include/common/hpack-tbl.h

/*
 * HPACK header table management (RFC7541) - type definitions and prototypes
 *
 * Copyright (C) 2014-2017 Willy Tarreau <willy@haproxy.org>
 * Copyright (C) 2017 HAProxy Technologies
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */
#ifndef _COMMON_HPACK_TBL_H
#define _COMMON_HPACK_TBL_H

#include <stdint.h>
#include <stdlib.h>
#include <common/config.h>
#include <common/http-hdr.h>
#include <common/ist.h>

/* Dynamic Headers Table, usable for tables up to 4GB long and values of 64kB-1.
 * The model can be improved by using offsets relative to the table entry's end
 * or to the end of the area, or by moving the descriptors at the end of the
 * table and the data at the beginning. This entry is 8 bytes long, which is 1/4
 * of the bookkeeping planned by the HPACK spec. Thus it saves 24 bytes per
 * header field, meaning that even with a single header, 24 extra bytes can be
 * stored (ie one such descriptor). At 29.2 average bytes per header field as
 * found in the hpack test case, that's slightly more than 1.5kB of space saved
 * from a 4kB block, resulting in contiguous space almost always being
 * available.
 *
 * Principle: the table is stored in a contiguous array containing both the
 * descriptors and the contents. Descriptors are stored at the beginning of the
 * array while contents are stored starting from the end. Most of the time there
 * is enough room left in the table to insert a new header field, thanks to the
 * savings on the descriptor size. Thus by inserting headers from the end it's
 * possible to maximize the delay before a collision of DTEs and data. In order
 * to always insert from the right, we need to keep a reference to the latest
 * inserted element and look before it. The last inserted cell's address defines
 * the lowest konwn address still in use, unless the area wraps in which case
 * the available space lies between the end of the tail and the beginning of the
 * head.
 *
 * In order to detect collisions between data blocks and DTEs, we also maintain
 * an index to the lowest element facing the DTE table, called "front". This one
 * is updated each time an element is inserted before it. Once the buffer wraps,
 * this element doesn't have to be updated anymore until it is released, in
 * which case the buffer doesn't wrap anymore and the front element becomes the
 * head again.
 *
 * Various heuristics are possible concerning the opportunity to wrap the
 * entries to limit the risk of collisions with the DTE, but experimentation
 * shows that thanks to the important savings made on the descriptors, the
 * likeliness of finding a large amount of free space at the end of the area is
 * much higher than the risk of colliding, so in the end the most naive
 * algorithms work pretty fine. Typical ratios of 1 collision per 2000 requests
 * have been observed.
 *
 * The defragmentation should be rare ; a study on live data shows on average
 * 29.2 bytes used per header field. This plus the 32 bytes overhead fix an
 * average of 66.9 header fields per 4kB table. This brings a 1606 bytes saving
 * using the current storage description, ensuring that oldest headers are
 * linearly removed by the sender before fragmentation occurs. This means that
 * for all smaller header fields there will not be any requirement to defragment
 * the area and most of the time it will even be possible to copy the old values
 * directly within the buffer after creating a new entry. On average within the
 * available space there will be enough room to store 1606/(29.2+8)=43 extra
 * header fields without switching to another place.
 *
 * The table header fits in the table itself, it only takes 16 bytes, so in the
 * worst case (1 single header) it's possible to store 4096 - 16 - 8 = 4072
 * data bytes, which is larger than the 4064 the protocol requires (4096 - 32).
 */

/* One dynamic table entry descriptor */
struct hpack_dte {
	uint32_t addr;  /* storage address, relative to the dte address */
	uint16_t nlen;  /* header name length */
	uint16_t vlen;  /* header value length */
};

/* Note: the table's head plus a struct hpack_dte must be smaller than or equal to 32
 * bytes so that a single large header can always fit. Here that's 16 bytes for
 * the header, plus 8 bytes per slot.
 * Note that when <used> == 0, front, head, and wrap are undefined.
 */
struct hpack_dht {
	uint32_t size;  /* allocated table size in bytes */
	uint32_t total; /* sum of nlen + vlen in bytes */
	uint16_t front; /* slot number of the first node after the idx table */
	uint16_t wrap;  /* number of allocated slots, wraps here */
	uint16_t head;  /* last inserted slot number */
	uint16_t used;  /* number of slots in use */
	struct hpack_dte dte[0]; /* dynamic table entries */
};

