Improve SSH handling

* Unify SSH code in SSHStore and LegacySSHStore.

* Fix a race starting the SSH master. We now wait synchronously for
  the SSH master to finish starting. This prevents the SSH clients
  from starting their own connections.

* Don't use a master if max-connections == 1.

* Add a "max-connections" store parameter.

* Add a "compress" store parameter.

src/libstore/ssh.cc

@@ -0,0 +1,93 @@
+#include "ssh.hh"
+
+namespace nix {
+
+std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string & command)
+{
+    startMaster();
+
+    Pipe in, out;
+    in.create();
+    out.create();
+
+    auto conn = std::make_unique<Connection>();
+    conn->sshPid = startProcess([&]() {
+        restoreSignals();
+
+        close(in.writeSide.get());
+        close(out.readSide.get());
+
+        if (dup2(in.readSide.get(), STDIN_FILENO) == -1)
+            throw SysError("duping over stdin");
+        if (dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
+            throw SysError("duping over stdout");
+
+        Strings args = { "ssh", host.c_str(), "-x", "-a" };
+        if (!keyFile.empty())
+            args.insert(args.end(), {"-i", keyFile});
+        if (compress)
+            args.push_back("-C");
+        if (useMaster)
+            args.insert(args.end(), {"-S", socketPath});
+        args.push_back(command);
+        execvp(args.begin()->c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("executing ‘%s’ on ‘%s’", command, host);
+    });
+
+
+    in.readSide = -1;
+    out.writeSide = -1;
+
+    conn->out = std::move(out.readSide);
+    conn->in = std::move(in.writeSide);
+
+    return conn;
+}
+
+void SSHMaster::startMaster()
+{
+    if (!useMaster || sshMaster != -1) return;
+
+    tmpDir = std::make_unique<AutoDelete>(createTempDir("", "nix", true, true, 0700));
+
+    socketPath = (Path) *tmpDir + "/ssh.sock";
+
+    Pipe out;
+    out.create();
+
+    sshMaster = startProcess([&]() {
+        restoreSignals();
+
+        close(out.readSide.get());
+
+        if (dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
+            throw SysError("duping over stdout");
+
+        Strings args =
+            { "ssh", host.c_str(), "-M", "-N", "-S", socketPath
+            , "-o", "LocalCommand=echo started"
+            , "-o", "PermitLocalCommand=yes"
+            };
+        if (!keyFile.empty())
+            args.insert(args.end(), {"-i", keyFile});
+        if (compress)
+            args.push_back("-C");
+
+        execvp(args.begin()->c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("starting SSH master");
+    });
+
+    out.writeSide = -1;
+
+    std::string reply;
+    try {
+        reply = readLine(out.readSide.get());
+    } catch (EndOfFile & e) { }
+
+    if (reply != "started")
+        throw Error("failed to start SSH master connection to ‘%s’", host);
+}
+
+}