apt-pkg/acquire.h

   1 // -*- mode: cpp; mode: fold -*-
   2 // Description                                                          /*{{{*/
   3 // $Id: acquire.h,v 1.29.2.1 2003/12/24 23:09:17 mdz Exp $
   4 /* ######################################################################
   5
   6    Acquire - File Acquiration
   7
   8    This module contians the Acquire system. It is responsible for bringing
   9    files into the local pathname space. It deals with URIs for files and
  10    URI handlers responsible for downloading or finding the URIs.
  11
  12    Each file to download is represented by an Acquire::Item class subclassed
  13    into a specialization. The Item class can add itself to several URI
  14    acquire queues each prioritized by the download scheduler. When the
  15    system is run the proper URI handlers are spawned and the the acquire
  16    queues are fed into the handlers by the schedular until the queues are
  17    empty. This allows for an Item to be downloaded from an alternate source
  18    if the first try turns out to fail. It also alows concurrent downloading
  19    of multiple items from multiple sources as well as dynamic balancing
  20    of load between the sources.
  21
  22    Schedualing of downloads is done on a first ask first get basis. This
  23    preserves the order of the download as much as possible. And means the
  24    fastest source will tend to process the largest number of files.
  25
  26    Internal methods and queues for performing gzip decompression,
  27    md5sum hashing and file copying are provided to allow items to apply
  28    a number of transformations to the data files they are working with.
  29
  30    ##################################################################### */
  31                                                                         /*}}}*/
  32
  33 /** \defgroup acquire Acquire system
  34  *
  35  *  \brief The Acquire system is responsible for retrieving files from
  36  *  local or remote URIs and postprocessing them (for instance,
  37  *  verifying their authenticity).  The core class in this system is
  38  *  pkgAcquire, which is responsible for managing the download queues
  39  *  during the download.  There is at least one download queue for
  40  *  each supported protocol; protocols such as http may provide one
  41  *  queue per host.
  42  *
  43  *  Each file to download is represented by a subclass of
  44  *  pkgAcquire::Item.  The files add themselves to the download
  45  *  queue(s) by providing their URI information to
  46  *  pkgAcquire::Item::QueueURI, which calls pkgAcquire::Enqueue.
  47  *
  48  *  Once the system is set up, the Run method will spawn subprocesses
  49  *  to handle the enqueued URIs; the scheduler will then take items
  50  *  from the queues and feed them into the handlers until the queues
  51  *  are empty.
  52  *
  53  *  \todo Acquire supports inserting an object into several queues at
  54  *  once, but it is not clear what its behavior in this case is, and
  55  *  no subclass of pkgAcquire::Item seems to actually use this
  56  *  capability.
  57  */
  58
  59 /** \addtogroup acquire
  60  *
  61  *  @{
  62  *
  63  *  \file acquire.h
  64  */
  65
  66 #ifndef PKGLIB_ACQUIRE_H
  67 #define PKGLIB_ACQUIRE_H
  68
  69 #include <vector>
  70 #include <string>
  71
  72 using std::vector;
  73 using std::string;
  74
  75
  76 #include <sys/time.h>
  77 #include <unistd.h>
  78
  79 class pkgAcquireStatus;
  80
  81 /** \brief The core download scheduler.
  82  *
  83  *  This class represents an ongoing download.  It manages the lists
  84  *  of active and pending downloads and handles setting up and tearing
  85  *  down download-related structures.
  86  *
  87  *  \todo Why all the protected data items and methods?
  88  */
  89 class pkgAcquire
  90 {
  91    public:
  92
  93    class Item;
  94    class Queue;
  95    class Worker;
  96    struct MethodConfig;
  97    struct ItemDesc;
  98    friend class Item;
  99    friend class Queue;
 100
 101    typedef vector<Item *>::iterator ItemIterator;
 102    typedef vector<Item *>::const_iterator ItemCIterator;
 103
 104    protected:
 105
 106    /** \brief A list of items to download.
 107     *
 108     *  This is built monotonically as items are created and only
 109     *  emptied when the download shuts down.
 110     */
 111    vector<Item *> Items;
 112
 113    /** \brief The head of the list of active queues.
 114     *
 115     *  \todo why a hand-managed list of queues instead of std::list or
 116     *  std::set?
 117     */
 118    Queue *Queues;
 119
 120    /** \brief The head of the list of active workers.
 121     *
 122     *  \todo why a hand-managed list of workers instead of std::list
 123     *  or std::set?
