Impove cache behaviour for TCP connections.

For ease of implementaion, dnsmasq has always forked a new process to
handle each incoming TCP connection. A side-effect of this is that any
DNS queries answered from TCP connections are not cached: when TCP
connections were rare, this was not a problem.  With the coming of
DNSSEC, it's now the case that some DNSSEC queries have answers which
spill to TCP, and if, for instance, this applies to the keys for the
root then those never get cached, and performance is very bad.  This
fix passes cache entries back from the TCP child process to the main
server process, and fixes the problem.

src/blockdata.c

@@ -61,7 +61,7 @@ void blockdata_report(void)
 	      blockdata_alloced * sizeof(struct blockdata));
 } 
 
-struct blockdata *blockdata_alloc(char *data, size_t len)
+static struct blockdata *blockdata_alloc_real(int fd, char *data, size_t len)
 {
   struct blockdata *block, *ret = NULL;
   struct blockdata **prev = &ret;
@@ -89,8 +89,17 @@ struct blockdata *blockdata_alloc(char *data, size_t len)
 	blockdata_hwm = blockdata_count; 
       
       blen = len > KEYBLOCK_LEN ? KEYBLOCK_LEN : len;
-      memcpy(block->key, data, blen);
-      data += blen;
+      if (data)
+	{
+	  memcpy(block->key, data, blen);
+	  data += blen;
+	}
+      else if (!read_write(fd, block->key, blen, 1))
+	{
+	  /* failed read free partial chain */
+	  blockdata_free(ret);
+	  return NULL;
+	}
       len -= blen;
       *prev = block;
       prev = &block->next;
@@ -100,6 +109,10 @@ struct blockdata *blockdata_alloc(char *data, size_t len)
   return ret;
 }
 
+struct blockdata *blockdata_alloc(char *data, size_t len)
+{
+  return blockdata_alloc_real(0, data, len);
+}
 
 void blockdata_free(struct blockdata *blocks)
 {
@@ -148,5 +161,21 @@ void *blockdata_retrieve(struct blockdata *block, size_t len, void *data)
 
   return data;
 }
- 
+
+
+void blockdata_write(struct blockdata *block, size_t len, int fd)
+{
+  for (; len > 0 && block; block = block->next)
+    {
+      size_t blen = len > KEYBLOCK_LEN ? KEYBLOCK_LEN : len;
+      read_write(fd, block->key, blen, 0);
+      len -= blen;
+    }
+}
+
+struct blockdata *blockdata_read(int fd, size_t len)
+{
+  return blockdata_alloc_real(fd, NULL, len);
+}
+
 #endif