--- a/src/lib-storage/index/imapc/imapc-mail.c	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-mail.c	Fri Apr 22 00:21:12 2016 +0300
@@ -96,6 +96,26 @@
 	return fix_broken_mail ? 0 : -1;
 }
 
+static uint64_t imapc_mail_get_modseq(struct mail *_mail)
+{
+	struct imapc_mailbox *mbox = (struct imapc_mailbox *)_mail->box;
+	struct imapc_msgmap *msgmap;
+	const uint64_t *modseqs;
+	unsigned int count;
+	uint32_t rseq;
+
+	if (!imapc_storage_has_modseqs(mbox->storage))
+		return index_mail_get_modseq(_mail);
+
+	msgmap = imapc_client_mailbox_get_msgmap(mbox->client_box);
+	if (imapc_msgmap_uid_to_rseq(msgmap, _mail->uid, &rseq)) {
+		modseqs = array_get(&mbox->rseq_modseqs, &count);
+		if (rseq <= count)
+			return modseqs[rseq-1];
+	}
+	return 1; /* unknown modseq */
+}
+
 static int imapc_mail_get_received_date(struct mail *_mail, time_t *date_r)
 {
 	struct index_mail *mail = (struct index_mail *)_mail;
@@ -561,7 +581,7 @@
 	index_mail_get_flags,
 	index_mail_get_keywords,
 	index_mail_get_keyword_indexes,
-	index_mail_get_modseq,
+	imapc_mail_get_modseq,
 	index_mail_get_pvt_modseq,
 	index_mail_get_parts,
 	index_mail_get_date,

--- a/src/lib-storage/index/imapc/imapc-mailbox.c	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-mailbox.c	Fri Apr 22 00:21:12 2016 +0300
@@ -2,6 +2,7 @@
 
 #include "lib.h"
 #include "ioloop.h"
+#include "mail-index-modseq.h"
 #include "imap-arg.h"
 #include "imap-seqset.h"
 #include "imap-util.h"
@@ -282,11 +283,12 @@
 	uint32_t lseq, rseq = reply->num;
 	struct imapc_fetch_request *const *fetch_requestp;
 	struct imapc_mail *const *mailp;
-	const struct imap_arg *list, *flags_list;
+	const struct imap_arg *list, *flags_list, *modseq_list;
 	const char *atom, *guid = NULL;
 	const struct mail_index_record *rec = NULL;
 	enum mail_flags flags;
 	uint32_t fetch_uid, uid;
+	uint64_t modseq = 0;
 	unsigned int i, j;
 	ARRAY_TYPE(const_string) keywords = ARRAY_INIT;
 	bool seen_flags = FALSE, have_labels = FALSE;
@@ -319,6 +321,15 @@
 					array_append(&keywords, &atom, 1);
 				}
 			}
+		} else if (strcasecmp(atom, "MODSEQ") == 0 &&
+			   imapc_storage_has_modseqs(mbox->storage)) {
+			/* (modseq-number) */
+			if (!imap_arg_get_list(&list[i+1], &modseq_list))
+				return;
+			if (!imap_arg_get_atom(&modseq_list[0], &atom) ||
+			    str_to_uint64(atom, &modseq) < 0 ||
+			    modseq_list[1].type != IMAP_ARG_EOL)
+				return;
 		} else if (strcasecmp(atom, "X-GM-MSGID") == 0 &&
 			   !mbox->initial_sync_done) {
 			if (imap_arg_get_atom(&list[i+1], &atom))
@@ -414,6 +425,11 @@
 		}
 		mail_index_keywords_unref(&kw);
 	}
+	if (modseq != 0) {
+		if (mail_index_modseq_lookup(mbox->delayed_sync_view, lseq) < modseq)
+			mail_index_update_modseq(mbox->delayed_sync_trans, lseq, modseq);
+		array_idx_set(&mbox->rseq_modseqs, rseq-1, &modseq);
+	}
 	if (guid != NULL) {
 		struct index_mailbox_context *ibox = INDEX_STORAGE_CONTEXT(&mbox->box);
 		const enum index_cache_field guid_cache_idx =
@@ -454,6 +470,7 @@
 	}
 	uid = imapc_msgmap_rseq_to_uid(msgmap, rseq);
 	imapc_msgmap_expunge(msgmap, rseq);
+	array_delete(&mbox->rseq_modseqs, rseq-1, 1);
 
