plugins/gzip/gzip.cc

/** @file

  Transforms content using gzip or deflate

  @section license License

  Licensed to the Apache Software Foundation (ASF) under one
  or more contributor license agreements.  See the NOTICE file
  distributed with this work for additional information
  regarding copyright ownership.  The ASF licenses this file
  to you under the Apache License, Version 2.0 (the
  "License"); you may not use this file except in compliance
  with the License.  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
 */

#include <string>
#include <string.h>
#include <zlib.h>
#include <ts/ts.h>
#include "debug_macros.h"
#include "misc.h"
#include "configuration.h"

#include "ink_defs.h"

using namespace std;
using namespace Gzip;

//FIXME: custom dictionaries would be nice. configurable/content-type?
//FIXME: look into autoscaling the compression level based on connection speed
// a gprs device might benefit from a higher compression ratio, whereas a desktop w. high bandwith
// might be served better with little or no compression at all
//FIXME: look into compressing from the task thread pool
//FIXME: make normalizing accept encoding configurable

// from mod_deflate:
// ZLIB's compression algorithm uses a
// 0-9 based scale that GZIP does where '1' is 'Best speed' 
// and '9' is 'Best compression'. Testing has proved level '6' 
// to be about the best level to use in an HTTP Server. 

const int ZLIB_COMPRESSION_LEVEL = 6;

int arg_idx_hooked;
int arg_idx_host_configuration;
int arg_idx_url_disallowed;


const char * global_hidden_header_name;
Configuration* config = NULL;
const char *dictionary = NULL;

static GzipData *
gzip_data_alloc(int compression_type)
{
  GzipData *data;
  int err;

  data = (GzipData *) TSmalloc(sizeof(GzipData));
  data->downstream_vio = NULL;
  data->downstream_buffer = NULL;
  data->downstream_reader = NULL;
  data->downstream_length = 0;
  data->state = transform_state_initialized;
  data->compression_type = compression_type;
  data->zstrm.next_in = Z_NULL;
  data->zstrm.avail_in = 0;
  data->zstrm.total_in = 0;
  data->zstrm.next_out = Z_NULL;
  data->zstrm.avail_out = 0;
  data->zstrm.total_out = 0;
  data->zstrm.zalloc = gzip_alloc;
  data->zstrm.zfree = gzip_free;
  data->zstrm.opaque = (voidpf) 0;
  data->zstrm.data_type = Z_ASCII;

  int window_bits = (compression_type == COMPRESSION_TYPE_GZIP) ? WINDOW_BITS_GZIP : WINDOW_BITS_DEFLATE;

  err = deflateInit2(&data->zstrm, ZLIB_COMPRESSION_LEVEL, Z_DEFLATED, window_bits, ZLIB_MEMLEVEL, Z_DEFAULT_STRATEGY);

  if (err != Z_OK) {
    fatal("gzip-transform: ERROR: deflateInit (%d)!", err);
  }

  if (dictionary) {
    err = deflateSetDictionary(&data->zstrm, (const Bytef *) dictionary, strlen(dictionary));
    if (err != Z_OK) {
      fatal("gzip-transform: ERROR: deflateSetDictionary (%d)!", err);
    }
  }

  return data;
}


static void
gzip_data_destroy(GzipData * data)
{
  TSReleaseAssert(data);

  //deflateEnd returnvalue ignore is intentional
  //it would spew log on every client abort
  deflateEnd(&data->zstrm);

  if (data->downstream_buffer) {
    TSIOBufferDestroy(data->downstream_buffer);
  }

  TSfree(data);
}

static TSReturnCode
gzip_content_encoding_header(TSMBuffer bufp, TSMLoc hdr_loc, const int compression_type)
{
  TSReturnCode ret;
  TSMLoc ce_loc;

  // Delete Content-Encoding if present???

