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.c

@@ -0,0 +1,83 @@
+#include "intr.h"
+
+#include "util/log.h"
+
+#include <assert.h>
+
+bool
+sc_intr_init(struct sc_intr *intr) {
+    bool ok = sc_mutex_init(&intr->mutex);
+    if (!ok) {
+        LOGE("Could not init intr mutex");
+        return false;
+    }
+
+    intr->socket = SC_INVALID_SOCKET;
+    intr->process = SC_PROCESS_NONE;
+
+    atomic_store_explicit(&intr->interrupted, false, memory_order_relaxed);
+
+    return true;
+}
+
+bool
+sc_intr_set_socket(struct sc_intr *intr, sc_socket socket) {
+    assert(intr->process == SC_PROCESS_NONE);
+
+    sc_mutex_lock(&intr->mutex);
+    bool interrupted =
+        atomic_load_explicit(&intr->interrupted, memory_order_relaxed);
+    if (!interrupted) {
+        intr->socket = socket;
+    }
+    sc_mutex_unlock(&intr->mutex);
+
+    return !interrupted;
+}
+
+bool
+sc_intr_set_process(struct sc_intr *intr, sc_pid pid) {
+    assert(intr->socket == SC_INVALID_SOCKET);
+
+    sc_mutex_lock(&intr->mutex);
+    bool interrupted =
+        atomic_load_explicit(&intr->interrupted, memory_order_relaxed);
+    if (!interrupted) {
+        intr->process = pid;
+    }
+    sc_mutex_unlock(&intr->mutex);
+
+    return !interrupted;
+}
+
+void
+sc_intr_interrupt(struct sc_intr *intr) {
+    sc_mutex_lock(&intr->mutex);
+
+    atomic_store_explicit(&intr->interrupted, true, memory_order_relaxed);
+
+    // No more than one component to interrupt
+    assert(intr->socket == SC_INVALID_SOCKET ||
+           intr->process == SC_PROCESS_NONE);
+
+    if (intr->socket != SC_INVALID_SOCKET) {
+        LOGD("Interrupting socket");
+        net_interrupt(intr->socket);
+        intr->socket = SC_INVALID_SOCKET;
+    }
+    if (intr->process != SC_PROCESS_NONE) {
+        LOGD("Interrupting process");
+        sc_process_terminate(intr->process);
+        intr->process = SC_PROCESS_NONE;
+    }
+
+    sc_mutex_unlock(&intr->mutex);
+}
+
+void
+sc_intr_destroy(struct sc_intr *intr) {
+    assert(intr->socket == SC_INVALID_SOCKET);
+    assert(intr->process == SC_PROCESS_NONE);
+
+    sc_mutex_destroy(&intr->mutex);
+}