From 513d1c5fa703817ae8fd08fab86709a9b5c111e4 Mon Sep 17 00:00:00 2001
From: Johannes Berg <johannes@sipsolutions.net>
Date: Wed, 5 Mar 2008 14:09:35 +0100
Subject: allow appends to spam if configured (offlineimap support)

---
 antispam-plugin.c      |  7 +++++++
 antispam-plugin.h      |  1 +
 antispam-storage-1.0.c | 51 +++++++++++++++++++++++++++++++++++++++++++++-----
 antispam.7             | 22 ++++++++++++++++++++++
 4 files changed, 76 insertions(+), 5 deletions(-)

diff --git a/antispam-plugin.c b/antispam-plugin.c
index cfec425..407b909 100644
--- a/antispam-plugin.c
+++ b/antispam-plugin.c
@@ -48,6 +48,7 @@ static char *default_spam_folders[] = {
 };
 static char **spam_folders = default_spam_folders;
 static char **unsure_folders = NULL;
+bool antispam_can_append_to_spam = FALSE;
 
 static bool mailbox_in_list(struct mailbox *box, char **list)
 {
@@ -126,6 +127,12 @@ void PLUGIN_FUNCTION(init)(void)
 	if (tmp)
 		spam_folders = p_strsplit(global_pool, tmp, ";");
 
+	tmp = get_setting("ALLOW_APPEND_TO_SPAM");
+	if (tmp && strcasecmp(tmp, "yes") == 0) {
+		antispam_can_append_to_spam = TRUE;
+		debug("allowing APPEND to spam folders");
+	}
+
 	if (spam_folders) {
 		iter = spam_folders;
 		while (*iter) {
diff --git a/antispam-plugin.h b/antispam-plugin.h
index 9b6bae5..98fed8f 100644
--- a/antispam-plugin.h
+++ b/antispam-plugin.h
@@ -50,5 +50,6 @@ bool mailbox_is_spam(struct mailbox *box);
 bool mailbox_is_trash(struct mailbox *box);
 bool mailbox_is_unsure(struct mailbox *box);
 const char *get_setting(const char *name);
+bool antispam_can_append_to_spam;
         
 #endif /* _ANTISPAM_PLUGIN_H */
diff --git a/antispam-storage-1.0.c b/antispam-storage-1.0.c
index e4ca9e8..e1b00fa 100644
--- a/antispam-storage-1.0.c
+++ b/antispam-storage-1.0.c
@@ -166,14 +166,55 @@ static int antispam_save_finish(struct mail_save_context *ctx,
 	case MMT_UNINTERESTING:
 		break;
 	case MMT_APPEND:
-		/* Disallow APPENDs to SPAM/UNSURE folders. */
-		if (mailbox_is_spam(save_dest_mail->box) ||
-		    mailbox_is_unsure(save_dest_mail->box)) {
+		/* Disallow APPENDs to UNSURE folders. */
+		if (mailbox_is_unsure(save_dest_mail->box)) {
 			ret = -1;
 			mail_storage_set_error(save_dest_mail->box->storage,
-					"Cannot APPEND to this folder.");
+					"Cannot APPEND to an UNSURE folder.");
+			break;
+		} else if (mailbox_is_spam(save_dest_mail->box)) {
+			/*
+			 * The client is APPENDing a message to a SPAM folder
+			 * so we try to train the backend on it. For most of
+			 * the backends, that can only succeed if the message
+			 * contains appropriate information.
+			 *
+			 * This happens especially when offlineimap is used and
+			 * the user moved a message to the SPAM folder while
+			 * offline---offlineimap cannot reproduce the COPY but
+			 * rather APPENDs the moved message on the next sync.
+			 *
+			 * This could be a bad if the spam headers were not
+			 * generated on our server, but since the user can
+			 * always APPEND to another folder and then COPY to a
+			 * SPAM folder backends need to be prepared for cases
+			 * like this anyway. With dspam, for example, the worst
+			 * that can happen is that the APPEND fails with a
+			 * training error from dspam.
+			 *
+			 * Unfortunately, we cannot handle the cases where
+			 *  (1) the user moved a message from one folder that
+			 *      contains SPAM to another folder containing SPAM
+			 *  (2) the user moved a message out of the SPAM folder
+			 *  (3) the user recovered a message from trash
+			 *
+			 * Because of these limitations, this behaviour needs
+			 * to be enabled with an option.
+			 */
+			if (!antispam_can_append_to_spam) {
+				ret = -1;
+				mail_storage_set_error(
+					save_dest_mail->box->storage,
+					"Cannot APPEND to a SPAM folder.");
+				break;
+			}
+			asbox->movetype = MMT_TO_SPAM;
+			/* fall through to default case to invoke backend */
+		} else {
+			/* neither UNSURE nor SPAM, regular folder */
+			break;
 		}
-		break;
+		/* fall through */
 	default:
 		ret = backend_handle_mail(ctx->transaction, ast, save_dest_mail,
 					  move_to_class(asbox->movetype));
diff --git a/antispam.7 b/antispam.7
index 855728c..b63ac1e 100644
--- a/antispam.7
+++ b/antispam.7
@@ -114,6 +114,11 @@ plugin {
     # semicolon-separated list of unsure folders (default unset i.e. none)
     # antispam_unsure =
 
+    # Whether to allow APPENDing to SPAM folders or not. Must be set to
+    # "yes" (case insensitive) to be activated. Before activating, please
+    # read the discussion below.
+    # antispam_allow_append_to_spam = no
+
     ###########################
     # BACKEND SPECIFIC OPTIONS
     #
@@ -161,6 +166,23 @@ plugin {
 }
 .fi
 
+.SH ALLOWING APPENDS?
+
+You should be careful with allowing APPENDs to SPAM folders. The reason
+for possibly allowing it is to allow not-SPAM --> SPAM transitions to work
+with offlineimap. However, because with APPEND the plugin cannot know the
+source of the message, multiple bad scenarios can happen:
+
+.IP " 1." 4
+SPAM --> SPAM transitions cannot be recognised and are trained
+
+.IP " 2." 4
+the same holds for Trash --> SPAM transitions
+
+.PP
+Additionally, because we cannot recognise SPAM --> not-SPAM transitions,
+training good messages will never work with APPEND.
+
 .SH AUTHORS
 
 Johannes Berg, Frank Cusack, Benedikt Boehm, Andreas Schneider
-- 
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