  if ((ret = TSMimeHdrFieldCreateNamed(bufp, hdr_loc, "Content-Encoding", sizeof("Content-Encoding") - 1, &ce_loc)) == TS_SUCCESS) {
    if (compression_type == COMPRESSION_TYPE_DEFLATE) {
      ret = TSMimeHdrFieldValueStringInsert(bufp, hdr_loc, ce_loc, -1, "deflate", sizeof("deflate") - 1);
    } else if (compression_type == COMPRESSION_TYPE_GZIP) {
      ret = TSMimeHdrFieldValueStringInsert(bufp, hdr_loc, ce_loc, -1, "gzip", sizeof("gzip") - 1);
    }
    if (ret == TS_SUCCESS) {
      ret = TSMimeHdrFieldAppend(bufp, hdr_loc, ce_loc);
    }
    TSHandleMLocRelease(bufp, hdr_loc, ce_loc);
  }

  if (ret != TS_SUCCESS) {
    error("cannot add the Content-Encoding header");
  }

  return ret;
}

static TSReturnCode
gzip_vary_header(TSMBuffer bufp, TSMLoc hdr_loc)
{
  TSReturnCode ret;
  TSMLoc ce_loc;

  ce_loc = TSMimeHdrFieldFind(bufp, hdr_loc, "Vary", sizeof("Vary") - 1);
  if (ce_loc) {
    int idx, count, len; 
    const char *value;

    count = TSMimeHdrFieldValuesCount(bufp, hdr_loc, ce_loc);
    for(idx=0; idx<count; idx++) {
      value = TSMimeHdrFieldValueStringGet(bufp, hdr_loc, ce_loc, idx, &len);
      if (len &&
          strncasecmp("Accept-Encoding", value, len) == 0) {
        // Bail, Vary: Accept-Encoding already sent from origin
        TSHandleMLocRelease(bufp, hdr_loc, ce_loc);
        return TS_SUCCESS;
      }
    }

    ret = TSMimeHdrFieldValueStringInsert(bufp, hdr_loc, ce_loc, -1, "Accept-Encoding", sizeof("Accept-Encoding") - 1);
    TSHandleMLocRelease(bufp, hdr_loc, ce_loc);
  } else {
    if ((ret = TSMimeHdrFieldCreateNamed(bufp, hdr_loc, "Vary", sizeof("Vary") - 1, &ce_loc)) == TS_SUCCESS) {
      if ((ret = TSMimeHdrFieldValueStringInsert(bufp, hdr_loc, ce_loc, -1, "Accept-Encoding", sizeof("Accept-Encoding") - 1)) == TS_SUCCESS) {
        ret = TSMimeHdrFieldAppend(bufp, hdr_loc, ce_loc);
      }

      TSHandleMLocRelease(bufp, hdr_loc, ce_loc);
    }
  }

  if (ret != TS_SUCCESS) {
    error("cannot add/update the Vary header");
  }

  return ret;
}

//FIXME: the etag alteration isn't proper. it should modify the value inside quotes
//       specify a very header..
static TSReturnCode
gzip_etag_header(TSMBuffer bufp, TSMLoc hdr_loc)
{
  TSReturnCode ret = TS_SUCCESS;
  TSMLoc ce_loc;

  ce_loc = TSMimeHdrFieldFind(bufp, hdr_loc, TS_MIME_FIELD_ETAG, TS_MIME_LEN_ETAG);

  if (ce_loc) {
    int changetag = 1;
    int strl;
    const char *strv = TSMimeHdrFieldValueStringGet(bufp, hdr_loc, ce_loc, -1, &strl);
    //do not alter weak etags.
    //FIXME: consider just making the etag weak for compressed content
    if (strl >= 2) {
      if ((strv[0] == 'w' || strv[0] == 'W') && strv[1] == '/') {
        changetag = 0;
      }
      if (changetag) {
        ret = TSMimeHdrFieldValueAppend(bufp, hdr_loc, ce_loc, 0, "-df", 3);
      }
    }
    TSHandleMLocRelease(bufp, hdr_loc, ce_loc);
  }

  if (ret != TS_SUCCESS) {
    error("cannot handle the %s header", TS_MIME_FIELD_ETAG);
  }

  return ret;
}

//FIXME: some things are potentially compressible. those responses
static void
gzip_transform_init(TSCont contp, GzipData * data)
{
  //update the vary, content-encoding, and etag response headers
  //prepare the downstream for transforming

