libknot/rdataset: add knot_rdataset_gather()

It's needed for https://gitlab.labs.nic.cz/knot/resolver/issues/93

src/libknot/rdataset.c

@@ -22,6 +22,7 @@
 #include "libknot/attribute.h"
 #include "libknot/rdataset.h"
 #include "libknot/errcode.h"
+#include "contrib/macros.h"
 #include "contrib/mempattern.h"
 
 static knot_rdata_t *rr_seek(knot_rdata_t *d, size_t pos)
@@ -240,6 +241,36 @@ int knot_rdataset_add(knot_rdataset_t *rrs, const knot_rdata_t *rr, knot_mm_t *m
 	return add_rr_at(rrs, rr, rrs->rr_count, mm);
 }
 
+_public_
+int knot_rdataset_gather(knot_rdataset_t *dst, knot_rdata_t **src, int src_len,
+		knot_mm_t *mm)
+{
+	if (unlikely(dst == NULL || src == NULL)) {
+		return KNOT_EINVAL;
+	}
+
+	int size = 0;
+	for (int i = 0; i < src_len; ++i) {
+		size += knot_rdata_array_size(knot_rdata_rdlen(src[i]));
+	}
+
+	knot_rdata_t *data = mm_alloc(mm, size);
+	if (unlikely(data == NULL)) {
+		return KNOT_ENOMEM;
+	}
+
+	mm_free(mm, dst->data);
+	dst->rr_count = src_len;
+	dst->data = data;
+
+	for (int i = 0; i < src_len; ++i) {
+		int len = knot_rdata_array_size(knot_rdata_rdlen(src[i]));
+		memcpy(data, src[i], len);
+		data += len;
+	}
+	return KNOT_EOK;
+}
+
 _public_
 int knot_rdataset_reserve(knot_rdataset_t *rrs, size_t size, knot_mm_t *mm)
 {

src/libknot/rdataset.h

@@ -71,6 +71,16 @@ int knot_rdataset_copy(knot_rdataset_t *dst, const knot_rdataset_t *src, knot_mm
  */
 knot_rdata_t *knot_rdataset_at(const knot_rdataset_t *rrs, size_t pos);
 
+/*!
+ * \brief Makes RRS structure from a list of resource records (in the given order).
+ * \param dst      RRS structure to write into.
+ * \param src      Array of records to copy from.
+ * \param src_len  The number of records to copy.
+ * \param mm       Memory context.
+ */
+int knot_rdataset_gather(knot_rdataset_t *dst, knot_rdata_t **src, int src_len,
+		knot_mm_t *mm);
+
 /*!
  * \brief Returns size of the structures holding the RR set.
  * \param rrs  RR array.