所以, 所谓的"安全"迭代器, 其在内部实现时: 在迭代过程中, 若字典正处于平滑扩容过程, 则暂停结点迁移, 直至迭代器运行结束. 这样虽然不能保证在迭代过程中插入的结点会被遍历到, 但至少保证在迭代起始时, 字典中持有的所有结点都会被遍历到.
这也是为什么dict结构中有一个iterators字段的原因: 该字段记录了运行于该字典上的安全迭代器的数目. 若该数目不为0, 字典是不会继续进行结点迁移平滑扩容的.
下面是字典的扩容操作中的核心代码, 我们以插入操作引起的扩容为例:
先是插入操作的外部逻辑:
如果插入时, 字典正处于平滑扩容过程中, 那么无论本次插入是否成功, 先迁移一个bucket索引中的结点至新表
在计算新插入结点键的bucket索引值时, 内部会探测哈希表是否需要扩容(若当前不在平滑扩容过程中)
int dictAdd(dict *d, void *key, void *val) { dictEntry *entry = dictAddRaw(d,key,NULL); // 调用dictAddRaw if (!entry) return DICT_ERR; dictSetVal(d, entry, val); return DICT_OK; } dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing) { long index; dictEntry *entry; dictht *ht; if (dictIsRehashing(d)) _dictRehashStep(d); // 若在平滑扩容过程中, 先步进迁移一个bucket索引 /* Get the index of the new element, or -1 if * the element already exists. */ // 在计算键在bucket中的索引值时, 内部会检查是否需要扩容 if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1) return NULL; /* Allocate the memory and store the new entry. * Insert the element in top, with the assumption that in a database * system it is more likely that recently added entries are accessed * more frequently. */ ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0]; entry = zmalloc(sizeof(*entry)); entry->next = ht->table[index]; ht->table[index] = entry; ht->used++; /* Set the hash entry fields. */ dictSetKey(d, entry, key); return entry; }下面是计算bucket索引值的函数, 内部会探测该哈希表是否需要扩容, 如果需要扩容(结点数目与bucket数组长度比例达到1:1), 就使字典进入平滑扩容过程:
static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing) { unsigned long idx, table; dictEntry *he; if (existing) *existing = NULL; /* Expand the hash table if needed */ if (_dictExpandIfNeeded(d) == DICT_ERR) // 探测是否需要扩容, 如果需要, 则开始扩容 return -1; for (table = 0; table <= 1; table++) { idx = hash & d->ht[table].sizemask; /* Search if this slot does not already contain the given key */ he = d->ht[table].table[idx]; while(he) { if (key==he->key || dictCompareKeys(d, key, he->key)) { if (existing) *existing = he; return -1; } he = he->next; } if (!dictIsRehashing(d)) break; } return idx; } /* Expand the hash table if needed */ static int _dictExpandIfNeeded(dict *d) { /* Incremental rehashing already in progress. Return. */ if (dictIsRehashing(d)) return DICT_OK; // 如果正在扩容过程中, 则什么也不做 /* If the hash table is empty expand it to the initial size. */ // 若字典中本无元素, 则初始化字典, 初始化时的bucket数组长度为4 if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE); /* If we reached the 1:1 ratio, and we are allowed to resize the hash * table (global setting) or we should avoid it but the ratio between * elements/buckets is over the "safe" threshold, we resize doubling * the number of buckets. */ // 若字典中元素的个数与bucket数组长度比值大于1:1时, 则调用dictExpand进入平滑扩容状态 if (d->ht[0].used >= d->ht[0].size && (dict_can_resize || d->ht[0].used/d->ht[0].size > dict_force_resize_ratio)) { return dictExpand(d, d->ht[0].used*2); } return DICT_OK; } int dictExpand(dict *d, unsigned long size) { dictht n; /* the new hash table */ // 新建一个dictht结构 unsigned long realsize = _dictNextPower(size); /* the size is invalid if it is smaller than the number of * elements already inside the hash table */ if (dictIsRehashing(d) || d->ht[0].used > size) return DICT_ERR; /* Rehashing to the same table size is not useful. */ if (realsize == d->ht[0].size) return DICT_ERR; /* Allocate the new hash table and initialize all pointers to NULL */ n.size = realsize; n.sizemask = realsize-1; n.table = zcalloc(realsize*sizeof(dictEntry*));// 初始化dictht下的table, 即bucket数组 n.used = 0; /* Is this the first initialization? If so it's not really a rehashing * we just set the first hash table so that it can accept keys. */ // 若是新字典初始化, 直接把dictht结构挂在ht[0]中 if (d->ht[0].table == NULL) { d->ht[0] = n; return DICT_OK; } // 否则, 把新dictht结构挂在ht[1]中, 并开启平滑扩容(置rehashidx为0, 字典处于非扩容状态时, 该字段值为-1) /* Prepare a second hash table for incremental rehashing */ d->ht[1] = n; d->rehashidx = 0; return DICT_OK; }