From b8a8c5061eada0ce3339b24ba1d587134552bc0c Mon Sep 17 00:00:00 2001

From: Stefan Eissing <stefan.eissing@greenbytes.de>

Date: Wed, 29 Jul 2020 14:41:38 +0200

Subject: [PATCH]  * Removing support for abandoned draft of http-wg regarding

 cache-digests.

---

diff --git a/mod_http2/h2_push.c b/mod_http2/h2_push.c

index 4a70674..8ae0b49 100644

--- a/mod_http2/h2_push.c

+++ b/mod_http2/h2_push.c

@@ -464,33 +464,6 @@ apr_array_header_t *h2_push_collect(apr_pool_t *p, const h2_request *req,

     return NULL;

 }

-/*******************************************************************************

- * push diary 

- *

- * - The push diary keeps track of resources already PUSHed via HTTP/2 on this

- *   connection. It records a hash value from the absolute URL of the resource

- *   pushed.

- * - Lacking openssl, it uses 'apr_hashfunc_default' for the value

- * - with openssl, it uses SHA256 to calculate the hash value

- * - whatever the method to generate the hash, the diary keeps a maximum of 64

- *   bits per hash, limiting the memory consumption to about 

- *      H2PushDiarySize * 8 

- *   bytes. Entries are sorted by most recently used and oldest entries are

- *   forgotten first.

- * - Clients can initialize/replace the push diary by sending a 'Cache-Digest'

- *   header. Currently, this is the base64url encoded value of the cache digest

- *   as specified in https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/

- *   This draft can be expected to evolve and the definition of the header

- *   will be added there and refined.

- * - The cache digest header is a Golomb Coded Set of hash values, but it may

- *   limit the amount of bits per hash value even further. For a good description

- *   of GCS, read here:

- *      http://giovanni.bajo.it/post/47119962313/golomb-coded-sets-smaller-than-bloom-filters

- * - The means that the push diary might be initialized with hash values of much

- *   less than 64 bits, leading to more false positives, but smaller digest size.

- ******************************************************************************/

- 

- 

 #define GCSLOG_LEVEL   APLOG_TRACE1

 typedef struct h2_push_diary_entry {

@@ -618,38 +591,48 @@ static int h2_push_diary_find(h2_push_diary *diary, apr_uint64_t hash)

     return -1;

 }

-static h2_push_diary_entry *move_to_last(h2_push_diary *diary, apr_size_t idx)

+static void move_to_last(h2_push_diary *diary, apr_size_t idx)

 {

     h2_push_diary_entry *entries = (h2_push_diary_entry*)diary->entries->elts;

     h2_push_diary_entry e;

-    apr_size_t lastidx = (apr_size_t)diary->entries->nelts;

+    int lastidx;

+    /* Move an existing entry to the last place */

+    if (diary->entries->nelts <= 0)

+        return;

+

     /* move entry[idx] to the end */

-    if (idx+1 < lastidx) {

+    lastidx = diary->entries->nelts - 1;

+    if (idx < lastidx) {

         e =  entries[idx];

-        memmove(entries+idx, entries+idx+1, sizeof(e) * (lastidx - idx));

+        memmove(entries+idx, entries+idx+1, sizeof(h2_push_diary_entry) * (lastidx - idx));

         entries[lastidx] = e;

     }

-    return &entries[lastidx];

 }

-static void h2_push_diary_append(h2_push_diary *diary, h2_push_diary_entry *e)

+static void remove_first(h2_push_diary *diary)

 {

-    h2_push_diary_entry *ne;

+    h2_push_diary_entry *entries = (h2_push_diary_entry*)diary->entries->elts;

+    int lastidx;

-    if (diary->entries->nelts < diary->N) {

-        /* append a new diary entry at the end */

-        APR_ARRAY_PUSH(diary->entries, h2_push_diary_entry) = *e;

-        ne = &APR_ARRAY_IDX(diary->entries, diary->entries->nelts-1, h2_push_diary_entry);

+    /* move remaining entries to index 0 */

+    lastidx = diary->entries->nelts - 1;

+    if (lastidx > 0) {

+        --diary->entries->nelts;

