refactor(3p/nix): Build nix-daemon as a separate binary

Skips over all the monobinary stuff and moves to a separate binary for
nix-daemon.

This also replaces the flag parsing logic with absl::flags. This
causes a behaviour change for --help, which no longer tries to display
a man page but instead shows the actual command-line help.

Note: This binary no longer links to the Boehm GC.

Change-Id: Ib852e994b82f2d56e91262878c10650e656427a9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1622
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/nix-daemon/nix-daemon.cc

@@ -0,0 +1,131 @@
+#include <filesystem>
+
+#include <absl/flags/flag.h>
+#include <absl/flags/parse.h>
+#include <absl/flags/usage_config.h>
+#include <absl/strings/str_format.h>
+#include <fcntl.h>
+#include <glog/logging.h>
+#include <grpcpp/security/server_credentials.h>
+#include <grpcpp/server.h>
+#include <grpcpp/server_builder_impl.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+
+#include "libmain/shared.hh"  // TODO(tazjin): can this be removed?
+#include "libstore/globals.hh"
+#include "libstore/store-api.hh"
+#include "libutil/util.hh"
+#include "nix-daemon-proto.hh"
+#include "nix-daemon/nix-daemon-proto.hh"
+#include "nix/legacy.hh"
+
+ABSL_FLAG(bool, pipe, false, "Use pipes for daemon communication");
+
+namespace nix::daemon {
+
+using grpc::Server;
+using grpc_impl::ServerBuilder;
+
+// TODO(grfn): There has to be a better way to do this - this was ported
+// verbatim from the old daemon implementation without much critical evaluation.
+static int ForwardToSocket(nix::Path socket_path) {
+  // Forward on this connection to the real daemon
+  int sockfd = socket(PF_UNIX, SOCK_STREAM, 0);
+  if (sockfd == -1) {
+    throw SysError("creating Unix domain socket");
+  }
+
+  auto socketDir = dirOf(socket_path);
+  if (chdir(socketDir.c_str()) == -1) {
+    throw SysError(format("changing to socket directory '%1%'") % socketDir);
+  }
+
+  auto socketName = baseNameOf(socket_path);
+  auto addr = sockaddr_un{};
+  addr.sun_family = AF_UNIX;
+  if (socketName.size() + 1 >= sizeof(addr.sun_path)) {
+    throw Error(format("socket name %1% is too long") % socketName);
+  }
+  strncpy(addr.sun_path, socketName.c_str(), sizeof(addr.sun_family));
+
+  if (connect(sockfd, reinterpret_cast<struct sockaddr*>(&addr),
+              sizeof(addr)) == -1) {
+    throw SysError(format("cannot connect to daemon at %1%") % socket_path);
+  }
+
+  auto nfds = (sockfd > STDIN_FILENO ? sockfd : STDIN_FILENO) + 1;
+  while (true) {
+    fd_set fds;
+    FD_ZERO(&fds);
+    FD_SET(sockfd, &fds);
+    FD_SET(STDIN_FILENO, &fds);
+    if (select(nfds, &fds, nullptr, nullptr, nullptr) == -1) {
+      throw SysError("waiting for data from client or server");
+    }
+    if (FD_ISSET(sockfd, &fds)) {
+      auto res = splice(sockfd, nullptr, STDOUT_FILENO, nullptr, SSIZE_MAX,
+                        SPLICE_F_MOVE);
+      if (res == -1) {
+        throw SysError("splicing data from daemon socket to stdout");
+      }
+      if (res == 0) {
+        throw EndOfFile("unexpected EOF from daemon socket");
+      }
+    }
+    if (FD_ISSET(STDIN_FILENO, &fds)) {
+      auto res = splice(STDIN_FILENO, nullptr, sockfd, nullptr, SSIZE_MAX,
+                        SPLICE_F_MOVE);
+      if (res == -1) {
+        throw SysError("splicing data from stdin to daemon socket");
+      }
+      if (res == 0) {
+        return 0;
+      }
+    }
+  }
+}
+
+int RunServer() {
+  Store::Params params;
+  params["path-info-cache-size"] = "0";
+  auto store = openStore(settings.storeUri, params);
+  auto worker = NewWorkerService(*store);
+
+  std::filesystem::path socket_path(settings.nixDaemonSocketFile);
+  std::filesystem::create_directories(socket_path.parent_path());
+  auto socket_addr = absl::StrFormat("unix://%s", socket_path);
+
+  ServerBuilder builder;
+  builder.AddListeningPort(socket_addr, grpc::InsecureServerCredentials());
+  builder.RegisterService(worker);
+
+  std::unique_ptr<Server> server(builder.BuildAndStart());
+  if (server) {
+    LOG(INFO) << "Nix daemon listening at " << socket_addr;
+    server->Wait();
+    return 0;
+  } else {
+    return 1;
+  }
+}
+
+}  // namespace nix::daemon
+
+int main(int argc, char** argv) {
+  absl::SetFlagsUsageConfig({.version_string = [] { return nix::nixVersion; }});
+  absl::ParseCommandLine(argc, argv);
+
+  if (absl::GetFlag(FLAGS_pipe)) {
+    if (nix::getStoreType() == nix::tDaemon) {
+      return nix::daemon::ForwardToSocket(nix::settings.nixDaemonSocketFile);
+    } else {
+      // TODO(grfn): Need to launch a server on stdin here - upstream calls
+      // processConnection(true, "root", 0);
+      LOG(ERROR) << "not implemented";
+      return 1;
+    }
+  }
+
+  return nix::daemon::RunServer();
+}