HTTPCache.cc

// -*- mode: c++; c-basic-offset:4 -*-

// This file is part of libdap, A C++ implementation of the OPeNDAP Data
// Access Protocol.

// Copyright (c) 2002,2003 OPeNDAP, Inc.
// Author: James Gallagher <jgallagher@opendap.org>
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
//
// You can contact OPeNDAP, Inc. at PO Box 112, Saunderstown, RI. 02874-0112.

#include "config.h"

// #define DODS_DEBUG
// #define DODS_DEBUG2
#undef USE_GETENV

#include <pthread.h>
#include <limits.h>
#include <unistd.h>   // for stat
#include <sys/types.h>  // for stat and mkdir
#include <sys/stat.h>

#include <cstring>
#include <cerrno>

#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>
#include <set>

#include "Error.h"
#include "InternalErr.h"
#include "ResponseTooBigErr.h"
#ifndef WIN32
#include "SignalHandler.h"
#endif
#include "HTTPCacheInterruptHandler.h"
#include "HTTPCacheTable.h"
#include "HTTPCache.h"
#include "HTTPCacheMacros.h"
#include "SignalHandlerRegisteredErr.h"

#include "util_mit.h"
#include "debug.h"

using namespace std;

namespace libdap {

HTTPCache *HTTPCache::_instance = 0;

// instance_mutex is used to ensure that only one instance is created.
// That is, it protects the body of the HTTPCache::instance() method. This
// mutex is initialized from within the static function once_init_routine()
// and the call to that takes place using pthread_once_init() where the mutex
// once_block is used to protect that call. All of this ensures that no matter
// how many threads call the instance() method, only one instance is ever
// made.
static pthread_mutex_t instance_mutex;
static pthread_once_t once_block = PTHREAD_ONCE_INIT;


#define NO_LM_EXPIRATION 24*3600 // 24 hours

#define DUMP_FREQUENCY 10 // Dump index every x loads

#define MEGA 0x100000L
#define CACHE_TOTAL_SIZE 20 // Default cache size is 20M
#define CACHE_FOLDER_PCT 10 // 10% of cache size for metainfo etc.
#define CACHE_GC_PCT 10  // 10% of cache size free after GC
#define MIN_CACHE_TOTAL_SIZE 5 // 5M Min cache size
#define MAX_CACHE_ENTRY_SIZE 3 // 3M Max size of single cached entry

static void
once_init_routine()
{
    int status;
    status = INIT(&instance_mutex);

    if (status != 0)
        throw InternalErr(__FILE__, __LINE__, "Could not initialize the HTTP Cache mutex. Exiting.");
}

HTTPCache *
HTTPCache::instance(const string &cache_root, bool force)
{
    int status = pthread_once(&once_block, once_init_routine);
    if (status != 0)
        throw InternalErr(__FILE__, __LINE__, "Could not initialize the HTTP Cache mutex. Exiting.");

    LOCK(&instance_mutex);

    DBG(cerr << "Entering instance(); (" << hex << _instance << dec << ")" << "... ");

    try {
        if (!_instance) {
            _instance = new HTTPCache(cache_root, force);

            DBG(cerr << "New instance: " << _instance << ", cache root: "
                << _instance->d_cache_root << endl);

            atexit(delete_instance);

#ifndef WIN32
            // Register the interrupt handler. If we've already registered
            // one, barf. If this becomes a problem, hack SignalHandler so
            // that we can chain these handlers... 02/10/04 jhrg
            //
            // Technically we're leaking memory here. However, since this
            // class is a singleton, we know that only three objects will
            // ever be created and they will all exist until the process
            // exits. We can let this slide... 02/12/04 jhrg
            EventHandler *old_eh = SignalHandler::instance()->register_handler(SIGINT, new HTTPCacheInterruptHandler, true);
            if (old_eh) {
                SignalHandler::instance()->register_handler(SIGINT, old_eh);
                throw SignalHandlerRegisteredErr(
                    "Could not register event handler for SIGINT without superseding an existing one.");
            }

            old_eh = SignalHandler::instance()->register_handler(SIGPIPE, new HTTPCacheInterruptHandler, true);
            if (old_eh) {
                SignalHandler::instance()->register_handler(SIGPIPE, old_eh);
                throw SignalHandlerRegisteredErr(
                    "Could not register event handler for SIGPIPE without superseding an existing one.");
            }

