Provide a new WPACKET function for filling in all the lengths

For the psk extension we need to fill in all the lengths of the message so
far, even though we haven't closed the WPACKET yet. This provides a function
to do that.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)

ssl/packet.c

diff --git a/ssl/packet.c b/ssl/packet.c
index 12321e7..87473fb 100644
--- a/ssl/packet.c
+++ b/ssl/packet.c
@@ -180,12 +180,13 @@
 
 
 /*
- * Internal helper function used by WPACKET_close() and WPACKET_finish() to
- * close a sub-packet and write out its length if necessary.
+ * Internal helper function used by WPACKET_close(), WPACKET_finish() and
+ * WPACKET_fill_lengths() to close a sub-packet and write out its length if
+ * necessary. If |doclose| is 0 then it goes through the motions of closing
+ * (i.e. it fills in all the lengths), but doesn't actually close anything.
  */
-static int wpacket_intern_close(WPACKET *pkt)
+static int wpacket_intern_close(WPACKET *pkt, WPACKET_SUB *sub, int doclose)
 {
-    WPACKET_SUB *sub = pkt->subs;
     size_t packlen = pkt->written - sub->pwritten;
 
     if (packlen == 0
@@ -194,6 +195,10 @@
 
     if (packlen == 0
             && sub->flags & WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH) {
+        /* We can't handle this case. Return an error */
+        if (!doclose)
+            return 0;
+
         /* Deallocate any bytes allocated for the length of the WPACKET */
         if ((pkt->curr - sub->lenbytes) == sub->packet_len) {
             pkt->written -= sub->lenbytes;
@@ -211,8 +216,28 @@
                               sub->lenbytes))
             return 0;
 
-    pkt->subs = sub->parent;
-    OPENSSL_free(sub);
+    if (doclose) {
+        pkt->subs = sub->parent;
+        OPENSSL_free(sub);
+    }
+
+    return 1;
+}
+
+int WPACKET_fill_lengths(WPACKET *pkt)
+{
+    WPACKET_SUB *sub;
+
+    assert(pkt->subs != NULL);
+    if (pkt->subs == NULL)
+        return 0;
+
+    sub = pkt->subs;
+    do {
+        if (!wpacket_intern_close(pkt, sub, 0))
+            return 0;
+        sub = sub->parent;
+    } while (sub != NULL);
 
     return 1;
 }
@@ -226,7 +251,7 @@
     if (pkt->subs == NULL || pkt->subs->parent == NULL)
         return 0;
 
-    return wpacket_intern_close(pkt);
+    return wpacket_intern_close(pkt, pkt->subs, 1);
 }
 
 int WPACKET_finish(WPACKET *pkt)
@@ -240,7 +265,7 @@
     if (pkt->subs == NULL || pkt->subs->parent != NULL)
         return 0;
 
-    ret = wpacket_intern_close(pkt);
+    ret = wpacket_intern_close(pkt, pkt->subs, 1);
     if (ret) {
         OPENSSL_free(pkt->subs);
         pkt->subs = NULL;

ssl/packet_locl.h

diff --git a/ssl/packet_locl.h b/ssl/packet_locl.h
index 61233d9..cd70265 100644
--- a/ssl/packet_locl.h
+++ b/ssl/packet_locl.h
@@ -700,6 +700,15 @@
 int WPACKET_finish(WPACKET *pkt);
 
 /*
+ * Iterates through all the sub-packets and writes out their lengths as if they
+ * were being closed. The lengths will be overwritten with the final lengths
+ * when the sub-packets are eventually closed (which may be different if more
+ * data is added to the WPACKET). This function will fail if a sub-packet is of
+ * 0 length and WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH is used.
+ */
+int WPACKET_fill_lengths(WPACKET *pkt);
+
+/*
  * Initialise a new sub-packet. Additionally |lenbytes| of data is preallocated
  * at the start of the sub-packet to store its length once we know it. Don't
  * call this directly. Use the convenience macros below instead.