proxy_cache.c

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    /* get etag */
    if ((etag = ap_table_get(c->hdrs, "Etag"))) {
        wetag = ap_pstrcat(r->pool, "W/", etag, NULL);
    }

    /* check for If-Match, If-Unmodified-Since */
    while (1) {

        /*
         * check If-Match and If-Unmodified-Since exist
         * 
         * If neither of these exist, the request is not conditional, and we
         * serve it normally
         */
        if (!c->im && BAD_DATE == c->ius) {
            break;
        }

        /*
         * check If-Match
         * 
         * we check if the Etag on the cached file is in the list of Etags in
         * the If-Match field. The comparison must be a strong comparison, so
         * the Etag cannot be marked as weak. If the comparision fails we
         * return 412 Precondition Failed.
         * 
         * if If-Match is specified AND If-Match is not a "*" AND Etag is
         * missing or weak or not in the list THEN return 412 Precondition
         * Failed
         */

        if (c->im) {
            if (strcmp(c->im, "*") &&
                (!etag || (strlen(etag) > 1 && 'W' == etag[0] && '/' == etag[1]) || !ap_proxy_liststr(c->im, etag, NULL))) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-Match specified, and it didn't - return 412");
            }
            else {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-Match specified, and it matched");
                break;
            }
        }

        /*
         * check If-Unmodified-Since
         * 
         * if If-Unmodified-Since is specified AND Last-Modified is specified
         * somewhere AND If-Unmodified-Since is in the past compared to
         * Last-Modified THEN return 412 Precondition Failed
         */
        if (BAD_DATE != c->ius && BAD_DATE != c->lmod) {
            if (c->ius < c->lmod) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-Unmodified-Since specified, but it wasn't - return 412");
            }
            else {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-Unmodified-Since specified, and it was unmodified");
                break;
            }
        }

        /* if cache file is being updated */
        if (c->origfp) {
            ap_proxy_write_headers(c, c->resp_line, c->hdrs);
            ap_proxy_send_fb(c->origfp, r, c, c->len, 1, 0, IOBUFSIZE);
            ap_proxy_cache_tidy(c);
        }
        else
            ap_pclosef(r->pool, ap_bfileno(cachefp, B_WR));

        ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "Use your cached copy, conditional precondition failed.");
        return HTTP_PRECONDITION_FAILED;
    }


    /* check for If-None-Match, If-Modified-Since */
    while (1) {

        /*
         * check for existance of If-None-Match and If-Modified-Since
         * 
         * if neither of these headers have been set, then the request is not
         * conditional, and we just send the cached response and be done with
         * it.
         */
        if (!c->inm && BAD_DATE == c->ims) {
            break;
        }

        /*
         * check If-None-Match
         * 
         * we check if the Etag on the cached file is in the list of Etags in
         * the If-None-Match field. The comparison must be a strong
         * comparison, so the Etag cannot be marked as weak. If the
         * comparision fails we return 412 Precondition Failed.
         * 
         * if If-None-Match is specified: if If-None-Match is a "*" THEN 304
         * else if Etag is specified AND we get a match THEN 304 else if Weak
         * Etag is specified AND we get a match THEN 304 else sent the
         * original object
         */
        if (c->inm) {
            if (!strcmp(c->inm, "*")) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-None-Match: * specified, return 304");
            }
            else if (etag && ap_proxy_liststr(c->inm, etag, NULL)) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-None-Match: specified and we got a strong match - return 304");
            }
            else if (wetag && ap_proxy_liststr(c->inm, wetag, NULL)) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-None-Match specified, and we got a weak match - return 304");
            }
            else
                break;
        }

        /*
         * check If-Modified-Since
         * 
         * if If-Modified-Since is specified AND Last-Modified is specified
         * somewhere: if last modification date is earlier than
         * If-Modified-Since THEN 304 else send the original object
         */
        if (BAD_DATE != c->ims && BAD_DATE != c->lmod) {
            if (c->ims >= c->lmod) {
                ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "If-Modified-Since specified and not modified, try return 304");
            }
            else
                break;
        }