 124     */
 125    Worker *Workers;
 126
 127    /** \brief The head of the list of acquire method configurations.
 128     *
 129     *  Each protocol (http, ftp, gzip, etc) via which files can be
 130     *  fetched can have a representation in this list.  The
 131     *  configuration data is filled in by parsing the 100 Capabilities
 132     *  string output by a method on startup (see
 133     *  pkgAcqMethod::pkgAcqMethod and pkgAcquire::GetConfig).
 134     *
 135     *  \todo why a hand-managed config dictionary instead of std::map?
 136     */
 137    MethodConfig *Configs;
 138
 139    /** \brief The progress indicator for this download. */
 140    pkgAcquireStatus *Log;
 141
 142    /** \brief The total size of the files which are to be fetched.
 143     *
 144     *  This is not necessarily the total number of bytes to download
 145     *  when, e.g., download resumption and list updates via patches
 146     *  are taken into account.
 147     */
 148    unsigned long ToFetch;
 149
 150    // Configurable parameters for the scheduler
 151
 152    /** \brief Represents the queuing strategy for remote URIs. */
 153    enum QueueStrategy {
 154      /** \brief Generate one queue for each protocol/host combination; downloads from
 155       *  multiple hosts can proceed in parallel.
 156       */
 157      QueueHost,
 158      /** \brief Generate a single queue for each protocol; serialize
 159       *  downloads from multiple hosts.
 160       */
 161      QueueAccess} QueueMode;
 162
 163    /** \brief If \b true, debugging information will be dumped to std::clog. */
 164    bool Debug;
 165    /** \brief If \b true, a download is currently in progress. */
 166    bool Running;
 167
 168    /** \brief Add the given item to the list of items. */
 169    void Add(Item *Item);
 170
 171    /** \brief Remove the given item from the list of items. */
 172    void Remove(Item *Item);
 173
 174    /** \brief Add the given worker to the list of workers. */
 175    void Add(Worker *Work);
 176
 177    /** \brief Remove the given worker from the list of workers. */
 178    void Remove(Worker *Work);
 179
 180    /** \brief Insert the given fetch request into the appropriate queue.
 181     *
 182     *  \param Item The URI to download and the item to download it
 183     *  for.  Copied by value into the queue; no reference to Item is
 184     *  retained.
 185     */
 186    void Enqueue(ItemDesc &Item);
 187
 188    /** \brief Remove all fetch requests for this item from all queues. */
 189    void Dequeue(Item *Item);
 190
 191    /** \brief Determine the fetch method and queue of a URI.
 192     *
 193     *  \param URI The URI to fetch.
 194     *
 195     *  \param[out] Config A location in which to place the method via
 196     *  which the URI is to be fetched.
 197     *
 198     *  \return the string-name of the queue in which a fetch request
 199     *  for the given URI should be placed.
 200     */
 201    string QueueName(string URI,MethodConfig const *&Config);
 202
 203    /** \brief Build up the set of file descriptors upon which select() should
 204     *  block.
 205     *
 206     *  The default implementation inserts the file descriptors
 207     *  corresponding to active downloads.
 208     *
 209     *  \param[out] Fd The largest file descriptor in the generated sets.
 210     *
 211     *  \param[out] RSet The set of file descriptors that should be
 212     *  watched for input.
 213     *
 214     *  \param[out] WSet The set of file descriptors that should be
 215     *  watched for output.
 216     */
 217    virtual void SetFds(int &Fd,fd_set *RSet,fd_set *WSet);
 218
 219    /** Handle input from and output to file descriptors which select()
 220     *  has determined are ready.  The default implementation
 221     *  dispatches to all active downloads.
 222     *
 223     *  \param RSet The set of file descriptors that are ready for
 224     *  input.
 225     *
 226     *  \param WSet The set of file descriptors that are ready for
 227     *  output.
 228     */
 229    virtual void RunFds(fd_set *RSet,fd_set *WSet);
 230
 231    /** \brief Check for idle queues with ready-to-fetch items.
 232     *
 233     *  Called by pkgAcquire::Queue::Done each time an item is dequeued
 234     *  but remains on some queues; i.e., another queue should start
 235     *  fetching it.
 236     */
 237    void Bump();
 238
 239    public:
 240
 241    /** \brief Retrieve information about a fetch method by name.
 242     *
 243     *  \param Access The name of the method to look up.
 244     *
 245     *  \return the method whose name is Access, or \b NULL if no such method exists.