 	imapc_mailbox_init_delayed_trans(mbox);
 	if (mail_index_lookup_seq(mbox->sync_view, uid, &lseq))
@@ -579,6 +596,19 @@
 }
 
 static void
+imapc_resp_text_highestmodseq(const struct imapc_untagged_reply *reply,
+			      struct imapc_mailbox *mbox)
+{
+	uint64_t highestmodseq;
+
+	if (mbox == NULL ||
+	    str_to_uint64(reply->resp_text_value, &highestmodseq) < 0)
+		return;
+
+	mbox->sync_highestmodseq = highestmodseq;
+}
+
+static void
 imapc_resp_text_permanentflags(const struct imapc_untagged_reply *reply,
 			       struct imapc_mailbox *mbox)
 {
@@ -646,6 +676,8 @@
 					 imapc_resp_text_uidvalidity);
 	imapc_mailbox_register_resp_text(mbox, "UIDNEXT",
 					 imapc_resp_text_uidnext);
+	imapc_mailbox_register_resp_text(mbox, "HIGHESTMODSEQ",
+					 imapc_resp_text_highestmodseq);
 	imapc_mailbox_register_resp_text(mbox, "PERMANENTFLAGS",
 					 imapc_resp_text_permanentflags);
 }

--- a/src/lib-storage/index/imapc/imapc-settings.c	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-settings.c	Fri Apr 22 00:21:12 2016 +0300
@@ -93,6 +93,7 @@
 	{ "proxyauth", IMAPC_FEATURE_PROXYAUTH },
 	{ "fetch-msn-workarounds", IMAPC_FEATURE_FETCH_MSN_WORKAROUNDS },
 	{ "fetch-fix-broken-mails", IMAPC_FEATURE_FETCH_FIX_BROKEN_MAILS },
+	{ "modseq", IMAPC_FEATURE_MODSEQ },
 	{ NULL, 0 }
 };
 

--- a/src/lib-storage/index/imapc/imapc-settings.h	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-settings.h	Fri Apr 22 00:21:12 2016 +0300
@@ -14,7 +14,8 @@
 	IMAPC_FEATURE_NO_EXAMINE		= 0x40,
 	IMAPC_FEATURE_PROXYAUTH			= 0x80,
 	IMAPC_FEATURE_FETCH_MSN_WORKAROUNDS	= 0x100,
-	IMAPC_FEATURE_FETCH_FIX_BROKEN_MAILS	= 0x200
+	IMAPC_FEATURE_FETCH_FIX_BROKEN_MAILS	= 0x200,
+	IMAPC_FEATURE_MODSEQ			= 0x400
 };
 /* </settings checks> */
 

--- a/src/lib-storage/index/imapc/imapc-storage.c	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-storage.c	Fri Apr 22 00:21:12 2016 +0300
@@ -55,6 +55,9 @@
 				  struct imapc_storage_client *client);
 static void imapc_untagged_namespace(const struct imapc_untagged_reply *reply,
 				     struct imapc_storage_client *client);
+static int imapc_mailbox_run_status(struct mailbox *box,
+				    enum mailbox_status_items items,
+				    struct mailbox_status *status_r);
 
 bool imap_resp_text_code_parse(const char *str, enum mail_error *error_r)
 {
@@ -72,6 +75,16 @@
 	return FALSE;
 }
 