            old_eh = SignalHandler::instance()->register_handler(SIGTERM, new HTTPCacheInterruptHandler, true);
            if (old_eh) {
                SignalHandler::instance()->register_handler(SIGTERM, old_eh);
                throw SignalHandlerRegisteredErr(
                    "Could not register event handler for SIGTERM without superseding an existing one.");
            }
#endif
        }
    }
    catch (...) {
        DBG2(cerr << "The constructor threw an Error!" << endl);
        UNLOCK(&instance_mutex);
        throw;
    }

    UNLOCK(&instance_mutex);
    DBGN(cerr << "returning " << hex << _instance << dec << endl);

    return _instance;
}

void
HTTPCache::delete_instance()
{
    DBG(cerr << "Entering delete_instance()..." << endl);

    if (HTTPCache::_instance) {
        DBG(cerr << "Deleting the cache: " << HTTPCache::_instance << endl);
        delete HTTPCache::_instance;
        HTTPCache::_instance = 0;

        //Now remove the signal handlers
        delete SignalHandler::instance()->remove_handler(SIGINT);
        delete SignalHandler::instance()->remove_handler(SIGPIPE);
        delete SignalHandler::instance()->remove_handler(SIGTERM);
    }

    DBG(cerr << "Exiting delete_instance()" << endl);
}

HTTPCache::HTTPCache(string cache_root, bool force) :
        d_locked_open_file(0),
        d_cache_enabled(false),
        d_cache_protected(false),

        d_cache_disconnected(DISCONNECT_NONE),

        d_expire_ignored(false),
        d_always_validate(false),
        d_total_size(CACHE_TOTAL_SIZE * MEGA),
        d_folder_size(CACHE_TOTAL_SIZE / CACHE_FOLDER_PCT),
        d_gc_buffer(CACHE_TOTAL_SIZE / CACHE_GC_PCT),
        d_max_entry_size(MAX_CACHE_ENTRY_SIZE * MEGA),
        d_default_expiration(NO_LM_EXPIRATION),
        d_max_age(-1),
        d_max_stale(-1),
        d_min_fresh(-1),
        d_http_cache_table(0)
{
    DBG(cerr << "Entering the constructor for " << this << "... ");
#if 0
    int status = pthread_once(&once_block, once_init_routine);
    if (status != 0)
        throw InternalErr(__FILE__, __LINE__, "Could not initialize the HTTP Cache mutex. Exiting.");
#endif
    INIT(&d_cache_mutex);

    // This used to throw an Error object if we could not get the
    // single user lock. However, that results in an invalid object. It's
    // better to have an instance that has default values. If we cannot get
    // the lock, make sure to set the cache as *disabled*. 03/12/03 jhrg
    //
    // I fixed this block so that the cache root is set before we try to get
    // the single user lock. That was the fix for bug #661. To make that
    // work, I had to move the call to create_cache_root out of
    // set_cache_root(). 09/08/03 jhrg

    set_cache_root(cache_root);
    int block_size;

    if (!get_single_user_lock(force))
        throw Error(internal_error, "Could not get single user lock for the cache");

#ifdef WIN32
    //  Windows is unable to provide us this information.  4096 appears
    //  a best guess.  It is likely to be in the range [2048, 8192] on
    //  windows, but will the level of truth of that statement vary over
    //  time ?
    block_size = 4096;
#else
    struct stat s;
    if (stat(cache_root.c_str(), &s) == 0)
        block_size = s.st_blksize;
    else
        throw Error(internal_error, "Could not set file system block size.");
#endif
    d_http_cache_table = new HTTPCacheTable(d_cache_root, block_size);
    d_cache_enabled = true;

    DBGN(cerr << "exiting" << endl);
}

HTTPCache::~HTTPCache()
{
    DBG(cerr << "Entering the destructor for " << this << "... ");

    try {
        if (startGC())
            perform_garbage_collection();

        d_http_cache_table->cache_index_write();
    }
    catch (Error &e) {
        // If the cache index cannot be written, we've got problems. However,
        // unless we're debugging, still free up the cache table in memory.
        // How should we let users know they cache index is not being
        // written?? 10/03/02 jhrg
        DBG(cerr << e.get_error_message() << endl);
    }

    delete d_http_cache_table;

    release_single_user_lock();

