Introduce interruptor tool

An interruptor instance will help to wake up a blocking call from
another thread (typically to terminate immediately on Ctrl+C).

app/src/util/intr.h

@@ -0,0 +1,78 @@
+#ifndef SC_INTR_H
+#define SC_INTR_H
+
+#include "common.h"
+
+#include <stdatomic.h>
+#include <stdbool.h>
+
+#include "net.h"
+#include "process.h"
+#include "thread.h"
+
+/**
+ * Interruptor to wake up a blocking call from another thread
+ *
+ * It allows to register a socket or a process before a blocking call, and
+ * interrupt/close from another thread to wake up the blocking call.
+ */
+struct sc_intr {
+    sc_mutex mutex;
+
+    sc_socket socket;
+    sc_pid process;
+
+    // Written protected by the mutex to avoid race conditions against
+    // sc_intr_set_socket() and sc_intr_set_process(), but can be read
+    // (atomically) without mutex
+    atomic_bool interrupted;
+};
+
+/**
+ * Initialize an interruptor
+ */
+bool
+sc_intr_init(struct sc_intr *intr);
+
+/**
+ * Set a socket as the interruptible component
+ *
+ * Call with SC_INVALID_SOCKET to unset.
+ */
+bool
+sc_intr_set_socket(struct sc_intr *intr, sc_socket socket);
+
+/**
+ * Set a process as the interruptible component
+ *
+ * Call with SC_PROCESS_NONE to unset.
+ */
+bool
+sc_intr_set_process(struct sc_intr *intr, sc_pid socket);
+
+/**
+ * Interrupt the current interruptible component
+ *
+ * Must be called from a different thread.
+ */
+void
+sc_intr_interrupt(struct sc_intr *intr);
+
+/**
+ * Read the interrupted state
+ *
+ * It is exposed as a static inline function because it just loads from an
+ * atomic.
+ */
+static inline bool
+sc_intr_is_interrupted(struct sc_intr *intr) {
+    return atomic_load_explicit(&intr->interrupted, memory_order_relaxed);
+}
+
+/**
+ * Destroy the interruptor
+ */
+void
+sc_intr_destroy(struct sc_intr *intr);
+
+#endif