+bool imapc_storage_has_modseqs(struct imapc_storage *storage)
+{
+	enum imapc_capability capa =
+		imapc_client_get_capabilities(storage->client->client);
+
+	return (capa & (IMAPC_CAPABILITY_CONDSTORE |
+			IMAPC_CAPABILITY_QRESYNC)) != 0 &&
+		IMAPC_HAS_FEATURE(storage, IMAPC_FEATURE_MODSEQ);
+}
+
 static struct mail_storage *imapc_storage_alloc(void)
 {
 	struct imapc_storage *storage;
@@ -603,6 +616,13 @@
 		return -1;
 	}
 
+	if (imapc_storage_has_modseqs(mbox->storage)) {
+		if (!array_is_created(&mbox->rseq_modseqs))
+			i_array_init(&mbox->rseq_modseqs, 32);
+		else
+			array_clear(&mbox->rseq_modseqs);
+	}
+
 	if (imapc_mailbox_select(mbox) < 0) {
 		mailbox_close(box);
 		return -1;
@@ -635,6 +655,8 @@
 		if (mail_index_transaction_commit(&mbox->delayed_sync_trans) < 0)
 			mailbox_set_index_error(&mbox->box);
 	}
+	if (array_is_created(&mbox->rseq_modseqs))
+		array_free(&mbox->rseq_modseqs);
 	if (mbox->sync_view != NULL)
 		mail_index_view_close(&mbox->sync_view);
 	if (mbox->to_idle_delay != NULL)
@@ -727,6 +749,9 @@
 			status->uidvalidity = num;
 		else if (strcasecmp(key, "UNSEEN") == 0)
 			status->unseen = num;
+		else if (strcasecmp(key, "HIGHESTMODSEQ") == 0 &&
+			 imapc_storage_has_modseqs(storage))
+			status->highest_modseq = num;
 	}
 }
 
@@ -765,15 +790,32 @@
 	}
 }
 
-static void imapc_mailbox_get_selected_status(struct imapc_mailbox *mbox,
-					      enum mailbox_status_items items,
-					      struct mailbox_status *status_r)
+static int imapc_mailbox_get_selected_status(struct imapc_mailbox *mbox,
+					     enum mailbox_status_items items,
+					     struct mailbox_status *status_r)
 {
+	int ret = 0;
+
 	index_storage_get_open_status(&mbox->box, items, status_r);
 	if ((items & STATUS_PERMANENT_FLAGS) != 0)
 		status_r->permanent_flags = mbox->permanent_flags;
 	if ((items & STATUS_FIRST_RECENT_UID) != 0)
 		status_r->first_recent_uid = mbox->highest_nonrecent_uid + 1;
+	if ((items & STATUS_HIGHESTMODSEQ) != 0) {
+		/* FIXME: this doesn't work perfectly. we're now just returning
+		   the HIGHESTMODSEQ from the current index, which may or may
+		   not be correct. with QRESYNC enabled we could be returning
+		   sync_highestmodseq, but that would require implementing
+		   VANISHED replies. and without QRESYNC we'd have to issue
+		   STATUS (HIGHESTMODSEQ), which isn't efficient since we get
+		   here constantly (after every IMAP command). */
+	}
+	if (imapc_storage_has_modseqs(mbox->storage)) {
+		/* even if local indexes are only in memory, we still
+		   have modseqs on the IMAP server itself. */
+		status_r->nonpermanent_modseqs = FALSE;
+	}
+	return ret;
 }
 
 static int imapc_mailbox_delete(struct mailbox *box)
@@ -802,6 +844,9 @@
 		str_append(str, " UIDVALIDITY");
 	if ((items & STATUS_UNSEEN) != 0)
 		str_append(str, " UNSEEN");
+	if ((items & STATUS_HIGHESTMODSEQ) != 0 &&
+	    imapc_storage_has_modseqs(mbox->storage))
+		str_append(str, " HIGHESTMODSEQ");
 
 	if (str_len(str) == 0) {
 		/* nothing requested */
@@ -833,14 +878,17 @@
 		status_r->have_guids = TRUE;
 