  TSVConn downstream_conn;
  TSMBuffer bufp;
  TSMLoc hdr_loc;

  data->state = transform_state_output;

  if (TSHttpTxnTransformRespGet(data->txn, &bufp, &hdr_loc) != TS_SUCCESS) {
    error("Error TSHttpTxnTransformRespGet");
    return;
  }

  if (gzip_content_encoding_header(bufp, hdr_loc, data->compression_type) == TS_SUCCESS &&
      gzip_vary_header(bufp, hdr_loc) == TS_SUCCESS &&
      gzip_etag_header(bufp, hdr_loc) == TS_SUCCESS) {
    downstream_conn = TSTransformOutputVConnGet(contp);
    data->downstream_buffer = TSIOBufferCreate();
    data->downstream_reader = TSIOBufferReaderAlloc(data->downstream_buffer);
    data->downstream_vio = TSVConnWrite(downstream_conn, contp, data->downstream_reader, INT64_MAX);
  }

  TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
}



static void
gzip_transform_one(GzipData * data, TSIOBufferReader upstream_reader, int amount)
{
  TSIOBufferBlock downstream_blkp;
  const char *upstream_buffer;
  char *downstream_buffer;
  int64_t upstream_length, downstream_length;
  int err;

  while (amount > 0) {
    downstream_blkp = TSIOBufferReaderStart(upstream_reader);
    if (!downstream_blkp) {
      error("couldn't get from IOBufferBlock");
      return;
    }

    upstream_buffer = TSIOBufferBlockReadStart(downstream_blkp, upstream_reader, &upstream_length);
    if (!upstream_buffer) {
      error("couldn't get from TSIOBufferBlockReadStart");
      return;
    }

    if (upstream_length > amount) {
      upstream_length = amount;
    }

    data->zstrm.next_in = (unsigned char *) upstream_buffer;
    data->zstrm.avail_in = upstream_length;

    while (data->zstrm.avail_in > 0) {
      downstream_blkp = TSIOBufferStart(data->downstream_buffer);
      downstream_buffer = TSIOBufferBlockWriteStart(downstream_blkp, &downstream_length);

      data->zstrm.next_out = (unsigned char *) downstream_buffer;
      data->zstrm.avail_out = downstream_length;

      err = deflate(&data->zstrm, Z_NO_FLUSH);

      if (err != Z_OK)
        warning("deflate() call failed: %d", err);

      if (downstream_length > data->zstrm.avail_out) {
        TSIOBufferProduce(data->downstream_buffer, downstream_length - data->zstrm.avail_out);
        data->downstream_length += (downstream_length - data->zstrm.avail_out);
      }

      if (data->zstrm.avail_out > 0) {
        if (data->zstrm.avail_in != 0) {
          error("gzip-transform: ERROR: avail_in is (%d): should be 0", data->zstrm.avail_in);
        }
      }
    }

    TSIOBufferReaderConsume(upstream_reader, upstream_length);
    amount -= upstream_length;
  }
}

static void
gzip_transform_finish(GzipData * data)
{
  if (data->state == transform_state_output) {
    TSIOBufferBlock downstream_blkp;
    char *downstream_buffer;
    int64_t downstream_length;
    int err;

    data->state = transform_state_finished;

    for (;;) {
      downstream_blkp = TSIOBufferStart(data->downstream_buffer);

      downstream_buffer = TSIOBufferBlockWriteStart(downstream_blkp, &downstream_length);
      data->zstrm.next_out = (unsigned char *) downstream_buffer;
      data->zstrm.avail_out = downstream_length;

      err = deflate(&data->zstrm, Z_FINISH);

      if (downstream_length > (int64_t) data->zstrm.avail_out) {
        TSIOBufferProduce(data->downstream_buffer, downstream_length - data->zstrm.avail_out);
        data->downstream_length += (downstream_length - data->zstrm.avail_out);
      }

      if (err == Z_OK) {        /* some more data to encode */
        continue;
      }

      if (err != Z_STREAM_END) {
        warning("deflate should report Z_STREAM_END");
      }
      break;
    }

    if (data->downstream_length != (int64_t) (data->zstrm.total_out)) {
      error("gzip-transform: ERROR: output lengths don't match (%d, %ld)", data->downstream_length,
            data->zstrm.total_out);
    }

