Better signals interface

This avoids some CPP and accidentally using Unix stuff in client code.

src/libutil/unix/signals-impl.hh

@@ -0,0 +1,111 @@
+#pragma once
+/**
+ * @file
+ *
+ * Implementation of some inline definitions for Unix signals, and also
+ * some extra Unix-only interfaces.
+ *
+ * (The only reason everything about signals isn't Unix-only is some
+ * no-op definitions are provided on Windows to avoid excess CPP in
+ * downstream code.)
+ */
+
+#include "types.hh"
+#include "error.hh"
+#include "logging.hh"
+#include "ansicolor.hh"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <dirent.h>
+#include <unistd.h>
+#include <signal.h>
+
+#include <boost/lexical_cast.hpp>
+
+#include <atomic>
+#include <functional>
+#include <map>
+#include <sstream>
+#include <optional>
+
+namespace nix {
+
+/* User interruption. */
+
+namespace unix {
+
+extern std::atomic<bool> _isInterrupted;
+
+extern thread_local std::function<bool()> interruptCheck;
+
+void _interrupted();
+
+/**
+ * Sets the signal mask. Like saveSignalMask() but for a signal set that doesn't
+ * necessarily match the current thread's mask.
+ * See saveSignalMask() to set the saved mask to the current mask.
+ */
+void setChildSignalMask(sigset_t *sigs);
+
+/**
+ * Start a thread that handles various signals. Also block those signals
+ * on the current thread (and thus any threads created by it).
+ * Saves the signal mask before changing the mask to block those signals.
+ * See saveSignalMask().
+ */
+void startSignalHandlerThread();
+
+/**
+ * Saves the signal mask, which is the signal mask that nix will restore
+ * before creating child processes.
+ * See setChildSignalMask() to set an arbitrary signal mask instead of the
+ * current mask.
+ */
+void saveSignalMask();
+
+/**
+ * To use in a process that already called `startSignalHandlerThread()`
+ * or `saveSignalMask()` first.
+ */
+void restoreSignals();
+
+void triggerInterrupt();
+
+}
+
+static inline void setInterrupted(bool isInterrupted)
+{
+    unix::_isInterrupted = isInterrupted;
+}
+
+static inline bool getInterrupted()
+{
+    return unix::_isInterrupted;
+}
+
+void inline checkInterrupt()
+{
+    using namespace unix;
+    if (_isInterrupted || (interruptCheck && interruptCheck()))
+        _interrupted();
+}
+
+/**
+ * A RAII class that causes the current thread to receive SIGUSR1 when
+ * the signal handler thread receives SIGINT. That is, this allows
+ * SIGINT to be multiplexed to multiple threads.
+ */
+struct ReceiveInterrupts
+{
+    pthread_t target;
+    std::unique_ptr<InterruptCallback> callback;
+
+    ReceiveInterrupts()
+        : target(pthread_self())
+        , callback(createInterruptCallback([&]() { pthread_kill(target, SIGUSR1); }))
+    { }
+};
+
+
+}