 246     */
 247    MethodConfig *GetConfig(string Access);
 248
 249    /** \brief Provides information on how a download terminated. */
 250    enum RunResult {
 251      /** \brief All files were fetched successfully. */
 252      Continue,
 253
 254      /** \brief Some files failed to download. */
 255      Failed,
 256
 257      /** \brief The download was cancelled by the user (i.e., #Log's
 258       *  pkgAcquireStatus::Pulse() method returned \b false).
 259       */
 260      Cancelled};
 261
 262    /** \brief Download all the items that have been Add()ed to this
 263     *  download process.
 264     *
 265     *  This method will block until the download completes, invoking
 266     *  methods on #Log to report on the progress of the download.
 267     *
 268     *  \param PulseInterval The method pkgAcquireStatus::Pulse will be
 269     *  invoked on #Log at intervals of PulseInterval milliseconds.
 270     *
 271     *  \return the result of the download.
 272     */
 273    RunResult Run(int PulseInterval=500000);
 274
 275    /** \brief Remove all items from this download process, terminate
 276     *  all download workers, and empty all queues.
 277     */
 278    void Shutdown();
 279
 280    /** \brief Get the first #Worker object.
 281     *
 282     *  \return the first active worker in this download process.
 283     */
 284    inline Worker *WorkersBegin() {return Workers;};
 285
 286    /** \brief Advance to the next #Worker object.
 287     *
 288     *  \return the worker immediately following I, or \b NULL if none
 289     *  exists.
 290     */
 291    Worker *WorkerStep(Worker *I);
 292
 293    /** \brief Get the head of the list of items. */
 294    inline ItemIterator ItemsBegin() {return Items.begin();};
 295
 296    /** \brief Get the end iterator of the list of items. */
 297    inline ItemIterator ItemsEnd() {return Items.end();};
 298
 299    // Iterate over queued Item URIs
 300    class UriIterator;
 301    /** \brief Get the head of the list of enqueued item URIs.
 302     *
 303     *  This iterator will step over every element of every active
 304     *  queue.
 305     */
 306    UriIterator UriBegin();
 307    /** \brief Get the end iterator of the list of enqueued item URIs. */
 308    UriIterator UriEnd();
 309
 310    /** Deletes each entry in the given directory that is not being
 311     *  downloaded by this object.  For instance, when downloading new
 312     *  list files, calling Clean() will delete the old ones.
 313     *
 314     *  \param Dir The directory to be cleaned out.
 315     *
 316     *  \return \b true if the directory exists and is readable.
 317     */
 318    bool Clean(string Dir);
 319
 320    /** \return the total size in bytes of all the items included in
 321     *  this download.
 322     */
 323    double TotalNeeded();
 324
 325    /** \return the size in bytes of all non-local items included in
 326     *  this download.
 327     */
 328    double FetchNeeded();
 329
 330    /** \return the amount of data to be fetched that is already
 331     *  present on the filesystem.
 332     */
 333    double PartialPresent();
 334
 335    /** \brief Construct a new pkgAcquire.
 336     *
 337     *  \param Log The progress indicator associated with this
 338     *  download, or \b NULL for none.  This object is not owned by the
 339     *  download process and will not be deleted when the pkgAcquire
 340     *  object is destroyed.  Naturally, it should live for at least as
 341     *  long as the pkgAcquire object does.
 342     */
 343    pkgAcquire(pkgAcquireStatus *Log = 0);
 344
 345    /** \brief Destroy this pkgAcquire object.
 346     *
 347     *  Destroys all queue, method, and item objects associated with
 348     *  this download.
 349     */
 350    virtual ~pkgAcquire();
 351 };
 352
 353 /** \brief Represents a single download source from which an item
 354  *  should be downloaded.
 355  *
 356  *  An item may have several assocated ItemDescs over its lifetime.
 357  */
 358 struct pkgAcquire::ItemDesc
 359 {
 360    /** \brief The URI from which to download this item. */
 361    string URI;
 362    /** brief A description of this item. */
 363    string Description;
 364    /** brief A shorter description of this item. */
 365    string ShortDesc;
 366    /** brief The underlying item which is to be downloaded. */
 367    Item *Owner;
 368 };
 369
 370 /** \brief A single download queue in a pkgAcquire object.
 371  *
 372  *  \todo Why so many protected values?