    gzip_log_ratio(data->zstrm.total_in, data->downstream_length);
  }
}


static void
gzip_transform_do(TSCont contp)
{
  TSVIO upstream_vio;
  GzipData *data;
  int64_t upstream_todo;
  int64_t upstream_avail;
  int64_t downstream_bytes_written;

  data = (GzipData*)TSContDataGet(contp);
  if (data->state == transform_state_initialized) {
    gzip_transform_init(contp, data);
  }

  upstream_vio = TSVConnWriteVIOGet(contp);
  downstream_bytes_written = data->downstream_length;

  if (!TSVIOBufferGet(upstream_vio)) {
    gzip_transform_finish(data);

    TSVIONBytesSet(data->downstream_vio, data->downstream_length);

    if (data->downstream_length > downstream_bytes_written) {
      TSVIOReenable(data->downstream_vio);
    }
    return;
  }

  upstream_todo = TSVIONTodoGet(upstream_vio);

  if (upstream_todo > 0) {
    upstream_avail = TSIOBufferReaderAvail(TSVIOReaderGet(upstream_vio));

    if (upstream_todo > upstream_avail) {
      upstream_todo = upstream_avail;
    }

    if (upstream_todo > 0) {
      gzip_transform_one(data, TSVIOReaderGet(upstream_vio), upstream_todo);
      TSVIONDoneSet(upstream_vio, TSVIONDoneGet(upstream_vio) + upstream_todo);
    }
  }

  if (TSVIONTodoGet(upstream_vio) > 0) {
    if (upstream_todo > 0) {
      if (data->downstream_length > downstream_bytes_written) {
        TSVIOReenable(data->downstream_vio);
      }
      TSContCall(TSVIOContGet(upstream_vio), TS_EVENT_VCONN_WRITE_READY, upstream_vio);
    }
  } else {
    gzip_transform_finish(data);
    TSVIONBytesSet(data->downstream_vio, data->downstream_length);

    if (data->downstream_length > downstream_bytes_written) {
      TSVIOReenable(data->downstream_vio);
    }

    TSContCall(TSVIOContGet(upstream_vio), TS_EVENT_VCONN_WRITE_COMPLETE, upstream_vio);
  }
}


static int
gzip_transform(TSCont contp, TSEvent event, void * /* edata ATS_UNUSED */)
{
  if (TSVConnClosedGet(contp)) {
    gzip_data_destroy((GzipData*)TSContDataGet(contp));
    TSContDestroy(contp);
    return 0;
  } else {
    switch (event) {
    case TS_EVENT_ERROR:{
        debug("gzip_transform: TS_EVENT_ERROR starts");
        TSVIO upstream_vio = TSVConnWriteVIOGet(contp);
        TSContCall(TSVIOContGet(upstream_vio), TS_EVENT_ERROR, upstream_vio);
      }
      break;
    case TS_EVENT_VCONN_WRITE_COMPLETE:
      TSVConnShutdown(TSTransformOutputVConnGet(contp), 0, 1);
      break;
    case TS_EVENT_VCONN_WRITE_READY:
      gzip_transform_do(contp);
      break;
    case TS_EVENT_IMMEDIATE:
      gzip_transform_do(contp);
      break;
    default:
      warning("unknown event [%d]", event);
      gzip_transform_do(contp);
      break;
    }
  }

  return 0;
}


static int
gzip_transformable(TSHttpTxn txnp, int server, HostConfiguration * host_configuration, int *compress_type)
{
  /* Server response header */
  TSMBuffer bufp;
  TSMLoc hdr_loc;
  TSMLoc field_loc;

  /* Client request header */
  TSMBuffer cbuf;
  TSMLoc chdr;
  TSMLoc cfield;

  const char *value;
  int nvalues;
  int i, compression_acceptable, len;

  TSHttpStatus resp_status;
  if (server) {
    TSHttpTxnServerRespGet(txnp, &bufp, &hdr_loc);
  } else {
    TSHttpTxnCachedRespGet(txnp, &bufp, &hdr_loc);
  }
  resp_status = TSHttpHdrStatusGet(bufp, hdr_loc);
  TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);

