mime: support reading shared mime-info database on unix systems

src/mime/testdata/test.types.globs2

@@ -0,0 +1,8 @@
+# Copyright 2021 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+
+# mime package test for globs2
+50:document/test:*.t3
+50:example/test:*.t4

src/mime/type_unix.go

@@ -17,13 +17,46 @@ func init() {
 	osInitMime = initMimeUnix
 }
 
+// See https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-0.21.html
+// for the FreeDesktop Shared MIME-info Database specification.
+var mimeGlobs = []string{
+	"/usr/local/share/mime/globs2",
+	"/usr/share/mime/globs2",
+}
+
+// Common locations for mime.types files on unix.
 var typeFiles = []string{
 	"/etc/mime.types",
 	"/etc/apache2/mime.types",
 	"/etc/apache/mime.types",
 	"/etc/httpd/conf/mime.types",
 }
 
+func loadMimeGlobsFile(filename string) error {
+	f, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		// Each line should be of format: weight:mimetype:*.ext
+		fields := strings.Split(scanner.Text(), ":")
+		if len(fields) < 3 || len(fields[0]) < 1 || len(fields[2]) < 2 {
+			continue
+		} else if fields[0][0] == '#' || fields[2][0] != '*' {
+			continue
+		}
+
+		setExtensionType(fields[2][1:], fields[1])
+	}
+	if err := scanner.Err(); err != nil {
+		panic(err)
+	}
+	return nil
+}
+
 func loadMimeFile(filename string) {
 	f, err := os.Open(filename)
 	if err != nil {
@@ -51,12 +84,20 @@ func loadMimeFile(filename string) {
 }
 
 func initMimeUnix() {
+	for _, filename := range mimeGlobs {
+		if err := loadMimeGlobsFile(filename); err == nil {
+			return // Stop checking more files if mimetype database is found.
+		}
+	}
+
+	// Fallback if no system-generated mimetype database exists.
 	for _, filename := range typeFiles {
 		loadMimeFile(filename)
 	}
 }
 
 func initMimeForTests() map[string]string {
+	mimeGlobs = []string{""}
 	typeFiles = []string{"testdata/test.types"}
 	return map[string]string{
 		".T1":  "application/test",

src/mime/type_unix_test.go

@@ -0,0 +1,39 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || darwin || dragonfly || freebsd || (js && wasm) || linux || netbsd || openbsd || solaris
+// +build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris
+
+package mime
+
+import (
+	"testing"
+)
+
+func initMimeUnixTest(t *testing.T) {
+	err := loadMimeGlobsFile("testdata/test.types.globs2")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	loadMimeFile("testdata/test.types")
+}
+
+func TestTypeByExtensionUNIX(t *testing.T) {
+	initMimeUnixTest(t)
+	typeTests := map[string]string{
+		".T1":  "application/test",
+		".t2":  "text/test; charset=utf-8",
+		".t3":  "document/test",
+		".t4":  "example/test",
+		".png": "image/png",
+	}
+
+	for ext, want := range typeTests {
+		val := TypeByExtension(ext)
+		if val != want {
+			t.Errorf("TypeByExtension(%q) = %q, want %q", ext, val, want)
+		}
+	}
+}