Avoid sending large UID EXPUNGE|FETCH|STORE and APPEND commands.

UID EXPUNGE|FETCH|STORE commands are now split into multiple (sequential)
commands when their set representation exceeds 4096 bytes in size.  Without
splitting logic set representations could grow arbitrarily large, and
exceed the server's maximum command size.

This adds roundtrips which could be eliminated by pipelining, but it's
unlikely to make any difference in typical synchronization work.  While set
representations seem to remain small in practice, they might grow
significantly if many non-contiguous UIDs were flagged and/or expunged, and
later synchronized at once.

Furthermore, for MULTIAPPEND-capable servers, the number of messages is
limited to 128 per APPEND command (also subject to a combined literal size of
1MiB like before).

These numbers are currently not configurable.  They're intentionally lower
than Dovecot's default maximum command size (64k) in order to avoid a
deadlock situation after sending 8k-long commands under COMPRESS=DEFLATE:
https://dovecot.org/pipermail/dovecot/2019-November/117522.html .

1 files changed, 43 insertions, 0 deletions

diff --git a/tests/split-set/t b/tests/split-set/t
new file mode 100644
index 0000000..5e8ea52
--- /dev/null
+++ b/tests/split-set/t
@@ -0,0 +1,43 @@
+N=2048
+
+# XXX with COMPRESS=DEFLATE dovecot-imapd 2.3.4 hangs when the command
+# line exceeds 'imap_max_line_length' (or 8192, whichever is smaller)
+# bytes, instead of returning a tagged BAD response.
+# https://dovecot.org/pipermail/dovecot/2019-November/117522.html
+
+# set UIDNEXT to 10^9 so all uids are 10 chars long, otherwise we'd need
+# to add many more messages to obtain large sets
+doveadm -u "local"  mailbox update --min-next-uid 1000000000 "INBOX"
+doveadm -u "remote" mailbox update --min-next-uid 1000000000 "INBOX"
+
+for ((i = 0; i < N; i++)); do
+    u="$(shuf -n1 -e "local" "remote")"
+    sample_message | deliver -u "$u"
+done
+
+interimap_init
+check_mailbox_status "INBOX"
+
+# mark every other message as \Seen on the local server
+for ((i = 0; i < N; i+=2)); do
+    doveadm -u "local" flags add "\\Seen" mailbox "INBOX" $((N-i))
+done
+
+# send the changes to the remote; this results into an UID STORE set
+# representation of size 11*N/2-1, which exceeds $imap_max_line_length
+interimap
+check_mailbox_status "INBOX"
+
+# now expunge every other message on the remote server; this results
+# into large UID STORE and UID EXPUNGE set representation
+for ((i = 0; i < N; i+=2)); do
+    doveadm -u "local" expunge mailbox "INBOX" $((N-i))
+    # add some more messages
+    u="$(shuf -n1 -e "local" "remote")"
+    sample_message | deliver -u "$u"
+done
+
+interimap || error
+check_mailbox_status "INBOX"
+
+# vim: set filetype=sh :