Create daemon module and use it from server

Need to run setup() after forking, otherwise each TaskRunner created
in setup() will dead-lock at exit as there are now two copies of
each of them but not of the threads causing the destructors to lock.

This made setup a little bit more complicated as it has to forward
the log and status to parent process but I turned out quite nice.

1 files changed, 45 insertions, 0 deletions

diff --git a/src/daemon.hh b/src/daemon.hh
new file mode 100644
index 0000000..10f0584
--- /dev/null
+++ b/src/daemon.hh
@@ -0,0 +1,45 @@
+#ifndef DAEMON_HH
+#define DAEMON_HH
+
+#include <memory>
+
+class Logger;
+
+class Daemon {
+public:
+  virtual ~Daemon() = default;
+
+  // Method is called after forking;
+  // if it returns true then parent process will detach and return success and
+  // run is executed.
+  // if it returns false then parent process will exit in failure and run is
+  // never executed.
+  // Anything logged to logger will write to calling logger regardless of return
+  // value.
+  // Note that stdin, stdout and stderr are all closed when this runs, you
+  // must use logger object if you want to output anything.
+  // Current directory is not modified.
+  virtual bool setup(Logger* logger) = 0;
+
+  // Method is called after forking and only after setup returns true.
+  // Note that stdin, stdout and stderr are all closed when this runs.
+  // Current directory is changed to root.
+  virtual bool run() = 0;
+
+  // Forks, runs setup() method before returning true or false.
+  // If setup() returns true then so does this method and run() is called.
+  // If setup() returns false then so does this method and run() is never
+  // called.
+  static bool fork_in_background(Logger* logger,
+                                 std::unique_ptr<Daemon> daemon);
+
+  // Doesn't fork, just runs setup() and if it returns true then
+  // also run() and waits for it to return.
+  static bool run_in_foreground(Logger* logger,
+                                std::unique_ptr<Daemon> daemon);
+
+protected:
+  Daemon() = default;
+};
+
+#endif  // DAEMON_HH