    DBGN(cerr << "exiting destructor." << endl);
    DESTROY(&d_cache_mutex);
}



bool
HTTPCache::stopGC() const
{
    return (d_http_cache_table->get_current_size() + d_folder_size < d_total_size - d_gc_buffer);
}

bool
HTTPCache::startGC() const
{
    DBG(cerr << "startGC, current_size: " << d_http_cache_table->get_current_size() << endl);
    return (d_http_cache_table->get_current_size() + d_folder_size > d_total_size);
}

void
HTTPCache::perform_garbage_collection()
{
    DBG(cerr << "Performing garbage collection" << endl);

    // Remove all the expired responses.
    expired_gc();

    // Remove entries larger than max_entry_size.
    too_big_gc();

    // Remove entries starting with zero hits, 1, ..., until stopGC()
    // returns true.
    hits_gc();
}

void
HTTPCache::expired_gc()
{
    if (!d_expire_ignored) {
        d_http_cache_table->delete_expired_entries();
    }
}

void
HTTPCache::hits_gc()
{
    int hits = 0;

    if (startGC()) {
        while (!stopGC()) {
            d_http_cache_table->delete_by_hits(hits);
            hits++;
        }
    }
}

void HTTPCache::too_big_gc() {
    if (startGC())
        d_http_cache_table->delete_by_size(d_max_entry_size);
}


bool HTTPCache::get_single_user_lock(bool force) 
{
    if (!d_locked_open_file) {
    FILE * fp = NULL;

    try {
        // It's OK to call create_cache_root if the directory already
        // exists.
        create_cache_root(d_cache_root);
    }
    catch (Error &e) {
        // We need to catch and return false because this method is
        // called from a ctor and throwing at this point will result in a
        // partially constructed object. 01/22/04 jhrg
        DBG(cerr << "Failure to create the cache root" << endl);
        return false;
    }

    // Try to read the lock file. If we can open for reading, it exists.
    string lock = d_cache_root + CACHE_LOCK;
    if ((fp = fopen(lock.c_str(), "r")) != NULL) {
        int res = fclose(fp);
        if (res) {
        DBG(cerr << "Failed to close " << (void *)fp << endl);
        }
        if (force)
        REMOVE(lock.c_str());
        else
        return false;
    }

    if ((fp = fopen(lock.c_str(), "w")) == NULL) {
        DBG(cerr << "Could not open for write access" << endl);
        return false;
    }

    d_locked_open_file = fp;
    return true;
    }

    DBG(cerr << "locked_open_file is true" << endl);
    return false;
}

void
HTTPCache::release_single_user_lock()
{
    if (d_locked_open_file) {
        int res = fclose(d_locked_open_file);
        if (res) {
            DBG(cerr << "Failed to close " << (void *)d_locked_open_file << endl) ;
        }
        d_locked_open_file = 0;
    }

    string lock = d_cache_root + CACHE_LOCK;
    REMOVE(lock.c_str());
}


string
HTTPCache::get_cache_root() const
{
    return d_cache_root;
}


void
HTTPCache::create_cache_root(const string &cache_root)
{
#ifdef WIN32
    string::size_type cur = cache_root[1] == ':' ? 3 : 1;
    typedef int mode_t;

    while ((cur = cache_root.find(DIR_SEPARATOR_CHAR, cur)) != string::npos) {
        string dir = cache_root.substr(0, cur);
        struct stat stat_info;
        if (stat(dir.c_str(), &stat_info) == -1) {
            DBG2(cerr << "Cache....... Creating " << dir << endl);
            mode_t mask = UMASK(0);
            if (MKDIR(dir.c_str(), 0777) < 0) {
                DBG2(cerr << "Error: can't create." << endl);
                UMASK(mask);
                throw Error(string("Could not create the directory for the cache. Failed when building path at ") + dir + string("."));
            }
            UMASK(mask);
        }
        else {
            DBG2(cerr << "Cache....... Found " << dir << endl);
        }
        cur++;
    }
#else
    // OSX and Linux