        /* are we updating the cache file? */
        if (c->origfp) {
            ap_proxy_write_headers(c, c->resp_line, c->hdrs);
            ap_proxy_send_fb(c->origfp, r, c, c->len, 1, 0, IOBUFSIZE);
            ap_proxy_cache_tidy(c);
        }
        else
            ap_pclosef(r->pool, ap_bfileno(cachefp, B_WR));

        ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "Use local copy, cached file hasn't changed");
        return HTTP_NOT_MODIFIED;
    }


    /* No conditional - just send it cousin! */
    ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "Local copy modified, send it");
    r->status_line = strchr(c->resp_line, ' ') + 1;
    r->status = c->status;

    /* Prepare and send headers to client */
    ap_proxy_table_replace(r->headers_out, c->hdrs);
    /* make sure our X-Cache header does not stomp on a previous header */
    ap_table_mergen(r->headers_out, "X-Cache", c->xcache);

    /* content type is already set in the headers */
    r->content_type = ap_table_get(r->headers_out, "Content-Type");

    ap_send_http_header(r);

    /* are we rewriting the cache file? */
    if (c->origfp) {
        ap_proxy_write_headers(c, c->resp_line, c->hdrs);
        ap_proxy_send_fb(c->origfp, r, c, c->len, r->header_only, 0, IOBUFSIZE);
        ap_proxy_cache_tidy(c);
        return OK;
    }

    /* no, we not */
    if (!r->header_only) {
        ap_proxy_send_fb(cachefp, r, NULL, c->len, 0, 0, IOBUFSIZE);
    }
    else {
        ap_pclosef(r->pool, ap_bfileno(cachefp, B_WR));
    }

    return OK;
}


/*
 * Call this to test for a resource in the cache
 * Returns DECLINED if we need to check the remote host
 * or an HTTP status code if successful
 *
 * Functions:
 *   if URL is cached then
 *      if cached file is not expired then
 *         if last modified after if-modified-since then send body
 *         else send 304 Not modified
 *      else if cached file is expired then
 *         if last modified after if-modified-since then add
 *            last modified date to request
 */
int ap_proxy_cache_check(request_rec *r, char *url, struct cache_conf * conf,
                             cache_req **cr)
{
    const char *datestr, *pragma_req = NULL, *pragma_cresp = NULL, *cc_req = NULL,
        *cc_cresp = NULL;
    cache_req *c;
    BUFF *cachefp;
    int i;
    void *sconf = r->server->module_config;
    proxy_server_conf *pconf =
    (proxy_server_conf *)ap_get_module_config(sconf, &proxy_module);
    const char *agestr = NULL;
    char *val;
    time_t age_c = 0;
    time_t age, maxage_req, maxage_cresp, maxage, smaxage, maxstale, minfresh;

    c = ap_pcalloc(r->pool, sizeof(cache_req));
    *cr = c;
    c->req = r;
    c->url = ap_pstrdup(r->pool, url);
    c->filename = NULL;
    c->tempfile = NULL;
    c->fp = NULL;
    c->origfp = NULL;
    c->version = 0;
    c->len = -1;
    c->req_hdrs = NULL;
    c->hdrs = NULL;
    c->xcache = NULL;

    /* get the If-Modified-Since date of the request, if it exists */
    c->ims = BAD_DATE;
    datestr = ap_table_get(r->headers_in, "If-Modified-Since");
    if (datestr != NULL) {
        /* this may modify the value in the original table */
        datestr = ap_proxy_date_canon(r->pool, datestr);
        c->ims = ap_parseHTTPdate(datestr);
        if (c->ims == BAD_DATE) /* bad or out of range date; remove it */
            ap_table_unset(r->headers_in, "If-Modified-Since");
    }

/* get the If-Unmodified-Since date of the request, if it exists */
    c->ius = BAD_DATE;
    datestr = ap_table_get(r->headers_in, "If-Unmodified-Since");
    if (datestr != NULL) {
        /* this may modify the value in the original table */
        datestr = ap_proxy_date_canon(r->pool, datestr);
        c->ius = ap_parseHTTPdate(datestr);
        if (c->ius == BAD_DATE) /* bad or out of range date; remove it */
            ap_table_unset(r->headers_in, "If-Unmodified-Since");
    }