/* supported hpack encoding/decoding errors */
enum {
	HPACK_ERR_NONE = 0,           /* no error */
	HPACK_ERR_ALLOC_FAIL,         /* memory allocation error */
	HPACK_ERR_UNKNOWN_OPCODE,     /* invalid first byte */
	HPACK_ERR_TRUNCATED,          /* truncated stream */
	HPACK_ERR_HUFFMAN,            /* huffman decoding error */
	HPACK_ERR_INVALID_PHDR,       /* invalid pseudo header field name */
	HPACK_ERR_MISPLACED_PHDR,     /* pseudo header field after a regular header field */
	HPACK_ERR_DUPLICATE_PHDR,     /* duplicate pseudo header field */
	HPACK_ERR_DHT_INSERT_FAIL,    /* failed to insert into DHT */
	HPACK_ERR_TOO_LARGE,          /* decoded request/response is too large */
	HPACK_ERR_MISSING_METHOD,     /* :method is missing */
	HPACK_ERR_MISSING_SCHEME,     /* :scheme is missing */
	HPACK_ERR_MISSING_PATH,       /* :path is missing */
	HPACK_ERR_MISSING_AUTHORITY,  /* :authority is missing with CONNECT */
	HPACK_ERR_SCHEME_NOT_ALLOWED, /* :scheme not allowed with CONNECT */
	HPACK_ERR_PATH_NOT_ALLOWED,   /* :path not allowed with CONNECT */
};

/* static header table as in RFC7541 Appendix A. [0] unused. */
#define HPACK_SHT_SIZE 62
extern const struct http_hdr hpack_sht[HPACK_SHT_SIZE];

extern int __hpack_dht_make_room(struct hpack_dht *dht, unsigned int needed);
extern int hpack_dht_insert(struct hpack_dht *dht, struct ist name, struct ist value);

/* return a pointer to the entry designated by index <idx> (starting at 1) or
 * NULL if this index is not there.
 */
static inline const struct hpack_dte *hpack_get_dte(const struct hpack_dht *dht, uint16_t idx)
{
	idx--;

	if (idx >= dht->used)
		return NULL;

	if (idx <= dht->head)
		idx = dht->head - idx;
	else
		idx = dht->head - idx + dht->wrap;

	return &dht->dte[idx];
}

/* returns non-zero if <idx> is valid for table <dht> */
static inline int hpack_valid_idx(const struct hpack_dht *dht, uint32_t idx)
{
	return idx < dht->used + HPACK_SHT_SIZE;
}

/* return a pointer to the header name for entry <dte>. */
static inline struct ist hpack_get_name(const struct hpack_dht *dht, const struct hpack_dte *dte)
{
	struct ist ret = {
		.ptr = (void *)dht + dte->addr,
		.len = dte->nlen,
	};
	return ret;
}

/* return a pointer to the header value for entry <dte>. */
static inline struct ist hpack_get_value(const struct hpack_dht *dht, const struct hpack_dte *dte)
{
	struct ist ret = {
		.ptr = (void *)dht + dte->addr + dte->nlen,
		.len = dte->vlen,
	};
	return ret;
}

/* takes an idx, returns the associated name */
static inline struct ist hpack_idx_to_name(const struct hpack_dht *dht, uint32_t idx)
{
	const struct hpack_dte *dte;

	if (idx < HPACK_SHT_SIZE)
		return hpack_sht[idx].n;

	dte = hpack_get_dte(dht, idx - HPACK_SHT_SIZE + 1);
	if (!dte)
		return ist("### ERR ###"); // error

	return hpack_get_name(dht, dte);
}

/* takes an idx, returns the associated value */
static inline struct ist hpack_idx_to_value(const struct hpack_dht *dht, uint32_t idx)
{
	const struct hpack_dte *dte;

	if (idx < HPACK_SHT_SIZE)
		return hpack_sht[idx].v;

	dte = hpack_get_dte(dht, idx - HPACK_SHT_SIZE + 1);
	if (!dte)
		return ist("### ERR ###"); // error

	return hpack_get_value(dht, dte);
}

/* Purges table dht until a header field of <needed> bytes fits according to
 * the protocol (adding 32 bytes overhead). Returns non-zero on success, zero
 * on failure (ie: table empty but still not sufficient).
 */
static inline int hpack_dht_make_room(struct hpack_dht *dht, unsigned int needed)
{
	if (dht->used * 32 + dht->total + needed + 32 <= dht->size)
		return 1;
	else if (!dht->used)
		return 0;

	return __hpack_dht_make_room(dht, needed);
}

/* allocate a dynamic headers table of <size> bytes and return it initialized */
static inline void hpack_dht_init(struct hpack_dht *dht, uint32_t size)
{
	dht->size = size;
	dht->total = 0;
	dht->used = 0;
}

/* allocate a dynamic headers table of <size> bytes and return it initialized */
static inline struct hpack_dht *hpack_dht_alloc(uint32_t size)
{
	struct hpack_dht *dht;

	dht = malloc(size);
	if (!dht)
		return dht;

	hpack_dht_init(dht, size);
	return dht;
}

/* free a dynamic headers table */
static inline void hpack_dht_free(struct hpack_dht *dht)
{
	free(dht);
}

#endif /* _COMMON_HPACK_TBL_H */