    // Save the mask
    mode_t mask = umask(0);

    // Ignore the error if the directory exists
    errno = 0;
    if (mkdir(cache_root.c_str(), 0777) < 0 && errno != EEXIST) {
        umask(mask);
        throw Error("Could not create the directory for the cache at '" + cache_root + "' (" + strerror(errno) + ").");
    }

    // Restore themask
    umask(mask);

#endif
}

void
HTTPCache::set_cache_root(const string &root)
{
    if (root != "") {
        d_cache_root = root;
        // cache root should end in /.
        if (d_cache_root[d_cache_root.size()-1] != DIR_SEPARATOR_CHAR)
            d_cache_root += DIR_SEPARATOR_CHAR;
    }
    else {
        // If no cache root has been indicated then look for a suitable
        // location.
#ifdef USE_GETENV
        char * cr = (char *) getenv("DODS_CACHE");
        if (!cr) cr = (char *) getenv("TMP");
        if (!cr) cr = (char *) getenv("TEMP");
        if (!cr) cr = (char*)CACHE_LOCATION;
        d_cache_root = cr;
#else
        d_cache_root = CACHE_LOCATION;
#endif

        if (d_cache_root[d_cache_root.size()-1] != DIR_SEPARATOR_CHAR)
            d_cache_root += DIR_SEPARATOR_CHAR;

        d_cache_root += CACHE_ROOT;
    }

    // Test d_hhtp_cache_table because this method can be called before that
    // instance is created and also can be called later to change the cache
    // root. jhrg 05.14.08
    if (d_http_cache_table)
        d_http_cache_table->set_cache_root(d_cache_root);
}

void
HTTPCache::set_cache_enabled(bool mode)
{
    lock_cache_interface();

    d_cache_enabled = mode;

    unlock_cache_interface();
}

bool
HTTPCache::is_cache_enabled() const
{
    DBG2(cerr << "In HTTPCache::is_cache_enabled: (" << d_cache_enabled << ")"
         << endl);
    return d_cache_enabled;
}

void
HTTPCache::set_cache_disconnected(CacheDisconnectedMode mode)
{
    lock_cache_interface();

    d_cache_disconnected = mode;

    unlock_cache_interface();
}

CacheDisconnectedMode
HTTPCache::get_cache_disconnected() const
{
    return d_cache_disconnected;
}

void
HTTPCache::set_expire_ignored(bool mode)
{
    lock_cache_interface();

    d_expire_ignored = mode;

    unlock_cache_interface();
}

/* Is the cache ignoring Expires headers returned with responses that have
   been cached? */

bool
HTTPCache::is_expire_ignored() const
{
    return d_expire_ignored;
}

void
HTTPCache::set_max_size(unsigned long size)
{
    lock_cache_interface();

    try {
        unsigned long new_size = size < MIN_CACHE_TOTAL_SIZE ?
                                 MIN_CACHE_TOTAL_SIZE * MEGA : size * MEGA;
        unsigned long old_size = d_total_size;
        d_total_size = new_size;
        d_folder_size = d_total_size / CACHE_FOLDER_PCT;
        d_gc_buffer = d_total_size / CACHE_GC_PCT;

        if (new_size < old_size && startGC()) {
            perform_garbage_collection();
            d_http_cache_table->cache_index_write();
        }
    }
    catch (...) {
        unlock_cache_interface();
        DBGN(cerr << "Unlocking interface." << endl);
        throw;
    }

    DBG2(cerr << "Cache....... Total cache size: " << d_total_size
         << " with " << d_folder_size
         << " bytes for meta information and folders and at least "
         << d_gc_buffer << " bytes free after every gc" << endl);

    unlock_cache_interface();
}

unsigned long
HTTPCache::get_max_size() const
{
    return d_total_size / MEGA;
}

void
HTTPCache::set_max_entry_size(unsigned long size)
{
    lock_cache_interface();

    try {
        unsigned long new_size = size * MEGA;
        if (new_size > 0 && new_size < d_total_size - d_folder_size) {
            unsigned long old_size = d_max_entry_size;
            d_max_entry_size = new_size;
            if (new_size < old_size && startGC()) {
                perform_garbage_collection();
                d_http_cache_table->cache_index_write();
            }
        }
    }
    catch (...) {
        unlock_cache_interface();
        throw;
    }

