Geary.RFC822.Message: Fix get_body() critical with multipart messages

Fixes a bad assumption MR !534 made about GMime structure and causing
a crash on sending multipart messages. Add unit tests to cover this
case.

test/engine/api/geary-email-test.vala

@@ -0,0 +1,52 @@
+/*
+ * Copyright © 2020 Michael Gratton <mike@vee.net>
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later). See the COPYING file in this distribution.
+ */
+
+class Geary.EmailTest: TestCase {
+
+
+    private const string BASIC_TEXT_PLAIN = "basic-text-plain.eml";
+    private const string BASIC_MULTIPART_ALTERNATIVE =
+        "basic-multipart-alternative.eml";
+
+
+    public EmailTest() {
+        base("Geary.EmailTest");
+        add_test("email_from_basic_message", email_from_basic_message);
+        add_test("email_from_multipart", email_from_multipart);
+    }
+
+    public void email_from_basic_message() throws GLib.Error {
+        var message = resource_to_message(BASIC_TEXT_PLAIN);
+        var email = new Email.from_message(new MockEmailIdentifer(0), message);
+
+        assert_non_null(email);
+        assert_non_null(email.subject);
+        assert_equal(email.subject.to_string(), "Re: Basic text/plain message");
+    }
+
+    public void email_from_multipart() throws GLib.Error {
+        var message = resource_to_message(BASIC_MULTIPART_ALTERNATIVE);
+        var email = new Email.from_message(new MockEmailIdentifer(0), message);
+
+        assert_non_null(email);
+        assert_non_null(email.subject);
+        assert_equal(email.subject.to_string(), "Re: Basic text/html message");
+    }
+
+    private RFC822.Message resource_to_message(string path) throws GLib.Error {
+        GLib.File resource =
+            GLib.File.new_for_uri(RESOURCE_URI).resolve_relative_path(path);
+
+        uint8[] contents;
+        resource.load_contents(null, out contents, null);
+
+        return new RFC822.Message.from_buffer(
+            new Geary.Memory.ByteBuffer(contents, contents.length)
+        );
+    }
+
+}

test/engine/rfc822/rfc822-message-test.vala

@@ -54,7 +54,8 @@ This is the second line.
         add_test("multipart_alternative_as_html",
                  multipart_alternative_as_html);
         add_test("get_header", get_header);
-        add_test("get_body", get_body);
+        add_test("get_body_single_part", get_body_single_part);
+        add_test("get_body_multipart", get_body_multipart);
         add_test("get_preview", get_preview);
         add_test("get_recipients", get_recipients);
         add_test("get_searchable_body", get_searchable_body);
@@ -185,10 +186,33 @@ This is the second line.
         assert(header.get_header("From") == "Alice <alice@example.net>");
     }
 
-    public void get_body() throws GLib.Error {
+    public void get_body_single_part() throws GLib.Error {
         Message message = resource_to_message(BASIC_TEXT_PLAIN);
         Text body = message.get_body();
-        assert(body.buffer.to_string().replace("\r", "") == BASIC_PLAIN_BODY);
+        assert_string(
+            body.buffer.to_string().replace("\r", "")
+        ).contains(
+            // should contain the body
+            BASIC_PLAIN_BODY
+        ).not_contains(
+            // should not contain headers (like the subject)
+            "Re: Basic text/plain message"
+        );
+    }
+
+    public void get_body_multipart() throws GLib.Error {
+        Message message = resource_to_message(BASIC_MULTIPART_ALTERNATIVE);
+        Text body = message.get_body();
+
+        assert_string(
+            body.buffer.to_string().replace("\r", "")
+        ).contains(
+            // should contain  the body
+            BASIC_PLAIN_BODY
+        ).not_contains(
+            // should not contain headers (like the subject)
+            "Re: Basic text/html message"
+        );
     }
 
     public void get_preview() throws GLib.Error {

test/meson.build

@@ -21,10 +21,11 @@ geary_test_engine_sources = [
 
   'engine/api/geary-account-information-test.vala',
   'engine/api/geary-attachment-test.vala',
+  'engine/api/geary-composed-email-test.vala',
+  'engine/api/geary-email-test.vala',
   'engine/api/geary-engine-test.vala',
   'engine/api/geary-folder-path-test.vala',
   'engine/api/geary-service-information-test.vala',
-  'engine/api/geary-composed-email-test.vala',
   'engine/app/app-conversation-test.vala',
   'engine/app/app-conversation-monitor-test.vala',
   'engine/app/app-conversation-set-test.vala',

test/test-engine.vala

@@ -90,6 +90,7 @@ int main(string[] args) {
     // last
     engine.add_suite(new Geary.RFC822.MessageTest().suite);
     engine.add_suite(new Geary.String.Test().suite);
+    engine.add_suite(new Geary.EmailTest().suite);
     engine.add_suite(new Geary.ComposedEmailTest().suite);
 
     /*