sim: manage: exclusively lock sim directory while killing other sim i…

…nstances

If two "manage start" were run it might have end up in situation where both
manage killed each other before any of them started Sim.

Fixes #30

subprojects/sim/src/manage.cc

@@ -12,6 +12,7 @@
 #include <simlib/process.hh>
 #include <simlib/string_view.hh>
 #include <simlib/syscalls.hh>
+#include <sys/file.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <thread>
@@ -96,6 +97,16 @@ struct SimPaths {
 
 static void stop() {
     SimPaths paths;
+    // Lock the sim directory during killing to avoid situation where two instances wanting to start
+    // sim kill each other before starting sim.
+    auto sim_dir = path_dirpath(paths.manage).to_string();
+    auto sim_dir_fd = FileDescriptor{sim_dir.c_str(), O_RDONLY};
+    if (flock(sim_dir_fd, LOCK_EX)) {
+        errlog("flock()", errmsg());
+        _exit(EXIT_FAILURE);
+    }
+    // Lock is freed when the sim_dir_fd is closed
+
     // First kill manage so that it won't restart servers
     kill_processes_by_exec({paths.manage}, std::chrono::seconds(1), true);
     // Kill servers