    DBG2(cerr << "Cache...... Max entry cache size is "
         << d_max_entry_size << endl);

    unlock_cache_interface();
}

unsigned long
HTTPCache::get_max_entry_size() const
{
    return d_max_entry_size / MEGA;
}

void
HTTPCache::set_default_expiration(const int exp_time)
{
    lock_cache_interface();

    d_default_expiration = exp_time;

    unlock_cache_interface();
}

int
HTTPCache::get_default_expiration() const
{
    return d_default_expiration;
}

void
HTTPCache::set_always_validate(bool validate)
{
    d_always_validate = validate;
}

bool
HTTPCache::get_always_validate() const
{
    return d_always_validate;
}

void
HTTPCache::set_cache_control(const vector<string> &cc)
{
    lock_cache_interface();

    try {
        d_cache_control = cc;

        vector<string>::const_iterator i;
        for (i = cc.begin(); i != cc.end(); ++i) {
            string header = (*i).substr(0, (*i).find(':'));
            string value = (*i).substr((*i).find(": ") + 2);
            if (header != "Cache-Control") {
                throw InternalErr(__FILE__, __LINE__, "Expected cache control header not found.");
            }
            else {
                if (value == "no-cache" || value == "no-store")
                    d_cache_enabled = false;
                else if (value.find("max-age") != string::npos) {
                    string max_age = value.substr(value.find("=" + 1));
                    d_max_age = parse_time(max_age.c_str());
                }
                else if (value == "max-stale")
                    d_max_stale = 0; // indicates will take anything;
                else if (value.find("max-stale") != string::npos) {
                    string max_stale = value.substr(value.find("=" + 1));
                    d_max_stale = parse_time(max_stale.c_str());
                }
                else if (value.find("min-fresh") != string::npos) {
                    string min_fresh = value.substr(value.find("=" + 1));
                    d_min_fresh = parse_time(min_fresh.c_str());
                }
            }
        }
    }
    catch (...) {
        unlock_cache_interface();
        throw;
    }

    unlock_cache_interface();
}


vector<string>
HTTPCache::get_cache_control()
{
    return d_cache_control;
}


bool
HTTPCache::is_url_in_cache(const string &url)
{
    DBG(cerr << "Is this url in the cache? (" << url << ")" << endl);

    HTTPCacheTable::CacheEntry *entry = d_http_cache_table->get_locked_entry_from_cache_table(url);
    bool status = entry != 0;
    if (entry) {
        entry->unlock_read_response();
    }
    return  status;
}

bool
is_hop_by_hop_header(const string &header)
{
    return header.find("Connection") != string::npos
           || header.find("Keep-Alive") != string::npos
           || header.find("Proxy-Authenticate") != string::npos
           || header.find("Proxy-Authorization") != string::npos
           || header.find("Transfer-Encoding") != string::npos
           || header.find("Upgrade") != string::npos;
}

void
HTTPCache::write_metadata(const string &cachename, const vector<string> &headers)
{
    string fname = cachename + CACHE_META;
    d_open_files.push_back(fname);

    FILE *dest = fopen(fname.c_str(), "w");
    if (!dest) {
        throw InternalErr(__FILE__, __LINE__,
                          "Could not open named cache entry file.");
    }

    vector<string>::const_iterator i;
    for (i = headers.begin(); i != headers.end(); ++i) {
        if (!is_hop_by_hop_header(*i)) {
            int s = fwrite((*i).c_str(), (*i).size(), 1, dest);
            if (s != 1) {
                fclose(dest);
                throw InternalErr(__FILE__, __LINE__, "could not write header: '" + (*i) + "' " + long_to_string(s));
            }
            s = fwrite("\n", 1, 1, dest);
            if (s != 1) {
                fclose(dest);
                throw InternalErr(__FILE__, __LINE__, "could not write header: " + long_to_string(s));
            }
        }
    }

    int res = fclose(dest);
    if (res) {
        DBG(cerr << "HTTPCache::write_metadata - Failed to close "
            << dest << endl);
    }

