imapc: Prefech - Initialize cached stream only if needed

If the mail is immediately accessed for other purposes (e.g. fetching
message flags), there's no need to go through all the trouble of
initializing the mail stream.

src/lib-storage/index/imapc/imapc-mail-fetch.c

@@ -368,6 +368,15 @@ imapc_mail_get_wanted_fetch_fields(struct imapc_mail *mail)
 	return fields;
 }
 
+void imapc_mail_try_init_stream_from_cache(struct imapc_mail *mail)
+{
+	struct mail *_mail = &mail->imail.mail.mail;
+	struct imapc_mailbox *mbox = (struct imapc_mailbox *)_mail->box;
+
+	if (mbox->prev_mail_cache.uid == _mail->uid)
+		imapc_mail_cache_get(mail, &mbox->prev_mail_cache);
+}
+
 bool imapc_mail_prefetch(struct mail *_mail)
 {
 	struct imapc_mail *mail = (struct imapc_mail *)_mail;
@@ -376,10 +385,13 @@ bool imapc_mail_prefetch(struct mail *_mail)
 	enum mail_fetch_field fields;
 	const char *const *headers = NULL;
 
-	if (mbox->prev_mail_cache.uid == _mail->uid)
-		imapc_mail_cache_get(mail, &mbox->prev_mail_cache);
 	/* try to get as much from cache as possible */
 	imapc_mail_update_access_parts(&mail->imail);
+	/* If mail is already cached we can avoid re-FETCHing the mail.
+	   However, don't initialize the stream if we don't actually want to
+	   access the mail. */
+	if (mail->imail.data.access_part != 0)
+		imapc_mail_try_init_stream_from_cache(mail);
 
 	fields = imapc_mail_get_wanted_fetch_fields(mail);
 	if (data->wanted_headers != NULL && data->stream == NULL &&

src/lib-storage/index/imapc/imapc-mail.c

@@ -302,6 +302,10 @@ imapc_mail_get_stream(struct mail *_mail, bool get_body,
 		data->hdr_size_set = FALSE;
 	}
 
+	/* See if we can get it from cache. If the wanted_fields/headers are
+	   set properly, this is usually already done by prefetching. */
+	imapc_mail_try_init_stream_from_cache(mail);
+
 	if (data->stream == NULL) {
 		if (!data->initialized) {
 			/* coming here from mail_set_seq() */

src/lib-storage/index/imapc/imapc-mail.h

@@ -33,6 +33,7 @@ imapc_mail_alloc(struct mailbox_transaction_context *t,
 		 struct mailbox_header_lookup_ctx *wanted_headers);
 int imapc_mail_fetch(struct mail *mail, enum mail_fetch_field fields,
 		     const char *const *headers);
+void imapc_mail_try_init_stream_from_cache(struct imapc_mail *mail);
 bool imapc_mail_prefetch(struct mail *mail);
 void imapc_mail_fetch_flush(struct imapc_mailbox *mbox);
 void imapc_mail_init_stream(struct imapc_mail *mail);