Remove Geary.RFC822.Part.get_effective_content_type

Just make the `content_type` property non-nullable and set a default
in the ctor instead, since it's unlikely to ever be null despite the
GMime API suggesting it could be. Update call sites to use the property
instead.

src/client/conversation-viewer/conversation-message.vala

@@ -929,7 +929,7 @@ public class ConversationMessage : Gtk.Grid, Geary.BaseInterface {
     // returns HTML that is placed into the document in the position
     // where the MIME part was found
     private string? inline_image_replacer(Geary.RFC822.Part part) {
-        Geary.Mime.ContentType content_type = part.get_effective_content_type();
+        Geary.Mime.ContentType content_type = part.content_type;
         if (content_type.media_type != "image" ||
             !this.web_view.can_show_mime_type(content_type.to_string())) {
             debug("Not displaying %s inline: unsupported Content-Type",

src/engine/imap-db/imap-db-attachment.vala

@@ -46,7 +46,7 @@ private class Geary.ImapDB.Attachment : Geary.Attachment {
 
         this(
             message_id,
-            part.get_effective_content_type(),
+            part.content_type,
             part.content_id,
             part.content_description,
             disposition,

src/engine/rfc822/rfc822-message-data.vala

@@ -396,7 +396,7 @@ public class Geary.RFC822.PreviewText : Geary.RFC822.Text {
         if (gpart != null) {
             Part part = new Part(gpart);
 
-            Mime.ContentType content_type = part.get_effective_content_type();
+            Mime.ContentType content_type = part.content_type;
             bool is_plain = content_type.is_type("text", "plain");
             bool is_html = content_type.is_type("text", "html");
 

src/engine/rfc822/rfc822-message.vala

@@ -519,7 +519,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
 
             is_matching_part = (
                 disposition != Mime.DispositionType.ATTACHMENT &&
-                part.get_effective_content_type().is_type("text", text_subtype)
+                part.content_type.is_type("text", text_subtype)
             );
         }
         return is_matching_part;
@@ -549,7 +549,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
                                                 ref string? body)
         throws RFC822Error {
         Part part = new Part(node);
-        Mime.ContentType content_type = part.get_effective_content_type();
+        Mime.ContentType content_type = part.content_type;
 
         // If this is a multipart, call ourselves recursively on the children
         GMime.Multipart? multipart = node as GMime.Multipart;
@@ -876,9 +876,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
 
             if (requested_disposition == Mime.DispositionType.UNSPECIFIED ||
                 actual_disposition == requested_disposition) {
-
+                Mime.ContentType content_type = part.content_type;
-                Mime.ContentType content_type =
-                    part.get_effective_content_type();
 
 #if WITH_TNEF_SUPPORT
                 if (content_type.is_type("application", "vnd.ms-tnef")) {

src/engine/rfc822/rfc822-part.vala

@@ -52,7 +52,7 @@ public class Geary.RFC822.Part : Object {
      *
      * See [[https://tools.ietf.org/html/rfc2045#section-5]]
      */
-    public Mime.ContentType? content_type { get; private set; }
+    public Mime.ContentType content_type { get; private set; }
 
     /**
      * The entity's Content-ID.
@@ -85,44 +85,34 @@ public class Geary.RFC822.Part : Object {
         this.source_object = source;
         this.source_part = source as GMime.Part;
 
-        GMime.ContentType? part_type = source.get_content_type();
-        if (part_type != null) {
-            this.content_type = new Mime.ContentType.from_gmime(part_type);
-        }
-
         this.content_id = source.get_content_id();
 
         this.content_description = (this.source_part != null)
             ? source_part.get_content_description() : null;
 
-        GMime.ContentDisposition? part_disposition = source.get_content_disposition();
+        GMime.ContentDisposition? part_disposition =
+            source.get_content_disposition();
         if (part_disposition != null) {
             this.content_disposition = new Mime.ContentDisposition.from_gmime(
                 part_disposition
             );
         }
-    }
 
-    /**
+        // Although the GMime API permits this to be null, it's not
-     * The entity's effective Content-Type.
+        // clear if it ever will be, since the API requires it to be
-     *
+        // specified at construction time.
-     * This returns the entity's content type if set, else returns
+        GMime.ContentType? part_type = source.get_content_type();
-     * {@link Geary.Mime.ContentType.DISPLAY_DEFAULT} this is a
+        if (part_type != null) {
-     * displayable (i.e. non-attachment) entity, or {@link
+            this.content_type = new Mime.ContentType.from_gmime(part_type);
-     * Geary.Mime.ContentType.ATTACHMENT_DEFAULT} if not.
+        } else {
-     */
-    public Mime.ContentType get_effective_content_type() {
-        Mime.ContentType? type = this.content_type;
-        if (type == null) {
             Mime.DispositionType disposition = Mime.DispositionType.UNSPECIFIED;
             if (this.content_disposition != null) {
                 disposition = this.content_disposition.disposition_type;
             }
-            type = (disposition != Mime.DispositionType.ATTACHMENT)
+            this.content_type = (disposition != Mime.DispositionType.ATTACHMENT)
                 ? Mime.ContentType.DISPLAY_DEFAULT
                 : Mime.ContentType.ATTACHMENT_DEFAULT;
         }
-        return type;
     }
 
     /**

test/engine/rfc822-part-test.vala

@@ -14,20 +14,16 @@ class Geary.RFC822.PartTest : TestCase {
 
     public PartTest() {
         base("Geary.RFC822.PartTest");
-        add_test("new_from_empty_mime_part", new_from_empty_mime_part);
+        add_test("new_from_minimal_mime_part", new_from_minimal_mime_part);
         add_test("new_from_complete_mime_part", new_from_complete_mime_part);
         add_test("write_to_buffer_plain", write_to_buffer_plain);
         add_test("write_to_buffer_plain_crlf", write_to_buffer_plain_crlf);
         add_test("write_to_buffer_plain_ical", write_to_buffer_plain_ical);
     }
 
-    public void new_from_empty_mime_part() throws Error {
+    public void new_from_minimal_mime_part() throws Error {
-        GMime.Part part = new_part(null, CR_BODY.data);
+        Part test = new Part(new_part("test/plain", CR_BODY.data));
-        part.set_header("Content-Type", "");
 
-        Part test = new Part(part);
-
-        assert_null(test.content_type, "content_type");
         assert_null_string(test.content_id, "content_id");
         assert_null_string(test.content_description, "content_description");
         assert_null(test.content_disposition, "content_disposition");