    d_open_files.pop_back();
}

void
HTTPCache::read_metadata(const string &cachename, vector<string> &headers)
{
    FILE *md = fopen(string(cachename + CACHE_META).c_str(), "r");
    if (!md) {
        throw InternalErr(__FILE__, __LINE__,
                          "Could not open named cache entry meta data file.");
    }

    char line[1024];
    while (!feof(md) && fgets(line, 1024, md)) {
        line[min(1024, static_cast<int>(strlen(line)))-1] = '\0'; // erase newline
        headers.push_back(string(line));
    }

    int res = fclose(md);
    if (res) {
        DBG(cerr << "HTTPCache::read_metadata - Failed to close "
            << md << endl);
    }
}

int
HTTPCache::write_body(const string &cachename, const FILE *src)
{
    d_open_files.push_back(cachename);

    FILE *dest = fopen(cachename.c_str(), "wb");
    if (!dest) {
        throw InternalErr(__FILE__, __LINE__,
                          "Could not open named cache entry file.");
    }

    // Read and write in 1k blocks; an attempt at doing this efficiently.
    // 09/30/02 jhrg
    char line[1024];
    size_t n;
    int total = 0;
    while ((n = fread(line, 1, 1024, const_cast<FILE *>(src))) > 0) {
        total += fwrite(line, 1, n, dest);
        DBG2(sleep(3));
    }

    if (ferror(const_cast<FILE *>(src)) || ferror(dest)) {
        int res = fclose(dest);
        res = res & unlink(cachename.c_str());
        if (res) {
            DBG(cerr << "HTTPCache::write_body - Failed to close/unlink "
                << dest << endl);
        }
        throw InternalErr(__FILE__, __LINE__,
                          "I/O error transferring data to the cache.");
    }

    rewind(const_cast<FILE *>(src));

    int res = fclose(dest);
    if (res) {
        DBG(cerr << "HTTPCache::write_body - Failed to close "
            << dest << endl);
    }

    d_open_files.pop_back();

    return total;
}

FILE *
HTTPCache::open_body(const string &cachename)
{
    DBG(cerr << "cachename: " << cachename << endl);

    FILE *src = fopen(cachename.c_str(), "rb"); // Read only
    if (!src)
    throw InternalErr(__FILE__, __LINE__, "Could not open cache file.");

    return src;
}

bool
HTTPCache::cache_response(const string &url, time_t request_time,
                          const vector<string> &headers, const FILE *body)
{
    lock_cache_interface();

    DBG(cerr << "Caching url: " << url << "." << endl);

    try {
        // If this is not an http or https URL, don't cache.
        if (url.find("http:") == string::npos &&
            url.find("https:") == string::npos) {
            unlock_cache_interface();
            return false;
        }

        // This does nothing if url is not already in the cache. It's
        // more efficient to do this than to first check and see if the entry
        // exists. 10/10/02 jhrg
        d_http_cache_table->remove_entry_from_cache_table(url);

        HTTPCacheTable::CacheEntry *entry = new HTTPCacheTable::CacheEntry(url);
        entry->lock_write_response();

        try {
            d_http_cache_table->parse_headers(entry, d_max_entry_size, headers); // etag, lm, date, age, expires, max_age.
            if (entry->is_no_cache()) {
                DBG(cerr << "Not cache-able; deleting HTTPCacheTable::CacheEntry: " << entry
                    << "(" << url << ")" << endl);
                entry->unlock_write_response();
                delete entry; entry = 0;
                unlock_cache_interface();
                return false;
            }

            // corrected_initial_age, freshness_lifetime, response_time.
            d_http_cache_table->calculate_time(entry, d_default_expiration, request_time);

            d_http_cache_table->create_location(entry); // cachename, cache_body_fd
            // move these write function to cache table
            entry->set_size(write_body(entry->get_cachename(), body));
            write_metadata(entry->get_cachename(), headers);
            d_http_cache_table->add_entry_to_cache_table(entry);
            entry->unlock_write_response();
        }
        catch (ResponseTooBigErr &e) {
            // Oops. Bummer. Clean up and exit.
            DBG(cerr << e.get_error_message() << endl);
            REMOVE(entry->get_cachename().c_str());
            REMOVE(string(entry->get_cachename() + CACHE_META).c_str());
            DBG(cerr << "Too big; deleting HTTPCacheTable::CacheEntry: " << entry << "(" << url
                << ")" << endl);
            entry->unlock_write_response();
            delete entry; entry = 0;
            unlock_cache_interface();
            return false;
        }