  //conservatively pick some statusses to compress
  if (!(resp_status == 200 || resp_status == 404 || resp_status == 500)) {
    info("http response status [%d] is not compressible", resp_status);
    return 0;
  }

  TSHttpTxnClientReqGet(txnp, &cbuf, &chdr);

  //the only compressible method is currently GET.
  int method_length;
  const char *method = TSHttpHdrMethodGet(cbuf, chdr, &method_length);
  if (!(method_length == TS_HTTP_LEN_GET && memcmp(method, TS_HTTP_METHOD_GET, TS_HTTP_LEN_GET) == 0)) {
    debug("method is not GET, not compressible");
    TSHandleMLocRelease(cbuf, TS_NULL_MLOC, chdr);
    return 0;
  }

  cfield = TSMimeHdrFieldFind(cbuf, chdr, TS_MIME_FIELD_ACCEPT_ENCODING, TS_MIME_LEN_ACCEPT_ENCODING);
  if (cfield != TS_NULL_MLOC) {
    compression_acceptable = 0;
    nvalues = TSMimeHdrFieldValuesCount(cbuf, chdr, cfield);
    for (i=0; i<nvalues; i++) {
      value = TSMimeHdrFieldValueStringGet(cbuf, chdr, cfield, i, &len);
      if (!value) {
        continue;
      }

      if (strncasecmp(value, "deflate", sizeof("deflate") - 1) == 0) {
        compression_acceptable = 1;
        *compress_type = COMPRESSION_TYPE_DEFLATE;
        break;
      } else if (strncasecmp(value, "gzip", sizeof("gzip") - 1) == 0) {
        compression_acceptable = 1;
        *compress_type = COMPRESSION_TYPE_GZIP;
        break;
      }
    }

    TSHandleMLocRelease(cbuf, chdr, cfield);
    TSHandleMLocRelease(cbuf, TS_NULL_MLOC, chdr);

    if (!compression_acceptable) {
      info("no acceptable encoding found in request header, not compressible");
      return 0;
    }
  } else {
    info("no acceptable encoding found in request header, not compressible");
    TSHandleMLocRelease(cbuf, chdr, cfield);
    TSHandleMLocRelease(cbuf, TS_NULL_MLOC, chdr);
    return 0;
  }

  if (server) {
    TSHttpTxnServerRespGet(txnp, &bufp, &hdr_loc);
  } else {
    TSHttpTxnCachedRespGet(txnp, &bufp, &hdr_loc);
  }

  /* If there already exists a content encoding then we don't want
     to do anything. */
  field_loc = TSMimeHdrFieldFind(bufp, hdr_loc, TS_MIME_FIELD_CONTENT_ENCODING, -1);
  if (field_loc) {
    info("response is already content encoded, not compressible");
    TSHandleMLocRelease(bufp, hdr_loc, field_loc);
    TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
    return 0;
  }

  /* We only want to do gzip compression on documents that have a
     content type of "text/" or "application/x-javascript". */
  field_loc = TSMimeHdrFieldFind(bufp, hdr_loc, TS_MIME_FIELD_CONTENT_TYPE, -1);
  if (!field_loc) {
    info("no content type header found, not compressible");
    TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
    return 0;
  }

  value = TSMimeHdrFieldValueStringGet(bufp, hdr_loc, field_loc, 0, &len);

  int rv = host_configuration->ContentTypeIsCompressible(value, len);
  if (!rv) { 
    info("content-type [%.*s] not compressible", len, value);
  }
  TSHandleMLocRelease(bufp, hdr_loc, field_loc);
  TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
  return rv;
}


static void
gzip_transform_add(TSHttpTxn txnp, int /* server ATS_UNUSED */, HostConfiguration * hc, int compress_type)
{
  int *tmp = (int *) TSHttpTxnArgGet(txnp, arg_idx_hooked);
  if (tmp) {
    //happens on cache_stale_hit
    debug("transform hook already set, bail");
    return;
  } else {
    TSHttpTxnArgSet(txnp, arg_idx_hooked, (void *) &GZIP_ONE);
    info("adding compression transform");
  }

  TSHttpTxnUntransformedRespCache(txnp, 1);

  if (!hc->cache()) {
    TSHttpTxnTransformedRespCache(txnp, 0);
  } else { 
    TSHttpTxnTransformedRespCache(txnp, 1);
  }