 373  */
 374 class pkgAcquire::Queue
 375 {
 376    friend class pkgAcquire;
 377    friend class pkgAcquire::UriIterator;
 378    friend class pkgAcquire::Worker;
 379
 380    /** \brief The next queue in the pkgAcquire object's list of queues. */
 381    Queue *Next;
 382
 383    protected:
 384
 385    /** \brief A single item placed in this queue. */
 386    struct QItem : pkgAcquire::ItemDesc
 387    {
 388       /** \brief The next item in the queue. */
 389       QItem *Next;
 390       /** \brief The worker associated with this item, if any. */
 391       pkgAcquire::Worker *Worker;
 392
 393       /** \brief Assign the ItemDesc portion of this QItem from
 394        *  another ItemDesc
 395        */
 396       void operator =(pkgAcquire::ItemDesc const &I)
 397       {
 398          URI = I.URI;
 399          Description = I.Description;
 400          ShortDesc = I.ShortDesc;
 401          Owner = I.Owner;
 402       };
 403    };
 404
 405    /** \brief The name of this queue. */
 406    string Name;
 407
 408    /** \brief The head of the list of items contained in this queue.
 409     *
 410     *  \todo why a by-hand list instead of an STL structure?
 411     */
 412    QItem *Items;
 413
 414    /** \brief The head of the list of workers associated with this queue.
 415     *
 416     *  \todo This is plural because support exists in Queue for
 417     *  multiple workers.  However, it does not appear that there is
 418     *  any way to actually associate more than one worker with a
 419     *  queue.
 420     *
 421     *  \todo Why not just use a std::set?
 422     */
 423    pkgAcquire::Worker *Workers;
 424
 425    /** \brief the download scheduler with which this queue is associated. */
 426    pkgAcquire *Owner;
 427
 428    /** \brief The number of entries in this queue that are currently
 429     *  being downloaded.
 430     */
 431    signed long PipeDepth;
 432
 433    /** \brief The maximum number of entries that this queue will
 434     *  attempt to download at once.
 435     */
 436    unsigned long MaxPipeDepth;
 437
 438    public:
 439
 440    /** \brief Insert the given fetch request into this queue.
 441     *
 442     *  \return \b true if the queuing was successful. May return
 443     *  \b false if the Item is already in the queue
 444     */
 445    bool Enqueue(ItemDesc &Item);
 446
 447    /** \brief Remove all fetch requests for the given item from this queue.
 448     *
 449     *  \return \b true if at least one request was removed from the queue.
 450     */
 451    bool Dequeue(Item *Owner);
 452
 453    /** \brief Locate an item in this queue.
 454     *
 455     *  \param URI A URI to match against.
 456     *  \param Owner A pkgAcquire::Worker to match against.
 457     *
 458     *  \return the first item in the queue whose URI is #URI and that
 459     *  is being downloaded by #Owner.
 460     */
 461    QItem *FindItem(string URI,pkgAcquire::Worker *Owner);
 462
 463    /** Presumably this should start downloading an item?
 464     *
 465     *  \todo Unimplemented.  Implement it or remove?
 466     */
 467    bool ItemStart(QItem *Itm,unsigned long Size);
 468
 469    /** \brief Remove the given item from this queue and set its state
 470     *  to pkgAcquire::Item::StatDone.
 471     *
 472     *  If this is the only queue containing the item, the item is also
 473     *  removed from the main queue by calling pkgAcquire::Dequeue.
 474     *
 475     *  \param Itm The item to remove.
 476     *
 477     *  \return \b true if no errors are encountered.
 478     */
 479    bool ItemDone(QItem *Itm);
 480
 481    /** \brief Start the worker process associated with this queue.
 482     *
 483     *  If a worker process is already associated with this queue,
 484     *  this is equivalent to calling Cycle().
 485     *
 486     *  \return \b true if the startup was successful.
 487     */
 488    bool Startup();
 489
 490    /** \brief Shut down the worker process associated with this queue.
 491     *
 492     *  \param Final If \b true, then the process is stopped unconditionally.
 493     *               Otherwise, it is only stopped if it does not need cleanup
 494     *               as indicated by the pkgAcqMethod::NeedsCleanup member of
 495     *               its configuration.
 496     *
 497     *  \return \b true.
 498     */
 499    bool Shutdown(bool Final);
 500
 501    /** \brief Send idle items to the worker process.
 502     *
 503     *  Fills up the pipeline by inserting idle items into the worker's queue.
 504     */
 505    bool Cycle();
 506
 507    /** \brief Check for items that could be enqueued.