 	if (box->opened) {
-		imapc_mailbox_get_selected_status(mbox, items, status_r);
+		if (imapc_mailbox_get_selected_status(mbox, items, status_r) < 0) {
+			/* can't do anything about this */
+		}
 	} else if ((items & (STATUS_FIRST_UNSEEN_SEQ | STATUS_KEYWORDS |
 			     STATUS_PERMANENT_FLAGS |
 			     STATUS_FIRST_RECENT_UID)) != 0) {
 		/* getting these requires opening the mailbox */
 		if (mailbox_open(box) < 0)
 			return -1;
-		imapc_mailbox_get_selected_status(mbox, items, status_r);
+		if (imapc_mailbox_get_selected_status(mbox, items, status_r) < 0)
+			return -1;
 	} else {
 		if (imapc_mailbox_run_status(box, items, status_r) < 0)
 			return -1;

--- a/src/lib-storage/index/imapc/imapc-storage.h	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-storage.h	Fri Apr 22 00:21:12 2016 +0300
@@ -108,9 +108,11 @@
 	enum mail_flags permanent_flags;
 	uint32_t highest_nonrecent_uid;
 
+	ARRAY(uint64_t) rseq_modseqs;
 	ARRAY_TYPE(uint32_t) delayed_expunged_uids;
 	uint32_t sync_uid_validity;
 	uint32_t sync_uid_next;
+	uint64_t sync_highestmodseq;
 	uint32_t sync_fetch_first_uid;
 	uint32_t sync_next_lseq;
 	uint32_t sync_next_rseq;
@@ -165,6 +167,7 @@
 void imapc_mail_cache_free(struct imapc_mail_cache *cache);
 int imapc_mailbox_select(struct imapc_mailbox *mbox);
 
+bool imapc_storage_has_modseqs(struct imapc_storage *storage);
 bool imap_resp_text_code_parse(const char *str, enum mail_error *error_r);
 void imapc_copy_error_from_reply(struct imapc_storage *storage,
 				 enum mail_error default_error,

--- a/src/lib-storage/index/imapc/imapc-sync.c	Fri Apr 22 20:31:02 2016 +0300
+++ b/src/lib-storage/index/imapc/imapc-sync.c	Fri Apr 22 00:21:12 2016 +0300
@@ -5,6 +5,7 @@
 #include "str.h"
 #include "imap-util.h"
 #include "mail-cache.h"
+#include "mail-index-modseq.h"
 #include "index-sync-private.h"
 #include "imapc-client.h"
 #include "imapc-msgmap.h"
@@ -245,6 +246,13 @@
 	}
 }
 
+static void imapc_sync_highestmodseq(struct imapc_sync_context *ctx)
+{
+	if (imapc_storage_has_modseqs(ctx->mbox->storage) &&
+	    mail_index_modseq_get_highest(ctx->sync_view) < ctx->mbox->sync_highestmodseq)
+		mail_index_update_highest_modseq(ctx->trans, ctx->mbox->sync_highestmodseq);
+}
+
 static void
 imapc_initial_sync_check(struct imapc_sync_context *ctx, bool nooped)
 {
@@ -311,6 +319,10 @@
 	string_t *cmd = t_str_new(64);
 
 	str_printfa(cmd, "UID FETCH %u:* (FLAGS", first_uid);
+	if (imapc_storage_has_modseqs(ctx->mbox->storage)) {
+		str_append(cmd, " MODSEQ");
+		mail_index_modseq_enable(ctx->mbox->box.index);
+	}
 	if (IMAPC_BOX_HAS_FEATURE(ctx->mbox, IMAPC_FEATURE_GMAIL_MIGRATION)) {
 		enum mailbox_info_flags flags;
 
@@ -393,8 +405,9 @@
 		imapc_mailbox_run(mbox);
 	array_free(&ctx->expunged_uids);
 
-	/* add uidnext after all appends */
+	/* add uidnext & highestmodseq after all appends */
 	imapc_sync_uid_next(ctx);
+	imapc_sync_highestmodseq(ctx);
 
 	if (!ctx->failed)
 		imapc_sync_expunge_eom(ctx);