+        memmove(entries, entries+1, sizeof(h2_push_diary_entry) * diary->entries->nelts);

     }

-    else {

-        /* replace content with new digest. keeps memory usage constant once diary is full */

-        ne = move_to_last(diary, 0);

-        *ne = *e;

+}

+

+static void h2_push_diary_append(h2_push_diary *diary, h2_push_diary_entry *e)

+{

+    while (diary->entries->nelts >= diary->N) {

+        remove_first(diary);

     }

+    /* append a new diary entry at the end */

+    APR_ARRAY_PUSH(diary->entries, h2_push_diary_entry) = *e;

     /* Intentional no APLOGNO */

     ap_log_perror(APLOG_MARK, GCSLOG_LEVEL, 0, diary->entries->pool,

-                  "push_diary_append: %"APR_UINT64_T_HEX_FMT, ne->hash);

+                  "push_diary_append: %"APR_UINT64_T_HEX_FMT, e->hash);

 }

 apr_array_header_t *h2_push_diary_update(h2_session *session, apr_array_header_t *pushes)

@@ -692,30 +675,12 @@ apr_array_header_t *h2_push_collect_update(h2_stream *stream,

                                            const struct h2_request *req, 

                                            const struct h2_headers *res)

 {

-    h2_session *session = stream->session;

-    const char *cache_digest = apr_table_get(req->headers, "Cache-Digest");

     apr_array_header_t *pushes;

-    apr_status_t status;

-    if (cache_digest && session->push_diary) {

-        status = h2_push_diary_digest64_set(session->push_diary, req->authority, 

-                                            cache_digest, stream->pool);

-        if (status != APR_SUCCESS) {

-            ap_log_cerror(APLOG_MARK, APLOG_DEBUG, status, session->c,

-                          H2_SSSN_LOG(APLOGNO(03057), session,

-                          "push diary set from Cache-Digest: %s"), cache_digest);

-        }

-    }

     pushes = h2_push_collect(stream->pool, req, stream->push_policy, res);

     return h2_push_diary_update(stream->session, pushes);

 }

-static apr_int32_t h2_log2inv(unsigned char log2)

-{

-    return log2? (1 << log2) : 1;

-}

-

-

 typedef struct {

     h2_push_diary *diary;

     unsigned char log2p;

@@ -830,11 +795,6 @@ apr_status_t h2_push_diary_digest_get(h2_push_diary *diary, apr_pool_t *pool,

     apr_size_t hash_count;

     nelts = diary->entries->nelts;

-    

-    if ((apr_uint32_t)nelts > APR_UINT32_MAX) {

-        /* should not happen */

-        return APR_ENOTIMPL;

-    }

     N = ceil_power_of_2(nelts);

     log2n = h2_log2(N);

@@ -896,166 +856,3 @@ apr_status_t h2_push_diary_digest_get(h2_push_diary *diary, apr_pool_t *pool,

     return APR_SUCCESS;

 }

-typedef struct {

-    h2_push_diary *diary;

-    apr_pool_t *pool;

-    unsigned char log2p;

-    const unsigned char *data;

-    apr_size_t datalen;

-    apr_size_t offset;

-    unsigned int bit;

-    apr_uint64_t last_val;

-} gset_decoder;

-

-static int gset_decode_next_bit(gset_decoder *decoder)

-{

-    if (++decoder->bit >= 8) {

-        if (++decoder->offset >= decoder->datalen) {

-            return -1;

-        }

-        decoder->bit = 0;

-    }

-    return (decoder->data[decoder->offset] & cbit_mask[decoder->bit])? 1 : 0;

-}

-

-static apr_status_t gset_decode_next(gset_decoder *decoder, apr_uint64_t *phash)

-{

-    apr_uint64_t flex = 0, fixed = 0, delta;

-    int i;

-    

-    /* read 1 bits until we encounter 0, then read log2n(diary-P) bits.

-     * On a malformed bit-string, this will not fail, but produce results

-     * which are pbly too large. Luckily, the diary will modulo the hash.