  TSVConn connp;
  GzipData *data;

  connp = TSTransformCreate(gzip_transform, txnp);
  data = gzip_data_alloc(compress_type);
  data->txn = txnp;

  TSContDataSet(connp, data);
  TSHttpTxnHookAdd(txnp, TS_HTTP_RESPONSE_TRANSFORM_HOOK, connp);
}

static int
cache_transformable(TSHttpTxn txnp)
{
  int obj_status;

  if (TSHttpTxnCacheLookupStatusGet(txnp, &obj_status) == TS_ERROR) {
    warning("Couldn't get cache status of object");
    return 0;
  }
  if (obj_status == TS_CACHE_LOOKUP_HIT_STALE) {
    info("stale cache hit");
    return 0;
  }
  if (obj_status == TS_CACHE_LOOKUP_HIT_FRESH) {
    info("fresh cache hit");
    return 1;
  }

  return 0;
}

HostConfiguration * 
find_host_configuration(TSHttpTxn /* txnp ATS_UNUSED */, TSMBuffer bufp, TSMLoc locp)
{
  TSMLoc fieldp = TSMimeHdrFieldFind(bufp, locp, TS_MIME_FIELD_HOST, TS_MIME_LEN_HOST);

  if (fieldp) {
    int strl;
    const char *strv = TSMimeHdrFieldValueStringGet(bufp, locp, fieldp, -1, &strl);
    TSHandleMLocRelease(bufp, locp, fieldp);

    HostConfiguration * host_configuration = config->Find(strv, strl);   
    return host_configuration;
  }

  return config->GlobalConfiguration();
}


static int
transform_plugin(TSCont /* contp ATS_UNUSED */, TSEvent event, void *edata)
{
  TSHttpTxn txnp = (TSHttpTxn) edata;
  int compress_type = COMPRESSION_TYPE_DEFLATE;

  switch (event) {
    case TS_EVENT_HTTP_READ_REQUEST_HDR:
      {
        TSMBuffer req_buf;
        TSMLoc req_loc;
        if (TSHttpTxnClientReqGet(txnp, &req_buf, &req_loc) == TS_SUCCESS) {
          int url_len;
          char * url = TSHttpTxnEffectiveUrlStringGet(txnp, &url_len);
          HostConfiguration * hc = find_host_configuration(txnp, req_buf, req_loc);
          //we could clone the hosting configuration here, to make it deletable on reload?
          TSHttpTxnArgSet(txnp, arg_idx_host_configuration, (void *) hc);

          if (!hc->enabled() || !hc->IsUrlAllowed(url, url_len)) {
            //FIXME: no double negatives
            TSHttpTxnArgSet(txnp, arg_idx_url_disallowed, (void *) &GZIP_ONE);
            info("url [%.*s] not allowed", url_len, url);
          } else {
            normalize_accept_encoding(txnp, req_buf, req_loc);	
          }
          TSfree(url);
          TSHandleMLocRelease(req_buf, TS_NULL_MLOC, req_loc);
        }
        TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
      }
      break;

    case TS_EVENT_HTTP_READ_RESPONSE_HDR:
      {
        //os: the accept encoding header needs to be restored..
        //otherwise the next request won't get a cache hit on this
        HostConfiguration * hc = (HostConfiguration*)TSHttpTxnArgGet(txnp, arg_idx_host_configuration);
        if (hc != NULL) { 
          if (hc->remove_accept_encoding()) {
            TSMBuffer req_buf;
            TSMLoc req_loc;
            if (TSHttpTxnServerReqGet(txnp, &req_buf, &req_loc) == TS_SUCCESS) {
              restore_accept_encoding(txnp, req_buf, req_loc, global_hidden_header_name);
              TSHandleMLocRelease(req_buf, TS_NULL_MLOC, req_loc);
            }
          }

          int allowed = !TSHttpTxnArgGet(txnp, arg_idx_url_disallowed);
          if ( allowed && gzip_transformable(txnp, 1, hc, &compress_type)) {
            gzip_transform_add(txnp, 1, hc, compress_type);
          }
        }
        TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
      }
      break;