 508     *
 509     *  Call this after an item placed in multiple queues has gone from
 510     *  the pkgAcquire::Item::StatFetching state to the
 511     *  pkgAcquire::Item::StatIdle state, to possibly refill an empty queue.
 512     *  This is an alias for Cycle().
 513     *
 514     *  \todo Why both this and Cycle()?  Are they expected to be
 515     *  different someday?
 516     */
 517    void Bump();
 518
 519    /** \brief Create a new Queue.
 520     *
 521     *  \param Name The name of the new queue.
 522     *  \param Owner The download process that owns the new queue.
 523     */
 524    Queue(string Name,pkgAcquire *Owner);
 525
 526    /** Shut down all the worker processes associated with this queue
 527     *  and empty the queue.
 528     */
 529    ~Queue();
 530 };
 531
 532 /** \brief Iterates over all the URIs being fetched by a pkgAcquire object. */
 533 class pkgAcquire::UriIterator
 534 {
 535    /** The next queue to iterate over. */
 536    pkgAcquire::Queue *CurQ;
 537    /** The item that we currently point at. */
 538    pkgAcquire::Queue::QItem *CurItem;
 539
 540    public:
 541
 542    inline void operator ++() {operator ++();};
 543
 544    void operator ++(int)
 545    {
 546       CurItem = CurItem->Next;
 547       while (CurItem == 0 && CurQ != 0)
 548       {
 549          CurItem = CurQ->Items;
 550          CurQ = CurQ->Next;
 551       }
 552    };
 553
 554    inline pkgAcquire::ItemDesc const *operator ->() const {return CurItem;};
 555    inline bool operator !=(UriIterator const &rhs) const {return rhs.CurQ != CurQ || rhs.CurItem != CurItem;};
 556    inline bool operator ==(UriIterator const &rhs) const {return rhs.CurQ == CurQ && rhs.CurItem == CurItem;};
 557
 558    /** \brief Create a new UriIterator.
 559     *
 560     *  \param Q The queue over which this UriIterator should iterate.
 561     */
 562    UriIterator(pkgAcquire::Queue *Q) : CurQ(Q), CurItem(0)
 563    {
 564       while (CurItem == 0 && CurQ != 0)
 565       {
 566          CurItem = CurQ->Items;
 567          CurQ = CurQ->Next;
 568       }
 569    }
 570 };
 571
 572 /** \brief Information about the properties of a single acquire method. */
 573 struct pkgAcquire::MethodConfig
 574 {
 575    /** \brief The next link on the acquire method list.
 576     *
 577     *  \todo Why not an STL container?
 578     */
 579    MethodConfig *Next;
 580
 581    /** \brief The name of this acquire method (e.g., http). */
 582    string Access;
 583
 584    /** \brief The implementation version of this acquire method. */
 585    string Version;
 586
 587    /** \brief If \b true, only one download queue should be created for this
 588     *  method.
 589     */
 590    bool SingleInstance;
 591
 592    /** \brief If \b true, this method supports pipelined downloading. */
 593    bool Pipeline;
 594
 595    /** \brief If \b true, the worker process should send the entire
 596     *  APT configuration tree to the fetch subprocess when it starts
 597     *  up.
 598     */
 599    bool SendConfig;
 600
 601    /** \brief If \b true, this fetch method does not require network access;
 602     *  all files are to be acquired from the local disk.
 603     */
 604    bool LocalOnly;
 605
 606    /** \brief If \b true, the subprocess has to carry out some cleanup
 607     *  actions before shutting down.
 608     *
 609     *  For instance, the cdrom method needs to unmount the CD after it
 610     *  finishes.
 611     */
 612    bool NeedsCleanup;
 613
 614    /** \brief If \b true, this fetch method acquires files from removable media. */
 615    bool Removable;
 616
 617    /** \brief Set up the default method parameters.
 618     *
 619     *  All fields are initialized to NULL, "", or \b false as
 620     *  appropriate.
 621     */
 622    MethodConfig();
 623 };
 624
 625 /** \brief A monitor object for downloads controlled by the pkgAcquire class.
 626  *
 627  *  \todo Why protected members?
 628  *
 629  *  \todo Should the double members be uint64_t?
 630  */
 631 class pkgAcquireStatus
 632 {
 633    protected:
 634
 635    /** \brief The last time at which this monitor object was updated. */
 636    struct timeval Time;
 637
 638    /** \brief The time at which the download started. */
 639    struct timeval StartTime;
 640
 641    /** \brief The number of bytes fetched as of the previous call to
 642     *  pkgAcquireStatus::Pulse, including local items.