-     */

-    while (1) {

-        int bit = gset_decode_next_bit(decoder);

-        if (bit == -1) {

-            return APR_EINVAL;

-        }

-        if (!bit) {

-            break;

-        }

-        ++flex;

-    }

-    

-    for (i = 0; i < decoder->log2p; ++i) {

-        int bit = gset_decode_next_bit(decoder);

-        if (bit == -1) {

-            return APR_EINVAL;

-        }

-        fixed = (fixed << 1) | bit;

-    }

-    

-    delta = (flex << decoder->log2p) | fixed;

-    *phash = delta + decoder->last_val;

-    decoder->last_val = *phash;

-    

-    /* Intentional no APLOGNO */

-    ap_log_perror(APLOG_MARK, GCSLOG_LEVEL, 0, decoder->pool,

-                  "h2_push_diary_digest_dec: val=%"APR_UINT64_T_HEX_FMT", delta=%"

-                  APR_UINT64_T_HEX_FMT", flex=%d, fixed=%"APR_UINT64_T_HEX_FMT, 

-                  *phash, delta, (int)flex, fixed);

-                  

-    return APR_SUCCESS;

-}

-

-/**

- * Initialize the push diary by a cache digest as described in 

- * https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/

- * .

- * @param diary the diary to set the digest into

- * @param data the binary cache digest

- * @param len the length of the cache digest

- * @return APR_EINVAL if digest was not successfully parsed

- */

-apr_status_t h2_push_diary_digest_set(h2_push_diary *diary, const char *authority, 

-                                      const char *data, apr_size_t len)

-{

-    gset_decoder decoder;

-    unsigned char log2n, log2p;

-    int N, i;

-    apr_pool_t *pool = diary->entries->pool;

-    h2_push_diary_entry e;

-    apr_status_t status = APR_SUCCESS;

-    

-    if (len < 2) {

-        /* at least this should be there */

-        return APR_EINVAL;

-    }

-    log2n = data[0];

-    log2p = data[1];

-    diary->mask_bits = log2n + log2p;

-    if (diary->mask_bits > 64) {

-        /* cannot handle */

-        return APR_ENOTIMPL;

-    }

-    

-    /* whatever is in the digest, it replaces the diary entries */

-    apr_array_clear(diary->entries);

-    if (!authority || !strcmp("*", authority)) {

-        diary->authority = NULL;

-    }

-    else if (!diary->authority || strcmp(diary->authority, authority)) {

-        diary->authority = apr_pstrdup(diary->entries->pool, authority);

-    }

-

-    N = h2_log2inv(log2n + log2p);

-

-    decoder.diary    = diary;

-    decoder.pool     = pool;

-    decoder.log2p    = log2p;

-    decoder.data     = (const unsigned char*)data;

-    decoder.datalen  = len;

-    decoder.offset   = 1;

-    decoder.bit      = 8;

-    decoder.last_val = 0;

-    

-    diary->N = N;

-    /* Determine effective N we use for storage */

-    if (!N) {

-        /* a totally empty cache digest. someone tells us that she has no

-         * entries in the cache at all. Use our own preferences for N+mask 

-         */

-        diary->N = diary->NMax;

-        return APR_SUCCESS;

-    }

-    else if (N > diary->NMax) {

-        /* Store not more than diary is configured to hold. We open us up

-         * to DOS attacks otherwise. */

-        diary->N = diary->NMax;

-    }

-    

-    /* Intentional no APLOGNO */

-    ap_log_perror(APLOG_MARK, GCSLOG_LEVEL, 0, pool,

-                  "h2_push_diary_digest_set: N=%d, log2n=%d, "

-                  "diary->mask_bits=%d, dec.log2p=%d", 

-                  (int)diary->N, (int)log2n, diary->mask_bits, 

-                  (int)decoder.log2p);

-                  

-    for (i = 0; i < diary->N; ++i) {

-        if (gset_decode_next(&decoder, &e.hash) != APR_SUCCESS) {

-            /* the data may have less than N values */

-            break;

-        }

-        h2_push_diary_append(diary, &e);