/* get the If-Match of the request, if it exists */
    c->im = ap_table_get(r->headers_in, "If-Match");

/* get the If-None-Match of the request, if it exists */
    c->inm = ap_table_get(r->headers_in, "If-None-Match");

/* find the filename for this cache entry */
    if (conf->root != NULL) {
        char hashfile[66];
        ap_proxy_hash(url, hashfile, pconf->cache.dirlevels, pconf->cache.dirlength);
        c->filename = ap_pstrcat(r->pool, conf->root, "/", hashfile, NULL);
    }
    else {
        c->filename = NULL;
        c->fp = NULL;
        ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "No CacheRoot, so no caching. Declining.");
        return DECLINED;
    }

/* find certain cache controlling headers */
    pragma_req = ap_table_get(r->headers_in, "Pragma");
    cc_req = ap_table_get(r->headers_in, "Cache-Control");

/* first things first - does the request allow us to return
 * cached information at all? If not, just decline the request.
 *
 * Note that there is a big difference between not being allowed
 * to cache a request (no-store) and not being allowed to return
 * a cached request without revalidation (max-age=0).
 *
 * Caching is forbidden under the following circumstances:
 *
 * - RFC2616 14.9.2 Cache-Control: no-store
 * we are not supposed to store this request at all. Behave as a tunnel.
 *
 */
    if (ap_proxy_liststr(cc_req, "no-store", NULL)) {

/* delete the previously cached file */
        if (c->filename)
            unlink(c->filename);
        c->fp = NULL;
        c->filename = NULL;
        ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "no-store forbids caching. Declining.");
        return DECLINED;
    }

/* if the cache file exists, open it */
    cachefp = NULL;
    ap_log_error(APLOG_MARK, APLOG_DEBUG | APLOG_NOERRNO, r->server, "Request for %s, pragma_req=%s, ims=%ld", url,
                 (pragma_req == NULL) ? "(unset)" : pragma_req, c->ims);
/* find out about whether the request can access the cache */
    if (c->filename != NULL && r->method_number == M_GET &&
        strlen(url) < 1024) {
        cachefp = ap_proxy_open_cachefile(r, c->filename);
    }


    /*
     * if a cache file exists, try reading body and headers from cache file
     */
    if (cachefp != NULL) {
        i = rdcache(r, cachefp, c);
        if (i == -1)
            ap_log_rerror(APLOG_MARK, APLOG_ERR, r,
                          "proxy: error reading cache file %s",
                          c->filename);
        else if (i == 0)
            ap_log_rerror(APLOG_MARK, APLOG_ERR | APLOG_NOERRNO, r,
                          "proxy: bad (short?) cache file: %s", c->filename);
        if (i != 1) {
            ap_pclosef(r->pool, ap_bfileno(cachefp, B_WR));
            cachefp = NULL;
        }
        if (c->hdrs) {
            cc_cresp = ap_table_get(c->hdrs, "Cache-Control");
            pragma_cresp = ap_table_get(c->hdrs, "Pragma");
            if ((agestr = ap_table_get(c->hdrs, "Age"))) {
                age_c = atoi(agestr);
            }
        }
    }

    /* if a cache file does not exist, create empty header array */
/* fixed?  in this case, we want to get the headers from the remote server
   it will be handled later if we don't do this (I hope ;-)

    if (cachefp == NULL)
        c->hdrs = ap_make_table(r->pool, 20);
*/
    /* FIXME: Shouldn't we check the URL somewhere? */

    /*
     * Check Content-Negotiation - Vary
     * 
     * At this point we need to make sure that the object we found in the cache
     * is the same object that would be delivered to the client, when the
     * effects of content negotiation are taken into effect.
     * 
     * In plain english, we want to make sure that a language-negotiated
     * document in one language is not given to a client asking for a
     * language negotiated document in a different language by mistake.
     * 
     * RFC2616 13.6 and 14.44 describe the Vary mechanism.
     */
    if (c->hdrs && c->req_hdrs) {
        char *vary = ap_pstrdup(r->pool, ap_table_get(c->hdrs, "Vary"));

        while (vary && *vary) {