Nginx 源码完全注释（10）ngx_radix_tree

ngx_radix_tree.h

// 未被使用的节点
#define NGX_RADIX_NO_VALUE   (uintptr_t) -1

typedef struct ngx_radix_node_s  ngx_radix_node_t;

struct ngx_radix_node_s {
    ngx_radix_node_t  *right; // 右子树的根节点
    ngx_radix_node_t  *left; // 左子树的根节点
    ngx_radix_node_t  *parent; // 父节点
    uintptr_t          value; // 值域
};

typedef struct {
    ngx_radix_node_t  *root; // 树根
    ngx_pool_t        *pool; // 该树所用的内存池
    ngx_radix_node_t  *free; // 空闲的节点由free开始连成一个链表，节点间通过right指针连接
    char              *start;
    size_t             size;
} ngx_radix_tree_t;

ngx_radix_tree.c

static void *ngx_radix_alloc(ngx_radix_tree_t *tree);

ngx_radix_tree_t *
ngx_radix_tree_create(ngx_pool_t *pool, ngx_int_t preallocate)
{
    uint32_t           key, mask, inc;
    ngx_radix_tree_t  *tree;

    // 为该树的结构体分配内存
    tree = ngx_palloc(pool, sizeof(ngx_radix_tree_t));
    if (tree == NULL) {
        return NULL;
    }

    // 初始化各成员
    tree->pool = pool;
    tree->free = NULL;
    tree->start = NULL;
    tree->size = 0;

    // 为根节点分配内存（实际上不一定有重新的内存分配操作，具体详见ngx_radix_alloc部分）
    tree->root = ngx_radix_alloc(tree);
    if (tree->root == NULL) {
        return NULL;
    }

    // 根节点的初始化
    tree->root->right = NULL;
    tree->root->left = NULL;
    tree->root->parent = NULL;
    tree->root->value = NGX_RADIX_NO_VALUE;

    // 如果指定的预分配节点数为 0，则直接返回这个树就好了
    if (preallocate == 0) {
        return tree;
    }

    /*
     * Preallocation of first nodes : 0, 1, 00, 01, 10, 11, 000, 001, etc.
     * increases TLB hits even if for first lookup iterations.
     * On 32-bit platforms the 7 preallocated bits takes continuous 4K,
     * 8 - 8K, 9 - 16K, etc.  On 64-bit platforms the 6 preallocated bits
     * takes continuous 4K, 7 - 8K, 8 - 16K, etc.  There is no sense to
     * to preallocate more than one page, because further preallocation
     * distributes the only bit per page.  Instead, a random insertion
     * may distribute several bits per page.
     *
     * Thus, by default we preallocate maximum
     *     6 bits on amd64 (64-bit platform and 4K pages)
     *     7 bits on i386 (32-bit platform and 4K pages)
     *     7 bits on sparc64 in 64-bit mode (8K pages)
     *     8 bits on sparc64 in 32-bit mode (8K pages)
     */

    // 下面这部分就很有意思了，你可以看上面的英文注释。简单说，一个 x bits 的值，对应其 Radix 树
    // 有 x + 1 层，那么节点的个数就是 2^(x+1) -1 个（数据结构常识，你也可以很容易证明这个结论）。
    if (preallocate == -1) {

        // 根据 pagesize 大小，确定可以分配多少个 radix 树结构
        switch (ngx_pagesize / sizeof(ngx_radix_tree_t)) {

        /* amd64 */
        case 128:
            preallocate = 6;
            break;

        /* i386, sparc64 */
        case 256:
            preallocate = 7;
            break;

        /* sparc64 in 32-bit mode */
        default:
            preallocate = 8;
        }
    }

    mask = 0;
    inc = 0x80000000;

    // preallocate 为几，最终 mask 就有几个最高位为1，其他为0。整个循环过程中 mask 不断右移并在
    // 最高位添置新 1。
    while (preallocate--) {

        key = 0;
        mask >>= 1;
        mask |= 0x80000000;

        do {
            if (ngx_radix32tree_insert(tree, key, mask, NGX_RADIX_NO_VALUE)
                != NGX_OK)
            {
                return NULL;
            }

            key += inc;

        } while (key);

        inc >>= 1;
    }

    return tree;
}

// mask 为掩码，用于截取 key 中的部分比特位，将其插入到 tree 数中，对应的值为 value
ngx_int_t
ngx_radix32tree_insert(ngx_radix_tree_t *tree, uint32_t key, uint32_t mask,
    uintptr_t value)
{
    uint32_t           bit;
    ngx_radix_node_t  *node, *next;

    bit = 0x80000000;

    node = tree->root;
    next = tree->root;

    while (bit & mask) {
        if (key & bit) {
            next = node->right;

