init: allow startup with -onlynet=onion -listenonion=1

It does not make sense to specify `-onlynet=onion` without providing a
Tor proxy (even if other `-onlynet=...` are given). This is checked
during startup. However, it was forgotten that a Tor proxy can also be
retrieved from "Tor control" to which we connect if `-listenonion=1`.

So, the full Tor proxy retrieval logic is:
1. get it from `-onion`
2. get it from `-proxy`
3. if `-listenonion=1`, then connect to "Tor control" and get the proxy
   from there (was forgotten before this change)

Fixes bitcoin#24980

Author: vasild

src/init.cpp

@@ -1307,13 +1307,20 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         onion_proxy = addrProxy;
     }
 
+    const bool onlynet_used_with_onion{args.IsArgSet("-onlynet") && IsReachable(NET_ONION)};
+
     // -onion can be used to set only a proxy for .onion, or override normal proxy for .onion addresses
     // -noonion (or -onion=0) disables connecting to .onion entirely
     // An empty string is used to not override the onion proxy (in which case it defaults to -proxy set above, or none)
     std::string onionArg = args.GetArg("-onion", "");
     if (onionArg != "") {
         if (onionArg == "0") { // Handle -noonion/-onion=0
             onion_proxy = Proxy{};
+            if (onlynet_used_with_onion) {
+                return InitError(
+                    _("Outbound connections restricted to Tor (-onlynet=onion) but the proxy for "
+                      "reaching the Tor network is explicitly forbidden: -onion=0"));
+            }
         } else {
             CService addr;
             if (!Lookup(onionArg, addr, 9050, fNameLookup) || !addr.IsValid()) {
@@ -1326,11 +1333,14 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     if (onion_proxy.IsValid()) {
         SetProxy(NET_ONION, onion_proxy);
     } else {
-        if (args.IsArgSet("-onlynet") && IsReachable(NET_ONION)) {
+        // If -listenonion is set, then we will (try to) connect to the Tor control port
+        // later from the torcontrol thread and may retrieve the onion proxy from there.
+        const bool listenonion_disabled{!args.GetBoolArg("-listenonion", DEFAULT_LISTEN_ONION)};
+        if (onlynet_used_with_onion && listenonion_disabled) {
             return InitError(
                 _("Outbound connections restricted to Tor (-onlynet=onion) but the proxy for "
-                  "reaching the Tor network is not provided (no -proxy= and no -onion= given) or "
-                  "it is explicitly forbidden (-onion=0)"));
+                  "reaching the Tor network is not provided: none of -proxy, -onion or "
+                  "-listenonion is given"));
         }
         SetReachable(NET_ONION, false);
     }

test/functional/feature_proxy.py

@@ -332,20 +332,27 @@ def networks_dict(d):
         msg = "Error: Invalid -i2psam address or hostname: 'def:xyz'"
         self.nodes[1].assert_start_raises_init_error(expected_msg=msg)
 
+        self.log.info("Test passing -onlynet=onion with -onion=0/-noonion raises expected init error")
         msg = (
             "Error: Outbound connections restricted to Tor (-onlynet=onion) but "
-            "the proxy for reaching the Tor network is not provided (no -proxy= "
-            "and no -onion= given) or it is explicitly forbidden (-onion=0)"
+            "the proxy for reaching the Tor network is explicitly forbidden: -onion=0"
         )
-        self.log.info("Test passing -onlynet=onion without -proxy or -onion raises expected init error")
-        self.nodes[1].extra_args = ["-onlynet=onion"]
-        self.nodes[1].assert_start_raises_init_error(expected_msg=msg)
-
-        self.log.info("Test passing -onlynet=onion with -onion=0/-noonion raises expected init error")
         for arg in ["-onion=0", "-noonion"]:
             self.nodes[1].extra_args = ["-onlynet=onion", arg]
             self.nodes[1].assert_start_raises_init_error(expected_msg=msg)
 
+        self.log.info("Test passing -onlynet=onion without -proxy, -onion or -listenonion raises expected init error")
+        self.nodes[1].extra_args = ["-onlynet=onion", "-listenonion=0"]
+        msg = (
+            "Error: Outbound connections restricted to Tor (-onlynet=onion) but the proxy for "
+            "reaching the Tor network is not provided: none of -proxy, -onion or -listenonion is given"
+        )
+        self.nodes[1].assert_start_raises_init_error(expected_msg=msg)
+
+        self.log.info("Test passing -onlynet=onion without -proxy or -onion but with -listenonion=1 is ok")
+        self.start_node(1, extra_args=["-onlynet=onion", "-listenonion=1"])
+        self.stop_node(1)
+
         self.log.info("Test passing unknown network to -onlynet raises expected init error")
         self.nodes[1].extra_args = ["-onlynet=abc"]
         msg = "Error: Unknown network specified in -onlynet: 'abc'"