    case TS_EVENT_HTTP_SEND_REQUEST_HDR:
      {
        HostConfiguration * hc = (HostConfiguration*)TSHttpTxnArgGet(txnp, arg_idx_host_configuration);
        if (hc!=NULL) {
          if (hc->remove_accept_encoding()) {
            TSMBuffer req_buf;
            TSMLoc req_loc;
            if (TSHttpTxnServerReqGet(txnp, &req_buf, &req_loc) == TS_SUCCESS) {
              hide_accept_encoding(txnp, req_buf, req_loc, global_hidden_header_name);
              TSHandleMLocRelease(req_buf, TS_NULL_MLOC, req_loc);
            }
          }
        }
        TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
      }
      break;

    case TS_EVENT_HTTP_CACHE_LOOKUP_COMPLETE:
      {
        int allowed = !TSHttpTxnArgGet(txnp, arg_idx_url_disallowed);
        HostConfiguration * hc = (HostConfiguration*)TSHttpTxnArgGet(txnp, arg_idx_host_configuration);
        if ( hc != NULL ) { 
          if (allowed && cache_transformable(txnp) && gzip_transformable(txnp, 0, hc, &compress_type)) {
            gzip_transform_add(txnp, 0, hc, compress_type);
          }
        }
        TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
      }
      break;

    default:
      fatal("gzip transform unknown event");
  }

  return 0;
}


static void
read_configuration(TSCont contp) {
  const char * path = (const char *)TSContDataGet(contp);
  Configuration * newconfig = Configuration::Parse(path);

  Configuration * oldconfig =__sync_lock_test_and_set(&config, newconfig);
  debug("config swapped,old config %p", oldconfig);

  //FIXME: we have leaked.
  //consider cloning or refcounting the configuration passed to the txn
  //to make deleting the old configuration possible
  //if (config != NULL )
  //  delete config;
}

static int
management_update(TSCont contp, TSEvent event, void * /* edata ATS_UNUSED */)
{
  TSReleaseAssert(event == TS_EVENT_MGMT_UPDATE);
  info("management update event received");
  read_configuration(contp);
  return 0;
}


void
TSPluginInit(int argc, const char *argv[])
{
  string config_path;

  if (argc > 2)  {
    fatal("the gzip plugin does not accept more than 1 plugin argument");
  } else if (argc == 2) { 
    config_path = std::string(argv[1]);
  }

  info("TSPluginInit %s", argv[0]);

  if (!register_plugin()) {
    fatal("The gzip plugin failed to register");
  }

  //if (argc == 2) {
  //  dictionary = load_dictionary(argv[1]);
  //}

  if (TSHttpArgIndexReserve("gzip", "for remembering if the hook was set", &arg_idx_hooked) != TS_SUCCESS) {
    fatal("failed to reserve an argument index");
  }
  if (TSHttpArgIndexReserve("gzip", "for storing if compression is applicable", &arg_idx_host_configuration) != TS_SUCCESS) {
    fatal("failed to reserve an argument index");
  }
  if (TSHttpArgIndexReserve("gzip", "for storing if compression is disallowed for this txn", &arg_idx_url_disallowed) != TS_SUCCESS) {
    fatal("failed to reserve an argument index");
  }

  global_hidden_header_name = init_hidden_header_name();

  TSCont management_contp = TSContCreate(management_update, NULL);
  //fixme: never freed. there is no shutdown event?
  char * p = (char*)TSmalloc(config_path.size()+1);
  strcpy(p,config_path.c_str());
  TSContDataSet(management_contp,(void*)p);
  TSMgmtUpdateRegister(management_contp, TAG);
  read_configuration(management_contp);

  TSCont transform_contp = TSContCreate(transform_plugin, NULL);
  TSHttpHookAdd(TS_HTTP_READ_REQUEST_HDR_HOOK, transform_contp);
  TSHttpHookAdd(TS_HTTP_READ_RESPONSE_HDR_HOOK, transform_contp);
  TSHttpHookAdd(TS_HTTP_SEND_REQUEST_HDR_HOOK, transform_contp);
  TSHttpHookAdd(TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK, transform_contp);

  info("loaded");
}