Run EventManager whenever writing to a file descriptor would block

This approach is not very elegant as it hooks into the event manager
deep inside the call graph, but solves the exiting issue and is an
okay stop gap solution until a better design comes up.

Fixes #4605

src/file.cc

@@ -5,6 +5,7 @@
 #include "exception.hh"
 #include "flags.hh"
 #include "option_types.hh"
+#include "event_manager.hh"
 #include "ranked_match.hh"
 #include "regex.hh"
 #include "string.hh"
@@ -256,13 +257,20 @@ void write(int fd, StringView data)
     const char* ptr = data.data();
     ssize_t count   = (int)data.length();
 
+    int flags = fcntl(fd, F_GETFL, 0);
+    fcntl(fd, F_SETFL, flags | O_NONBLOCK);
+    auto restore_flags = on_scope_end([&] { fcntl(fd, F_SETFL, flags); });
+
     while (count)
     {
-        ssize_t written = ::write(fd, ptr, count);
+        if (ssize_t written = ::write(fd, ptr, count); written != -1)
+        {
             ptr += written;
             count -= written;
-
-        if (written == -1)
+        }
+        else if (errno == EAGAIN and EventManager::has_instance())
+            EventManager::instance().handle_next_events(EventMode::Urgent, nullptr, false);
+        else
             throw file_access_error(format("fd: {}", fd), strerror(errno));
     }
 }

test/regression/4605-fifo-hang/cmd

@@ -0,0 +1 @@
+

test/regression/4605-fifo-hang/rc

@@ -0,0 +1,5 @@
+exec 5000oabcdefghi<esc>%yppppp
+nop %sh{
+    echo "write $kak_response_fifo" > "$kak_command_fifo"
+    strace -o /tmp/strace.log cat "$kak_response_fifo"
+}