Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libstore/ssh.hh

@@ -0,0 +1,71 @@
+#pragma once
+///@file
+
+#include "sync.hh"
+#include "processes.hh"
+#include "file-system.hh"
+
+namespace nix {
+
+class SSHMaster
+{
+private:
+
+    const std::string host;
+    bool fakeSSH;
+    const std::string keyFile;
+    /**
+     * Raw bytes, not Base64 encoding.
+     */
+    const std::string sshPublicHostKey;
+    const bool useMaster;
+    const bool compress;
+    const Descriptor logFD;
+
+    struct State
+    {
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
+        Pid sshMaster;
+#endif
+        std::unique_ptr<AutoDelete> tmpDir;
+        Path socketPath;
+    };
+
+    Sync<State> state_;
+
+    void addCommonSSHOpts(Strings & args);
+    bool isMasterRunning();
+
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
+    Path startMaster();
+#endif
+
+public:
+
+    SSHMaster(
+        std::string_view host,
+        std::string_view keyFile,
+        std::string_view sshPublicHostKey,
+        bool useMaster, bool compress, Descriptor logFD = INVALID_DESCRIPTOR);
+
+    struct Connection
+    {
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
+        Pid sshPid;
+#endif
+        AutoCloseFD out, in;
+    };
+
+    /**
+     * @param command The command (arg vector) to execute.
+     *
+     * @param extraSShArgs Extra args to pass to SSH (not the command to
+     * execute). Will not be used when "fake SSHing" to the local
+     * machine.
+     */
+    std::unique_ptr<Connection> startCommand(
+        Strings && command,
+        Strings && extraSshArgs = {});
+};
+
+}