feat(util): add GLib command stream for nonblocking line reads

Add a small GLib-backed helper for command stdout that integrates with the main loop instead of blocking on getline() in a worker thread.

The helper keeps the existing child setup semantics used by Waybar commands, including process groups, parent-death signaling, and WAYBAR_OUTPUT_NAME propagation.

This is the foundation for moving long-running custom commands away from manual poll/read logic in the module itself.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

src/util/command_line_stream.cpp

@@ -0,0 +1,209 @@
+#include "util/command_line_stream.hpp"
+
+#include <fcntl.h>
+#include <signal.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <array>
+#include <cerrno>
+#include <cstring>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+#ifdef __linux__
+#include <sys/prctl.h>
+#endif
+#ifdef __FreeBSD__
+#include <sys/procctl.h>
+#endif
+
+#include "util/command.hpp"
+
+namespace {
+
+void prepareChild(const std::string& output_name) {
+  sigset_t mask;
+  sigfillset(&mask);
+
+  const auto err = pthread_sigmask(SIG_UNBLOCK, &mask, nullptr);
+  if (err != 0) {
+    spdlog::error("pthread_sigmask in LineStream failed: {}", std::strerror(err));
+  }
+
+  int deathsig = SIGTERM;
+#ifdef __linux__
+  if (prctl(PR_SET_PDEATHSIG, deathsig) != 0) {
+    spdlog::error("prctl(PR_SET_PDEATHSIG) in LineStream failed: {}", std::strerror(errno));
+  }
+#endif
+#ifdef __FreeBSD__
+  if (procctl(P_PID, 0, PROC_PDEATHSIG_CTL, reinterpret_cast<void*>(&deathsig)) == -1) {
+    spdlog::error("procctl(PROC_PDEATHSIG_CTL) in LineStream failed: {}", std::strerror(errno));
+  }
+#endif
+
+  if (setpgid(0, 0) != 0) {
+    spdlog::error("setpgid in LineStream failed: {}", std::strerror(errno));
+  }
+  if (!output_name.empty()) {
+    setenv("WAYBAR_OUTPUT_NAME", output_name.c_str(), 1);
+  }
+}
+
+void emitBufferedLines(std::string& buffer,
+                       const waybar::util::command::LineStream::OutputCallback& on_output,
+                       bool flush_trailing_line) {
+  for (auto newline_pos = buffer.find('\n'); newline_pos != std::string::npos;
+       newline_pos = buffer.find('\n')) {
+    auto line = buffer.substr(0, newline_pos);
+    buffer.erase(0, newline_pos + 1);
+    if (!line.empty() && line.back() == '\r') {
+      line.pop_back();
+    }
+    on_output(line);
+  }
+
+  if (flush_trailing_line && !buffer.empty()) {
+    auto line = std::move(buffer);
+    buffer.clear();
+    if (!line.empty() && line.back() == '\r') {
+      line.pop_back();
+    }
+    on_output(line);
+  }
+}
+
+}  // namespace
+
+waybar::util::command::LineStream::LineStream(std::string output_name, OutputCallback on_output,
+                                              ExitCallback on_exit)
+    : output_name_(std::move(output_name)),
+      on_output_(std::move(on_output)),
+      on_exit_(std::move(on_exit)),
+      pid_(0),
+      stdout_fd_(-1) {}
+
+waybar::util::command::LineStream::~LineStream() { stop(); }
+
+void waybar::util::command::LineStream::start(const std::string& cmd) {
+  stop();
+
+  std::vector<std::string> argv{"/bin/sh", "-c", cmd};
+  Glib::spawn_async_with_pipes(
+      "", argv, Glib::SPAWN_DO_NOT_REAP_CHILD | Glib::SPAWN_CLOEXEC_PIPES,
+      sigc::bind(sigc::ptr_fun(&prepareChild), output_name_), &pid_, nullptr, &stdout_fd_, nullptr);
+
+  const auto flags = fcntl(stdout_fd_, F_GETFL, 0);
+  if (flags == -1 || fcntl(stdout_fd_, F_SETFL, flags | O_NONBLOCK) == -1) {
+    const auto saved_errno = errno;
+    stop();
+    throw std::runtime_error("Unable to configure child stdout: " +
+                             std::string(std::strerror(saved_errno)));
+  }
+
+  stdout_connection_ =
+      Glib::signal_io().connect(sigc::mem_fun(*this, &LineStream::handleStdout), stdout_fd_,
+                                Glib::IO_IN | Glib::IO_HUP | Glib::IO_ERR | Glib::IO_NVAL);
+  child_connection_ =
+      Glib::signal_child_watch().connect(sigc::mem_fun(*this, &LineStream::handleExit), pid_);
+}
+
+void waybar::util::command::LineStream::stop() {
+  stdout_connection_.disconnect();
+  child_connection_.disconnect();
+
+  if (pid_ != 0) {
+    killpg(pid_, SIGTERM);
+    waitpid(pid_, nullptr, 0);
+    Glib::spawn_close_pid(pid_);
+    pid_ = 0;
+  }
+
+  closeStdout();
+  buffer_.clear();
+}
+
+bool waybar::util::command::LineStream::running() const { return pid_ != 0; }
+
+bool waybar::util::command::LineStream::handleStdout(Glib::IOCondition condition) {
+  const auto should_flush =
+      static_cast<bool>(condition & (Glib::IO_HUP | Glib::IO_ERR | Glib::IO_NVAL));
+  drainStdout(should_flush);
+
+  if (!running() || should_flush) {
+    closeStdout();
+    return false;
+  }
+
+  return true;
+}
+
+void waybar::util::command::LineStream::handleExit(Glib::Pid pid, int status) {
+  child_connection_.disconnect();
+
+  if (stdout_fd_ != -1) {
+    drainStdout(true);
+    stdout_connection_.disconnect();
+    closeStdout();
+  }
+
+  if (pid_ == pid) {
+    Glib::spawn_close_pid(pid_);
+    pid_ = 0;
+  } else {
+    Glib::spawn_close_pid(pid);
+  }
+
+  on_exit_(statusToExitCode(status));
+}
+
+void waybar::util::command::LineStream::closeStdout() {
+  if (stdout_fd_ != -1) {
+    ::close(stdout_fd_);
+    stdout_fd_ = -1;
+  }
+}
+
+void waybar::util::command::LineStream::drainStdout(bool flush_trailing_line) {
+  if (stdout_fd_ == -1) {
+    return;
+  }
+
+  std::array<char, 4096> chunk = {};
+  while (true) {
+    const auto bytes_read = ::read(stdout_fd_, chunk.data(), chunk.size());
+    if (bytes_read > 0) {
+      buffer_.append(chunk.data(), static_cast<size_t>(bytes_read));
+      emitBufferedLines(buffer_, on_output_, false);
+      continue;
+    }
+
+    if (bytes_read == 0) {
+      emitBufferedLines(buffer_, on_output_, flush_trailing_line);
+      return;
+    }
+
+    if (errno == EINTR) {
+      continue;
+    }
+    if (errno == EAGAIN || errno == EWOULDBLOCK) {
+      return;
+    }
+
+    spdlog::error("Reading command stdout failed: {}", std::strerror(errno));
+    emitBufferedLines(buffer_, on_output_, flush_trailing_line);
+    return;
+  }
+}
+
+int waybar::util::command::LineStream::statusToExitCode(int status) {
+  if (WIFEXITED(status)) {
+    return WEXITSTATUS(status);
+  }
+  if (WIFSIGNALED(status)) {
+    return 128 + WTERMSIG(status);
+  }
+  return 1;
+}