imap: support PREAUTH

It is a defined possible greeting at server startup that means the
connection is already authenticated. See
https://tools.ietf.org/html/rfc3501#section-7.1.4

Test 846 added to verify.

Fixes #1818
Closes #1820

lib/imap.c

@@ -254,6 +254,8 @@ static bool imap_endofresp(struct connectdata *conn, char *line, size_t len,
       *resp = 'N';
     else if(len >= 3 && !memcmp(line, "BAD", 3))
       *resp = 'B';
+    else if(len >= 7 && !memcmp(line, "PREAUTH", 7))
+      *resp = 'P';
     else {
       failf(conn->data, "Bad tagged response");
       *resp = -1;
@@ -563,9 +565,10 @@ static CURLcode imap_perform_authentication(struct connectdata *conn)
   struct imap_conn *imapc = &conn->proto.imapc;
   saslprogress progress;
 
-  /* Check we have enough data to authenticate with and end the
-     connect phase if we don't */
-  if(!Curl_sasl_can_authenticate(&imapc->sasl, conn)) {
+  /* Check if already authenticated OR if there is enough data to authenticate
+     with and end the connect phase if we don't */
+  if(imapc->preauth ||
+     !Curl_sasl_can_authenticate(&imapc->sasl, conn)) {
     state(conn, IMAP_STOP);
     return result;
   }
@@ -789,19 +792,21 @@ static CURLcode imap_state_servergreet_resp(struct connectdata *conn,
                                             int imapcode,
                                             imapstate instate)
 {
-  CURLcode result = CURLE_OK;
   struct Curl_easy *data = conn->data;
-
   (void)instate; /* no use for this yet */
 
-  if(imapcode != 'O') {
-    failf(data, "Got unexpected imap-server response");
-    result = CURLE_WEIRD_SERVER_REPLY;
+  if(imapcode == 'P') {
+    /* PREAUTH */
+    struct imap_conn *imapc = &conn->proto.imapc;
+    imapc->preauth = TRUE;
+    infof(data, "PREAUTH connection, already authenticated!\n");
+  }
+  else if(imapcode != 'O') {
+    failf(data, "Got unexpected imap-server response");
+    return CURLE_WEIRD_SERVER_REPLY;
   }
-  else
-    result = imap_perform_capability(conn);
 
-  return result;
+  return imap_perform_capability(conn);
 }
 
 /* For CAPABILITY responses */

lib/imap.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 2009 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 2009 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -71,6 +71,7 @@ struct imap_conn {
   struct pingpong pp;
   imapstate state;            /* Always use imap.c:state() to change state! */
   bool ssldone;               /* Is connect() over SSL done? */
+  bool preauth;               /* Is this connection PREAUTH? */
   struct SASL sasl;           /* SASL-related parameters */
   unsigned int preftype;      /* Preferred authentication type */
   int cmdid;                  /* Last used command ID */

tests/data/Makefile.inc

@@ -87,7 +87,7 @@ test809 test810 test811 test812 test813 test814 test815 test816 test817 \
 test818 test819 test820 test821 test822 test823 test824 test825 test826 \
 test827 test828 test829 test830 test831 test832 test833 test834 test835 \
 test836 test837 test838 test839 test840 test841 test842 test843 test844 \
-test845 \
+test845 test846 \
 \
 test850 test851 test852 test853 test854 test855 test856 test857 test858 \
 test859 test860 test861 test862 test863 test864 test865 test866 test867 \

tests/data/test846

@@ -0,0 +1,50 @@
+<testcase>
+<info>
+<keywords>
+IMAP
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<servercmd>
+REPLY welcome * PREAUTH ready to serve already!
+REPLY CAPABILITY * CAPABILITY IMAP4REV1 I18NLEVEL=1 LITERAL+ IDLE UIDPLUS NAMESPACE CHILDREN MAILBOX-REFERRALS BINARY UNSELECT ESEARCH WITHIN SCAN SORT THREAD=REFERENCES THREAD=ORDEREDSUBJECT MULTIAPPEND SASL-IR LOGIN-REFERRALS STARTTLS LOGINDISABLED\r\nA001 OK CAPABILITY completed
+</servercmd>
+<data>
+From: me@somewhere
+To: fake@nowhere
+
+body
+
+--
+  yours sincerely
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+imap
+</server>
+ <name>
+IMAP PREAUTH response
+ </name>
+ <command>
+'imap://%HOSTIP:%IMAPPORT/846/;UID=1' -u notused:still-provided
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+A001 CAPABILITY
+A002 SELECT 846
+A003 FETCH 1 BODY[]
+A004 LOGOUT
+</protocol>
+</verify>
+</testcase>