feat: proxy support for web-connections (#5038)

* feature: proxy support for web-connections

* forgotten changes

* fix code-styles

* –Documentation for new setting "proxy-url"

* Create property proxyUrl for class tr_web. Lazy creation of tr_web object in tr_session after loaded settings from file

* Update docs/Editing-Configuration-Files.md

Simplify the documentation text

Co-authored-by: ThinkChaos <ThinkChaos@users.noreply.github.com>

* Fix merge error

* Fix merge error.

* Simplify tr_web's lifecycle. Fix error. Rename quark to sneak_case-style

* Fix parameter value test

---------

Co-authored-by: ThinkChaos <ThinkChaos@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>

docs/Editing-Configuration-Files.md

@@ -81,6 +81,7 @@ Here is a sample of the three basic types: respectively Boolean, Number and Stri
  * **message-level:** Number (0 = None, 1 = Critical, 2 = Error, 3 = Warn, 4 = Info, 5 = Debug, 6 = Trace; default = 4) Set verbosity of Transmission's log messages.
  * **pex-enabled:** Boolean (default = true) Enable [Peer Exchange (PEX)](https://en.wikipedia.org/wiki/Peer_exchange).
  * **pidfile:** String Path to file in which daemon PID will be stored (_transmission-daemon only_)
+ * **proxy_url:** String (default = "") Proxy for HTTP(S) requests (for example, requests to tracker). Format `[scheme]://[host]:[port]`, where `scheme` is one of: `http`, `https`, `socks4`, `socks4h`, `socks5`, `socks5h`. If unspecified, or empty, no proxy is used. For more information see [curl proxy documentation](https://curl.se/libcurl/c/CURLOPT_PROXY.html)
  * **scrape-paused-torrents-enabled:** Boolean (default = true)
  * **script-torrent-added-enabled:** Boolean (default = false) Run a script when a torrent is added to Transmission. Environmental variables are passed in as detailed on the [Scripts](./Scripts.md) page.
  * **script-torrent-added-filename:** String (default = "") Path to script.

libtransmission/quark.cc

@@ -263,6 +263,7 @@ auto constexpr MyStatic = std::array<std::string_view, TR_N_KEYS>{
     "private"sv,
     "progress"sv,
     "prompt-before-exit"sv,
+    "proxy_url"sv,
     "queue-move-bottom"sv,
     "queue-move-down"sv,
     "queue-move-top"sv,

libtransmission/quark.h

@@ -265,6 +265,7 @@ enum
     TR_KEY_private,
     TR_KEY_progress,
     TR_KEY_prompt_before_exit,
+    TR_KEY_proxy_url,
     TR_KEY_queue_move_bottom,
     TR_KEY_queue_move_down,
     TR_KEY_queue_move_top,

libtransmission/session.cc

@@ -349,6 +349,11 @@ size_t tr_session::WebMediator::clamp(int torrent_id, size_t byte_count) const
     return tor == nullptr ? 0U : tor->bandwidth().clamp(TR_DOWN, byte_count);
 }
 
+std::optional<std::string_view> tr_session::WebMediator::proxyUrl() const
+{
+    return session_->settings_.proxy_url;
+}
+
 void tr_session::WebMediator::run(tr_web::FetchDoneFunc&& func, tr_web::FetchResponse&& response) const
 {
     session_->run_in_session_thread(std::move(func), std::move(response));

libtransmission/session.h

@@ -263,6 +263,7 @@ private:
         [[nodiscard]] std::optional<std::string> bind_address_V6() const override;
         [[nodiscard]] std::optional<std::string_view> userAgent() const override;
         [[nodiscard]] size_t clamp(int torrent_id, size_t byte_count) const override;
+        [[nodiscard]] std::optional<std::string_view> proxyUrl() const override;
         [[nodiscard]] time_t now() const override;
         // runs the tr_web::fetch response callback in the libtransmission thread
         void run(tr_web::FetchDoneFunc&& func, tr_web::FetchResponse&& response) const override;
@@ -436,6 +437,7 @@ public:
         std::string script_torrent_added_filename;
         std::string script_torrent_done_filename;
         std::string script_torrent_done_seeding_filename;
+        std::string proxy_url;
         tr_encryption_mode encryption_mode = TR_ENCRYPTION_PREFERRED;
         tr_log_level log_level = TR_LOG_INFO;
         tr_mode_t umask = 022;
@@ -481,6 +483,7 @@ public:
                 { TR_KEY_port_forwarding_enabled, &port_forwarding_enabled },
                 { TR_KEY_preallocation, &preallocation_mode },
                 { TR_KEY_preferred_transport, &preferred_transport },
+                { TR_KEY_proxy_url, &proxy_url },
                 { TR_KEY_queue_stalled_enabled, &queue_stalled_enabled },
                 { TR_KEY_queue_stalled_minutes, &queue_stalled_minutes },
                 { TR_KEY_ratio_limit, &ratio_limit },

libtransmission/web.cc

@@ -621,6 +621,11 @@ public:
             (void)curl_easy_setopt(e, CURLOPT_COOKIEFILE, file.c_str());
         }
 
+        if (auto const& proxyUrl = mediator.proxyUrl().value_or(""); !std::empty(proxyUrl))
+        {
+            (void)curl_easy_setopt(e, CURLOPT_PROXY, proxyUrl.data());
+        }
+
         if (auto const& range = task.range(); range)
         {
             /* don't bother asking the server to compress webseed fragments */

libtransmission/web.h

@@ -149,6 +149,12 @@ public:
             return byte_count;
         }
 
+        // Return the preferred proxy url
+        [[nodiscard]] virtual std::optional<std::string_view> proxyUrl() const
+        {
+            return std::nullopt;
+        }
+
         // Invoke the user-provided fetch callback
         virtual void run(FetchDoneFunc&& func, FetchResponse&& response) const
         {