libstore: add log-fd to ssh-ng as well

this way we can get ssh error message if connection setup fails.

Change-Id: Ifc001f77ec0477fb9786f7767a47f3745d6475ff

lix/libstore/machines.cc

@@ -71,12 +71,12 @@ try {
 
     StoreConfig::Params storeParams;
     if (storeUri.starts_with("ssh://")) {
-        pipe.create();
-        storeParams["log-fd"] = std::to_string(pipe.writeSide.get());
         storeParams["max-connections"] = "1";
     }
 
     if (storeUri.starts_with("ssh://") || storeUri.starts_with("ssh-ng://")) {
+        pipe.create();
+        storeParams["log-fd"] = std::to_string(pipe.writeSide.get());
         if (sshKey != "")
             storeParams["ssh-key"] = sshKey;
         if (sshPublicHostKey != "")

lix/libstore/ssh-store.cc

@@ -28,27 +28,51 @@ struct SSHStoreConfig : virtual RemoteStoreConfig, virtual CommonSSHStoreConfig
     }
 };
 
+struct SSHStoreConfigWithLog : SSHStoreConfig
+{
+    SSHStoreConfigWithLog(const Params & params)
+        : StoreConfig(params)
+        , RemoteStoreConfig(params)
+        , CommonSSHStoreConfig(params)
+        , SSHStoreConfig(params)
+    {
+    }
+
+    // Hack for getting ssh errors into build-remote.
+    // Intentionally not in `SSHStoreConfig` so that it doesn't appear in
+    // the documentation
+    const Setting<int> logFD{
+        this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"
+    };
+};
+
 class SSHStore final : public RemoteStore
 {
-    SSHStoreConfig config_;
+    SSHStoreConfigWithLog config_;
 
 public:
-    SSHStore(const std::string & scheme, const std::string & host, SSHStoreConfig config)
+    SSHStore(const std::string & scheme, const std::string & host, SSHStoreConfigWithLog config)
         : Store(config)
         , RemoteStore(config)
         , config_(std::move(config))
         , host(host)
-        , ssh(
-            host,
-            config_.port,
-            config_.sshKey,
-            config_.sshPublicHostKey,
-            config_.compress)
+        , ssh(host,
+              config_.port,
+              config_.sshKey,
+              config_.sshPublicHostKey,
+              config_.compress,
+              config_.logFD)
     {
     }
 
-    SSHStoreConfig & config() override { return config_; }
-    const SSHStoreConfig & config() const override { return config_; }
+    SSHStoreConfigWithLog & config() override
+    {
+        return config_;
+    }
+    const SSHStoreConfigWithLog & config() const override
+    {
+        return config_;
+    }
 
     static std::set<std::string> uriSchemes() { return {"ssh-ng"}; }
 

lix/libstore/ssh.hh

@@ -30,8 +30,12 @@ private:
     void addCommonSSHOpts(Strings & args);
 
 public:
-
-    SSH(const std::string & host, const std::optional<uint16_t> port, const std::string & keyFile, const std::string & sshPublicHostKey, bool compress, int logFD = -1);
+    SSH(const std::string & host,
+        const std::optional<uint16_t> port,
+        const std::string & keyFile,
+        const std::string & sshPublicHostKey,
+        bool compress,
+        int logFD);
 
     struct Connection
     {