        if (d_http_cache_table->get_new_entries() > DUMP_FREQUENCY) {
            if (startGC())
                perform_garbage_collection();

            d_http_cache_table->cache_index_write(); // resets new_entries
        }
    }
    catch (...) {
        unlock_cache_interface();
        throw;
    }

    unlock_cache_interface();

    return true;
}

vector<string>
HTTPCache::get_conditional_request_headers(const string &url)
{
    lock_cache_interface();

    HTTPCacheTable::CacheEntry *entry = 0;
    vector<string> headers;

    DBG(cerr << "Getting conditional request headers for " << url << endl);

    try {
        entry = d_http_cache_table->get_locked_entry_from_cache_table(url);
        if (!entry)
            throw Error(internal_error, "There is no cache entry for the URL: " + url);

        if (entry->get_etag() != "")
            headers.push_back(string("If-None-Match: ") + entry->get_etag());

        if (entry->get_lm() > 0) {
            time_t lm = entry->get_lm();
            headers.push_back(string("If-Modified-Since: ")
                              + date_time_str(&lm));
        }
        else if (entry->get_max_age() > 0) {
            time_t max_age = entry->get_max_age();
            headers.push_back(string("If-Modified-Since: ")
                              + date_time_str(&max_age));
        }
        else if (entry->get_expires() > 0) {
            time_t expires = entry->get_expires();
            headers.push_back(string("If-Modified-Since: ")
                              + date_time_str(&expires));
        }
        entry->unlock_read_response();
        unlock_cache_interface();
    }
    catch (...) {
    unlock_cache_interface();
    if (entry) {
        entry->unlock_read_response();
    }
    throw;
    }

    return headers;
}

struct HeaderLess: binary_function<const string&, const string&, bool>
{
    bool operator()(const string &s1, const string &s2) const {
        return s1.substr(0, s1.find(':')) < s2.substr(0, s2.find(':'));
    }
};

void
HTTPCache::update_response(const string &url, time_t request_time,
                           const vector<string> &headers)
{
    lock_cache_interface();

    HTTPCacheTable::CacheEntry *entry = 0;
    DBG(cerr << "Updating the response headers for: " << url << endl);

    try {
        entry = d_http_cache_table->get_write_locked_entry_from_cache_table(url);
        if (!entry)
            throw Error(internal_error, "There is no cache entry for the URL: " + url);

        // Merge the new headers with the exiting HTTPCacheTable::CacheEntry object.
        d_http_cache_table->parse_headers(entry, d_max_entry_size, headers);

        // Update corrected_initial_age, freshness_lifetime, response_time.
        d_http_cache_table->calculate_time(entry, d_default_expiration, request_time);

        // Merge the new headers with those in the persistent store. How:
        // Load the new headers into a set, then merge the old headers. Since
        // set<> ignores duplicates, old headers with the same name as a new
        // header will got into the bit bucket. Define a special compare
        // functor to make sure that headers are compared using only their
        // name and not their value too.
        set<string, HeaderLess> merged_headers;

        // Load in the new headers
        copy(headers.begin(), headers.end(),
             inserter(merged_headers, merged_headers.begin()));

        // Get the old headers and load them in.
        vector<string> old_headers;
        read_metadata(entry->get_cachename(), old_headers);
        copy(old_headers.begin(), old_headers.end(),
             inserter(merged_headers, merged_headers.begin()));

        // Read the values back out. Use reverse iterators with back_inserter
        // to preserve header order. NB: vector<> does not support push_front
        // so we can't use front_inserter(). 01/09/03 jhrg
        vector<string> result;
        copy(merged_headers.rbegin(), merged_headers.rend(),
             back_inserter(result));

        write_metadata(entry->get_cachename(), result);
        entry->unlock_write_response();
        unlock_cache_interface();
    }
    catch (...) {
        if (entry) {
            entry->unlock_read_response();
        }
        unlock_cache_interface();
        throw;
    }
}

bool
HTTPCache::is_url_valid(const string &url)
{
    lock_cache_interface();

    bool freshness;
    HTTPCacheTable::CacheEntry *entry = 0;

