MINOR: peers: accept to learn strings of different lengths

While analysing old bug (9d9179b) with Emeric, we first believed
that the fix was wrong and that there was a potential for learning
one extra character in the peers learning code for strings due to
the use of table->key_size instead of table->key_size-1. In fact it
cannot happen with a normally behaving sender because the key sizes
are compared when synchronizing the table.

But this unveiled a suboptimal handling of strings. It can be quite
common to see admins reload haproxy to increase some key sizes when
seeing that user agents or cookies get truncated, or conversely to
reduce them after seeing they take too much memory and are never full.
The problem is that this will get rid of the table's contents because
of the size mismatch. While this is understandable for properly
formatted data (eg: IP addresses, integers, SSLIDs...) it's too bad
for strings.

So instead, make an exception to accept string of incompatible lengths
and let the synchronization code truncate them to the appropriate size
just as if the keys were learned normally.

Thanks to this change, it is now possible to change the "len" parameter
of a string stick-table and restart without losing its contents.

src/peers.c

@@ -390,8 +390,13 @@ switchstate:
 				for (st = curpeers->tables; st; st = st->next) {
 					/* If table name matches */
 					if (strcmp(st->table->id, trash.str) == 0) {
-						/* If key size mismatches */
-						if (key_size != st->table->key_size) {
+						/* Check key size mismatches, except for strings
+						 * which may be truncated as long as they fit in
+						 * a buffer.
+						 */
+						if (key_size != st->table->key_size &&
+						    (key_type != STKTABLE_TYPE_STRING ||
+						     1 + 4 + 4 + key_size - 1 >= trash.size)) {
 							si->applet.st0 = PEER_SESSION_EXIT;
 							si->applet.st1 = PEER_SESSION_ERRSIZE;
 							goto switchstate;
@@ -625,32 +630,55 @@ switchstate:
 						pushack = ntohl(netinteger);
 					}
 
-					/* read key. We try to read it directly
-					 * to the target memory location so that
-					 * we are certain there is always enough
-					 * space. However, if the allocation fails,
-					 * we fall back to trash and we ignore the
-					 * input data not to block the sync of other
-					 * tables (think about a full table with
-					 * "nopurge" for example).
+					/* Read key. The string keys are read in two steps, the first step
+					 * consists in reading whatever fits into the table directly into
+					 * the pre-allocated key. The second step consists in simply
+					 * draining all exceeding data. This can happen for example after a
+					 * config reload with a smaller key size for the stick table than
+					 * what was previously set, or when facing the impossibility to
+					 * allocate a new stksess (for example when the table is full with
+					 * "nopurge").
 					 */
 					if (ps->table->table->type == STKTABLE_TYPE_STRING) {
+						unsigned int to_read, to_store;
+
 						/* read size first */
 						reql = bo_getblk(si->ob, (char *)&netinteger, sizeof(netinteger), totl);
 						if (reql <= 0) /* closed or EOL not found */
 							goto incomplete;
 
 						totl += reql;
-						newts = stksess_new(ps->table->table, NULL);
-						reql = bo_getblk(si->ob, newts ? (char *)newts->key.key : trash.str, ntohl(netinteger), totl);
-						if (reql <= 0) /* closed or EOL not found */
+
+						to_store = 0;
+						to_read = ntohl(netinteger);
+
+						if (to_read + totl > si->ob->buf->size) {
+							/* impossible to read a key this large, abort */
+							reql = -1;
 							goto incomplete;
+						}
 
-						/* always truncate the string to the approprite size */
+						newts = stksess_new(ps->table->table, NULL);
 						if (newts)
-							newts->key.key[MIN(ntohl(netinteger), ps->table->table->key_size)] = 0;
+							to_store = MIN(to_read, ps->table->table->key_size - 1);
 
-						totl += reql;
+						/* we read up to two blocks, the first one goes into the key,
+						 * the rest is drained into the trash.
+						 */
+						if (to_store) {
+							reql = bo_getblk(si->ob, (char *)newts->key.key, to_store, totl);
+							if (reql <= 0) /* closed or incomplete */
+								goto incomplete;
+							newts->key.key[reql] = 0;
+							totl += reql;
+							to_read -= reql;
+						}
+						if (to_read) {
+							reql = bo_getblk(si->ob, trash.str, to_read, totl);
+							if (reql <= 0) /* closed or incomplete */
+								goto incomplete;
+							totl += reql;
+						}
 					}
 					else if (ps->table->table->type == STKTABLE_TYPE_INTEGER) {
 						newts = stksess_new(ps->table->table, NULL);