refactor: make announce key param unique per-torrent (#4508)

libtransmission/announcer-common.h

@@ -63,12 +63,8 @@ struct tr_announce_request
     /* the port we listen for incoming peers on */
     tr_port port;
 
-    /* Per-session key. BEP 0015 defines this key as a 32-bit number and
-     * explains its purpose: "Clients that resolve hostnames to v4 and v6
-     * and then announce to both should use the same key for both so that
-     * trackers that care about accurate statistics-keeping can match the
-     * two announces." */
-    uint32_t key;
+    // see discussion of tr_announce_key_t that type's declaration
+    tr_announce_key_t key;
 
     /* the number of peers we'd like to get back in the response */
     int numwant = 0;

libtransmission/announcer-udp.cc

@@ -5,6 +5,7 @@
 
 #include <algorithm> // for std::find_if()
 #include <cerrno> // for errno, EAFNOSUPPORT
+#include <climits> // for CHAR_BIT
 #include <cstring> // for memset()
 #include <ctime>
 #include <future>
@@ -170,6 +171,9 @@ struct tau_announce_request
     tau_announce_request(uint32_t announce_ip, tr_announce_request const& in, tr_announce_response_func on_response)
         : on_response_{ std::move(on_response) }
     {
+        // https://www.bittorrent.org/beps/bep_0015.html sets key size at 32 bits
+        static_assert(sizeof(tr_announce_request::key) * CHAR_BIT == 32);
+
         response.seeders = -1;
         response.leechers = -1;
         response.downloads = -1;

libtransmission/announcer.cc

@@ -235,8 +235,6 @@ public:
 
     tr_session* const session;
 
-    uint32_t const key = tr_rand_obj<uint32_t>();
-
 private:
     void flushCloseMessages()
     {
@@ -946,7 +944,7 @@ void tr_announcerAddBytes(tr_torrent* tor, int type, uint32_t n_bytes)
     req.leftUntilComplete = tor->hasMetainfo() ? tor->totalSize() - tor->hasTotal() : INT64_MAX;
     req.event = event;
     req.numwant = event == TR_ANNOUNCE_EVENT_STOPPED ? 0 : Numwant;
-    req.key = announcer->key;
+    req.key = tor->announce_key();
     req.partial_seed = tor->isPartialSeed();
     tier->buildLogName(req.log_name, sizeof(req.log_name));
     return req;

libtransmission/torrent.h

@@ -24,6 +24,7 @@
 #include "bitfield.h"
 #include "block-info.h"
 #include "completion.h"
+#include "crypto-utils.h"
 #include "file-piece-map.h"
 #include "interned-string.h"
 #include "log.h"
@@ -788,6 +789,11 @@ public:
         }
     }
 
+    [[nodiscard]] constexpr auto announce_key() const noexcept
+    {
+        return announce_key_;
+    }
+
     tr_torrent_metainfo metainfo_;
 
     tr_bandwidth bandwidth_;
@@ -920,7 +926,11 @@ private:
     }
 
     tr_verify_state verify_state_ = TR_VERIFY_NONE;
+
     float verify_progress_ = -1;
+
+    tr_announce_key_t announce_key_ = tr_rand_obj<tr_announce_key_t>();
+
     tr_interned_string bandwidth_group_;
 
     bool needs_completeness_check_ = true;

libtransmission/tr-macros.h

@@ -7,6 +7,7 @@
 
 #include <array>
 #include <cstddef> // size_t
+#include <cstdint> // uint32_t
 
 ///
 
@@ -88,6 +89,20 @@
 // Mostly to enforce better formatting
 #define TR_ARG_TUPLE(...) __VA_ARGS__
 
+// https://www.bittorrent.org/beps/bep_0007.html
+// "The client SHOULD include a key parameter in its announces. The key
+// should remain the same for a particular infohash during a torrent
+// session. Together with the peer_id this allows trackers to uniquely
+// identify clients for the purpose of statistics-keeping when they
+// announce from multiple IP.
+// The key should be generated so it has at least 32bits worth of entropy."
+//
+// https://www.bittorrent.org/beps/bep_0015.html
+// "Clients that resolve hostnames to v4 and v6 and then announce to both
+// should use the same [32-bit integer] key for both so that trackers that
+// care about accurate statistics-keeping can match the two announces."
+using tr_announce_key_t = uint32_t;
+
 // https://www.bittorrent.org/beps/bep_0003.html
 // A string of length 20 which this downloader uses as its id. Each
 // downloader generates its own id at random at the start of a new