    DBG(cerr << "Is this URL valid? (" << url << ")" << endl);

    try {
        if (d_always_validate) {
            unlock_cache_interface();
            return false;  // force re-validation.
        }

        entry = d_http_cache_table->get_locked_entry_from_cache_table(url);
        if (!entry)
            throw Error(internal_error, "There is no cache entry for the URL: " + url);

        // If we supported range requests, we'd need code here to check if
        // there was only a partial response in the cache. 10/02/02 jhrg

        // In case this entry is of type "must-revalidate" then we consider it
        // invalid.
        if (entry->get_must_revalidate()) {
            entry->unlock_read_response();
            unlock_cache_interface();
            return false;
        }

        time_t resident_time = time(NULL) - entry->get_response_time();
        time_t current_age = entry->get_corrected_initial_age() + resident_time;

        // Check that the max-age, max-stale, and min-fresh directives
        // given in the request cache control header is followed.
        if (d_max_age >= 0 && current_age > d_max_age) {
            DBG(cerr << "Cache....... Max-age validation" << endl);
            entry->unlock_read_response();
            unlock_cache_interface();
            return false;
        }
        if (d_min_fresh >= 0
            && entry->get_freshness_lifetime() < current_age + d_min_fresh) {
            DBG(cerr << "Cache....... Min-fresh validation" << endl);
            entry->unlock_read_response();
            unlock_cache_interface();
            return false;
        }

        freshness = (entry->get_freshness_lifetime()
                     + (d_max_stale >= 0 ? d_max_stale : 0) > current_age);
        entry->unlock_read_response();
        unlock_cache_interface();
    }
    catch (...) {
        if (entry) {
            entry->unlock_read_response();
        }
        unlock_cache_interface();
        throw;
    }

    return freshness;
}

FILE * HTTPCache::get_cached_response(const string &url,
        vector<string> &headers, string &cacheName) {
    lock_cache_interface();

    FILE *body = 0;
    HTTPCacheTable::CacheEntry *entry = 0;

    DBG(cerr << "Getting the cached response for " << url << endl);

    try {
        entry = d_http_cache_table->get_locked_entry_from_cache_table(url);
        if (!entry) {
            unlock_cache_interface();
            return 0;
        }

        cacheName = entry->get_cachename();
        read_metadata(entry->get_cachename(), headers);

        DBG(cerr << "Headers just read from cache: " << endl);
        DBGN(copy(headers.begin(), headers.end(), ostream_iterator<string>(cerr, "\n")));

        body = open_body(entry->get_cachename());

        DBG(cerr << "Returning: " << url << " from the cache." << endl);

        d_http_cache_table->bind_entry_to_data(entry, body);
    }
    catch (...) {
        // Why make this unlock operation conditional on entry?
        if (entry)
            unlock_cache_interface();
        if (body != 0)
            fclose(body);
        throw;
    }

    unlock_cache_interface();

    return body;
}

FILE *
HTTPCache::get_cached_response(const string &url, vector<string> &headers)
{
    string discard_name;
    return get_cached_response(url, headers, discard_name);
}

FILE *
HTTPCache::get_cached_response(const string &url)
{
    string discard_name;
    vector<string> discard_headers;
    return get_cached_response(url, discard_headers, discard_name);
}

void
HTTPCache::release_cached_response(FILE *body)
{
    lock_cache_interface();

    try {
        // fclose(body); This results in a seg fault on linux jhrg 8/27/13
        d_http_cache_table->uncouple_entry_from_data(body);
    }
    catch (...) {
        unlock_cache_interface();
        throw;
    }

    unlock_cache_interface();
}

void
HTTPCache::purge_cache()
{
    lock_cache_interface();

    try {
        if (d_http_cache_table->is_locked_read_responses())
            throw Error(internal_error, "Attempt to purge the cache with entries in use.");

        d_http_cache_table->delete_all_entries();
    }
    catch (...) {
        unlock_cache_interface();
        throw;
    }

    unlock_cache_interface();
}

} // namespace libdap