mem leak due to `ares__hosts_entry_to_hostent()` allocation strategy (#824)

`ares__hosts_entry_to_hostent()` would allocate a separate buffer for
each address, but `ares_free_hostent()` expects a single allocation to
hold all addresses.

This PR fixes this issue and simplifies the logic by using the
already-existing `ares__addrinfo2hostent()` to write the hostent instead
of coming up with yet another way to write the structure.

Fixes #823
Fix By: Brad House (@bradh352)

src/lib/ares__hosts_file.c

@@ -857,124 +857,6 @@ ares_status_t ares__hosts_search_host(ares_channel_t *channel,
   return ARES_SUCCESS;
 }
 
-ares_status_t ares__hosts_entry_to_hostent(const ares_hosts_entry_t *entry,
-                                           int family, struct hostent **hostent)
-{
-  ares_status_t       status;
-  size_t              naliases;
-  ares__llist_node_t *node;
-  size_t              idx;
-
-  *hostent = ares_malloc_zero(sizeof(**hostent));
-  if (*hostent == NULL) {
-    status = ARES_ENOMEM;
-    goto fail;
-  }
-
-  (*hostent)->h_addrtype = (HOSTENT_ADDRTYPE_TYPE)family;
-
-  /* Copy IP addresses that match the address family */
-  idx = 0;
-  for (node = ares__llist_node_first(entry->ips); node != NULL;
-       node = ares__llist_node_next(node)) {
-    struct ares_addr addr;
-    const void      *ptr     = NULL;
-    size_t           ptr_len = 0;
-    const char      *ipaddr  = ares__llist_node_val(node);
-    char           **temp    = NULL;
-
-    memset(&addr, 0, sizeof(addr));
-
-    addr.family = family;
-    ptr         = ares_dns_pton(ipaddr, &addr, &ptr_len);
-    if (ptr == NULL) {
-      continue;
-    }
-
-    /* If family == AF_UNSPEC, then we want to inherit this for future
-     * conversions as we can only support a single address class */
-    if (family == AF_UNSPEC) {
-      family                 = addr.family;
-      (*hostent)->h_addrtype = (HOSTENT_ADDRTYPE_TYPE)addr.family;
-    }
-
-    temp = ares_realloc_zero((*hostent)->h_addr_list,
-                             (idx + 1) * sizeof(*(*hostent)->h_addr_list),
-                             (idx + 2) * sizeof(*(*hostent)->h_addr_list));
-    if (temp == NULL) {
-      status = ARES_ENOMEM;
-      goto fail;
-    }
-
-    (*hostent)->h_addr_list = temp;
-
-    (*hostent)->h_addr_list[idx] = ares_malloc(ptr_len);
-    if ((*hostent)->h_addr_list[idx] == NULL) {
-      status = ARES_ENOMEM;
-      goto fail;
-    }
-
-    memcpy((*hostent)->h_addr_list[idx], ptr, ptr_len);
-    idx++;
-    (*hostent)->h_length = (HOSTENT_LENGTH_TYPE)ptr_len;
-  }
-
-  /* entry didn't match address class */
-  if (idx == 0) {
-    status = ARES_ENOTFOUND;
-    goto fail;
-  }
-
-  /* Copy main hostname */
-  (*hostent)->h_name = ares_strdup(ares__llist_first_val(entry->hosts));
-  if ((*hostent)->h_name == NULL) {
-    status = ARES_ENOMEM;
-    goto fail;
-  }
-
-  /* Copy aliases */
-  naliases = ares__llist_len(entry->hosts) - 1;
-
-  /* Cap at 100, some people use https://github.com/StevenBlack/hosts and we
-   * don't need 200k+ aliases */
-  if (naliases > 100) {
-    naliases = 100;
-  }
-
-  (*hostent)->h_aliases =
-    ares_malloc_zero((naliases + 1) * sizeof(*(*hostent)->h_aliases));
-  if ((*hostent)->h_aliases == NULL) {
-    status = ARES_ENOMEM;
-    goto fail;
-  }
-
-  /* Copy all entries to the alias except the first */
-  idx  = 0;
-  node = ares__llist_node_first(entry->hosts);
-  node = ares__llist_node_next(node);
-  while (node != NULL) {
-    (*hostent)->h_aliases[idx] = ares_strdup(ares__llist_node_val(node));
-    if ((*hostent)->h_aliases[idx] == NULL) {
-      status = ARES_ENOMEM;
-      goto fail;
-    }
-    idx++;
-
-    /* Break out if artificially capped */
-    if (idx == naliases) {
-      break;
-    }
-    node = ares__llist_node_next(node);
-  }
-
-  return ARES_SUCCESS;
-
-fail:
-  ares_free_hostent(*hostent);
-  *hostent = NULL;
-  return status;
-}
-
 static ares_status_t
   ares__hosts_ai_append_cnames(const ares_hosts_entry_t    *entry,
                                struct ares_addrinfo_cname **cnames_out)
@@ -1068,10 +950,12 @@ ares_status_t ares__hosts_entry_to_addrinfo(const ares_hosts_entry_t *entry,
       return ARES_EBADFAMILY;
   }
 
-  ai->name = ares_strdup(name);
-  if (ai->name == NULL) {
-    status = ARES_ENOMEM;
-    goto done;
+  if (name != NULL) {
+    ai->name = ares_strdup(name);
+    if (ai->name == NULL) {
+      status = ARES_ENOMEM;
+      goto done;
+    }
   }
 
   for (node = ares__llist_node_first(entry->ips); node != NULL;
@@ -1117,3 +1001,35 @@ done:
 
   return status;
 }
+
+ares_status_t ares__hosts_entry_to_hostent(const ares_hosts_entry_t *entry,
+                                           int family, struct hostent **hostent)
+{
+  ares_status_t         status;
+  struct ares_addrinfo *ai = ares_malloc_zero(sizeof(*ai));
+
+  *hostent = NULL;
+
+  if (ai == NULL) {
+    return ARES_ENOMEM;
+  }
+
+  status = ares__hosts_entry_to_addrinfo(entry, NULL, family, 0, ARES_TRUE, ai);
+  if (status != ARES_SUCCESS) {
+    goto done;
+  }
+
+  status = ares__addrinfo2hostent(ai, family, hostent);
+  if (status != ARES_SUCCESS) {
+    goto done;
+  }
+
+done:
+  ares_freeaddrinfo(ai);
+  if (status != ARES_SUCCESS) {
+    ares_free_hostent(*hostent);
+    *hostent = NULL;
+  }
+
+  return status;
+}