 643     */
 644    double LastBytes;
 645
 646    /** \brief The current rate of download as of the most recent call
 647     *  to pkgAcquireStatus::Pulse, in bytes per second.
 648     */
 649    double CurrentCPS;
 650
 651    /** \brief The number of bytes fetched as of the most recent call
 652     *  to pkgAcquireStatus::Pulse, including local items.
 653     */
 654    double CurrentBytes;
 655
 656    /** \brief The total number of bytes that need to be fetched.
 657     *
 658     *  \warning This member is inaccurate, as new items might be
 659     *  enqueued while the download is in progress!
 660     */
 661    double TotalBytes;
 662
 663    /** \brief The total number of bytes accounted for by items that
 664     *  were successfully fetched.
 665     */
 666    double FetchedBytes;
 667
 668    /** \brief The amount of time that has elapsed since the download
 669     *   started.
 670     */
 671    unsigned long ElapsedTime;
 672
 673    /** \brief The total number of items that need to be fetched.
 674     *
 675     *  \warning This member is inaccurate, as new items might be
 676     *  enqueued while the download is in progress!
 677     */
 678    unsigned long TotalItems;
 679
 680    /** \brief The number of items that have been successfully downloaded. */
 681    unsigned long CurrentItems;
 682
 683    public:
 684
 685    /** \brief If \b true, the download scheduler should call Pulse()
 686     *  at the next available opportunity.
 687     */
 688    bool Update;
 689
 690    /** \brief If \b true, extra Pulse() invocations will be performed.
 691     *
 692     *  With this option set, Pulse() will be called every time that a
 693     *  download item starts downloading, finishes downloading, or
 694     *  terminates with an error.
 695     */
 696    bool MorePulses;
 697
 698    /** \brief Invoked when a local or remote file has been completely fetched.
 699     *
 700     *  \param Size The size of the file fetched.
 701     *
 702     *  \param ResumePoint How much of the file was already fetched.
 703     */
 704    virtual void Fetched(unsigned long Size,unsigned long ResumePoint);
 705
 706    /** \brief Invoked when the user should be prompted to change the
 707     *         inserted removable media.
 708     *
 709     *  This method should not return until the user has confirmed to
 710     *  the user interface that the media change is complete.
 711     *
 712     *  \param Media The name of the media type that should be changed.
 713     *
 714     *  \param Drive The identifying name of the drive whose media
 715     *               should be changed.
 716     *
 717     *  \return \b true if the user confirms the media change, \b
 718     *  false if it is cancelled.
 719     *
 720     *  \todo This is a horrible blocking monster; it should be CPSed
 721     *  with prejudice.
 722     */
 723    virtual bool MediaChange(string Media,string Drive) = 0;
 724
 725    /** \brief Invoked when an item is confirmed to be up-to-date.
 726
 727     * For instance, when an HTTP download is informed that the file on
 728     * the server was not modified.
 729     */
 730    virtual void IMSHit(pkgAcquire::ItemDesc &/*Itm*/) {};
 731
 732    /** \brief Invoked when some of an item's data is fetched. */
 733    virtual void Fetch(pkgAcquire::ItemDesc &/*Itm*/) {};
 734
 735    /** \brief Invoked when an item is successfully and completely fetched. */
 736    virtual void Done(pkgAcquire::ItemDesc &/*Itm*/) {};
 737
 738    /** \brief Invoked when the process of fetching an item encounters
 739     *  a fatal error.
 740     */
 741    virtual void Fail(pkgAcquire::ItemDesc &/*Itm*/) {};
 742
 743    /** \brief Periodically invoked while the Acquire process is underway.
 744     *
 745     *  Subclasses should first call pkgAcquireStatus::Pulse(), then
 746     *  update their status output.  The download process is blocked
 747     *  while Pulse() is being called.
 748     *
 749     *  \return \b false if the user asked to cancel the whole Acquire process.
 750     *
 751     *  \see pkgAcquire::Run
 752     */
 753    virtual bool Pulse(pkgAcquire *Owner);
 754
 755    /** \brief Invoked when the Acquire process starts running. */
 756    virtual void Start();
 757
 758    /** \brief Invoked when the Acquire process stops running. */
 759    virtual void Stop();
 760
 761    /** \brief Initialize all counters to 0 and the time to the current time. */
 762    pkgAcquireStatus();
 763    virtual ~pkgAcquireStatus() {};
 764 };
 765
 766 /** @} */
 767
 768 #endif