write-commands: generate proper array indexes for local bson documents.

We were generating empty keys here, which is fine, but not really
expected by the lower components. Let's go ahead and just generate
the key values.

Author: Christian Hergert

src/mongoc/mongoc-write-command-private.h

@@ -76,6 +76,7 @@ typedef struct
    bson_t       writeConcernErrors;
    bool         failed;
    bson_error_t error;
+   uint32_t     upsert_append_count;
 } mongoc_write_result_t;
 
 

src/mongoc/mongoc-write-command.c

@@ -851,14 +851,22 @@ _mongoc_write_result_append_upsert (mongoc_write_result_t *result,
                                     const bson_value_t    *value)
 {
    bson_t child;
+   const char *keyptr = NULL;
+   char key[12];
+   int len;
 
    BSON_ASSERT (result);
    BSON_ASSERT (value);
 
-   bson_append_document_begin (&result->upserted, "", -1, &child);
+   len = bson_uint32_to_string (result->upsert_append_count, &keyptr, key,
+                                sizeof key);
+
+   bson_append_document_begin (&result->upserted, keyptr, len, &child);
    BSON_APPEND_INT32 (&child, "index", idx);
    BSON_APPEND_VALUE (&child, "_id", value);
    bson_append_document_end (&result->upserted, &child);
+
+   result->upsert_append_count++;
 }
 
 
@@ -992,7 +1000,11 @@ _mongoc_write_result_merge_arrays (mongoc_write_result_t *result, /* IN */
    bson_iter_t citer;
    int32_t idx;
    int32_t count = 0;
+   int32_t aridx;
    bson_t child;
+   const char *keyptr = NULL;
+   char key[12];
+   int len;
 
    ENTRY;
 
@@ -1001,11 +1013,14 @@ _mongoc_write_result_merge_arrays (mongoc_write_result_t *result, /* IN */
    BSON_ASSERT (iter);
    BSON_ASSERT (BSON_ITER_HOLDS_ARRAY (iter));
 
+   aridx = bson_count_keys (dest);
+
    if (bson_iter_recurse (iter, &ar)) {
       while (bson_iter_next (&ar)) {
          if (BSON_ITER_HOLDS_DOCUMENT (&ar) &&
              bson_iter_recurse (&ar, &citer)) {
-            bson_append_document_begin (dest, "", 0, &child);
+            len = bson_uint32_to_string (aridx++, &keyptr, key, sizeof key);
+            bson_append_document_begin (dest, keyptr, len, &child);
             while (bson_iter_next (&citer)) {
                if (BSON_ITER_IS_KEY (&citer, "index")) {
                   idx = bson_iter_int32 (&citer) + result->offset;