-    }

-    

-    /* Intentional no APLOGNO */

-    ap_log_perror(APLOG_MARK, GCSLOG_LEVEL, 0, pool,

-                  "h2_push_diary_digest_set: diary now with %d entries, mask_bits=%d", 

-                  (int)diary->entries->nelts, diary->mask_bits);

-    return status;

-}

-

-apr_status_t h2_push_diary_digest64_set(h2_push_diary *diary, const char *authority, 

-                                        const char *data64url, apr_pool_t *pool)

-{

-    const char *data;

-    apr_size_t len = h2_util_base64url_decode(&data, data64url, pool);

-    /* Intentional no APLOGNO */

-    ap_log_perror(APLOG_MARK, GCSLOG_LEVEL, 0, pool,

-                  "h2_push_diary_digest64_set: digest=%s, dlen=%d", 

-                  data64url, (int)len);

-    return h2_push_diary_digest_set(diary, authority, data, len);

-}

-

diff --git a/mod_http2/h2_push.h b/mod_http2/h2_push.h

index 0533853..5dc189f 100644

--- a/mod_http2/h2_push.h

+++ b/mod_http2/h2_push.h

@@ -35,6 +35,44 @@ typedef enum {

     H2_PUSH_DIGEST_SHA256

 } h2_push_digest_type;

+/*******************************************************************************

+ * push diary 

+ *

+ * - The push diary keeps track of resources already PUSHed via HTTP/2 on this

+ *   connection. It records a hash value from the absolute URL of the resource

+ *   pushed.

+ * - Lacking openssl, 

+ * - with openssl, it uses SHA256 to calculate the hash value, otherwise it

+ *   falls back to apr_hashfunc_default()

+ * - whatever the method to generate the hash, the diary keeps a maximum of 64

+ *   bits per hash, limiting the memory consumption to about 

+ *      H2PushDiarySize * 8 

+ *   bytes. Entries are sorted by most recently used and oldest entries are

+ *   forgotten first.

+ * - While useful by itself to avoid duplicated PUSHes on the same connection,

+ *   the original idea was that clients provided a 'Cache-Digest' header with

+ *   the values of *their own* cached resources. This was described in

+ *   <https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/> 

+ *   and some subsequent revisions that tweaked values but kept the overall idea.

+ * - The draft was abandoned by the IETF http-wg, as support from major clients,

+ *   e.g. browsers, was lacking for various reasons.

+ * - For these reasons, mod_h2 abandoned its support for client supplied values

+ *   but keeps the diary. It seems to provide value for applications using PUSH,

+ *   is configurable in size and defaults to a very moderate amount of memory

+ *   used.

+ * - The cache digest header is a Golomb Coded Set of hash values, but it may

+ *   limit the amount of bits per hash value even further. For a good description

+ *   of GCS, read here:

+ *   <http://giovanni.bajo.it/post/47119962313/golomb-coded-sets-smaller-than-bloom-filters>

+ ******************************************************************************/

+ 

+ 

+/*

+ * The push diary is based on the abandoned draft 

+ * <https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/>

+ * that describes how to use golomb filters.

+ */

+

 typedef struct h2_push_diary h2_push_diary;

 typedef void h2_push_digest_calc(h2_push_diary *diary, apr_uint64_t *phash, h2_push *push);

@@ -101,20 +139,4 @@ apr_status_t h2_push_diary_digest_get(h2_push_diary *diary, apr_pool_t *p,

                                       int maxP, const char *authority, 

                                       const char **pdata, apr_size_t *plen);

-/**

- * Initialize the push diary by a cache digest as described in 

- * https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/

- * .

- * @param diary the diary to set the digest into

- * @param authority the authority to set the data for

- * @param data the binary cache digest

- * @param len the length of the cache digest

- * @return APR_EINVAL if digest was not successfully parsed

- */

-apr_status_t h2_push_diary_digest_set(h2_push_diary *diary, const char *authority, 

-                                      const char *data, apr_size_t len);

-

-apr_status_t h2_push_diary_digest64_set(h2_push_diary *diary, const char *authority, 

-                                        const char *data64url, apr_pool_t *pool);

-

 #endif /* defined(__mod_h2__h2_push__) */