        } else {
            next = node->left;
        }

        // 当前节点为叶子节点，停止循环查找
        if (next == NULL) {
            break;
        }

        bit >>= 1;
        node = next;
    }

    // next 不为 NULL，是因 bit & mask 为 0 退出上面的 while 的
    if (next) {
        if (node->value != NGX_RADIX_NO_VALUE) {
            return NGX_BUSY;
        }

        node->value = value;
        return NGX_OK;
    }

    // next 为 NULL，从 tree 新分配一个节点
    while (bit & mask) {
        next = ngx_radix_alloc(tree);
        if (next == NULL) {
            return NGX_ERROR;
        }

        next->right = NULL;
        next->left = NULL;
        next->parent = node;
        next->value = NGX_RADIX_NO_VALUE;

        if (key & bit) {
            node->right = next;

        } else {
            node->left = next;
        }

        bit >>= 1;
        node = next;
    }

    node->value = value;

    return NGX_OK;
}

// 节点从 Radix 树中删除后，会放入到 free 链表中
ngx_int_t
ngx_radix32tree_delete(ngx_radix_tree_t *tree, uint32_t key, uint32_t mask)
{
    uint32_t           bit;
    ngx_radix_node_t  *node;

    bit = 0x80000000;
    node = tree->root;

    while (node && (bit & mask)) {
        // key 该位为 1，表示接下来找右子树
        if (key & bit) {
            node = node->right;
        // key 该位为 0，表示接下来找左子树
        } else {
            node = node->left;
        }

        bit >>= 1;
    }

    // 要删除的节点不存在
    if (node == NULL) {
        return NGX_ERROR;
    }

    // 要删除的节点还有子节点
    if (node->right || node->left) {
        if (node->value != NGX_RADIX_NO_VALUE) {
            node->value = NGX_RADIX_NO_VALUE;
            return NGX_OK;
        }

        // 要删除的节点有子树，但是该节点的值为无效值，则视为错误
        return NGX_ERROR;
    }

    for ( ;; ) {
        // 如果该节点是右节点
        if (node->parent->right == node) {
            node->parent->right = NULL;
        // 如果该节点是左节点
        } else {
            node->parent->left = NULL;
        }

        node->right = tree->free;
        tree->free = node;

        node = node->parent;

        if (node->right || node->left) {
            break;
        }

        if (node->value != NGX_RADIX_NO_VALUE) {
            break;
        }

        // node 为根节点
        if (node->parent == NULL) {
            break;
        }
    }

    return NGX_OK;
}

// 在 tree 树中查找 key 值，key 是一个无符号的32位整数，每一位对应从树根开始
// 查找时选择左子树（0）还是右子树（1）
uintptr_t
ngx_radix32tree_find(ngx_radix_tree_t *tree, uint32_t key)
{
    uint32_t           bit;
    uintptr_t          value;
    ngx_radix_node_t  *node;

    // 初始状态下最高位为1，用于后面的“与”操作，确定左右子树
    bit = 0x80000000;
    value = NGX_RADIX_NO_VALUE;
    node = tree->root; // 从树根开始

    // 理论上最多循环32次（key为32位），实际上查找到node为NULL，则表明上一轮循环中已经是叶子节点
    while (node) {
        if (node->value != NGX_RADIX_NO_VALUE) {
            value = node->value;
        }

        // 该位为 1 则右子树
        if (key & bit) {
            node = node->right;

        // 该位为 0 则左子树
        } else {
            node = node->left;
        }

        bit >>= 1;
    }

    // 返回找到的节点的值
    return value;
}

static void *
ngx_radix_alloc(ngx_radix_tree_t *tree)
{
    char  *p;

    // 创建Radix树时会调用，此时free为NULL，不会进入该if分支
    // 插入时调用到这里，free 值非零，则返回 free
    if (tree->free) {
        p = (char *) tree->free;
        tree->free = tree->free->right;
        return p;
    }

    // 创建Radix树时会调用，此时tree->size为0，会进入该if分支
    if (tree->size < sizeof(ngx_radix_node_t)) {
        // 以ngx_pagesize大小内存对齐的方式，从内存池tree->pool中分配ngx_pagesize大小的内存给start
        // ngx_pagesize 是在 src/os/unix/ngx_posix_init.c 和 src/os/win32/ngx_win32_init.c
        // 的 ngx_os_init() 函数中初始化的。pagesize 的值与处理器架构有关。
        tree->start = ngx_pmemalign(tree->pool, ngx_pagesize, ngx_pagesize);
        if (tree->start == NULL) {
            return NULL;
        }

        // tree->size 为刚才分配的内存大小
        tree->size = ngx_pagesize;
    }

    // tree->start 加上 ngx_radix_node_t 将要占用的大小
    // tree->size 减去 ngx_radix_node_t 将要占用的大小
    p = tree->start;
    tree->start += sizeof(ngx_radix_node_t);
    tree->size -= sizeof(ngx_radix_node_t);

    // 虽然返回值类型是 void*，但是调用处都会转为 ngx_radix_node_t
    return p;
}