensionsByType(%q): %v", tt.typ, err)
+			continue
+		}
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("ExtensionsByType(%q) = %q; want %q", tt.typ, got, tt.want)
+		}
+	}
+}