lockfile: handle same PID case and add hint for container user

src/core/download_store.cc

@@ -37,14 +37,23 @@ DownloadStore::enable(bool lock) {
     m_lockfile.set_path(std::string());
 
   if (!m_lockfile.try_lock()) {
-    if (torrent::utils::error_number::current().is_bad_path())
+    if (torrent::utils::error_number::current().is_bad_path()) {
       throw torrent::input_error(
         "Could not lock session directory: \"" + m_path + "\", " +
         torrent::utils::error_number::current().message());
-    else
-      throw torrent::input_error("Could not lock session directory: \"" +
-                                 m_path + "\", held by \"" +
-                                 m_lockfile.locked_by_as_string() + "\".");
+    } else {
+      auto msg = "Could not lock session directory: \"" + m_path +
+                 "\", held by \"" + m_lockfile.locked_by_as_string() + "\".";
+
+      if (::getpid() < 10) {
+        // Hint for container users
+        msg += '\n';
+        msg += "Hint: use a consistent hostname so rTorrent can safely handle "
+               "stale locks.";
+      }
+
+      throw torrent::input_error(msg);
+    }
   }
 }
 

src/utils/lockfile.cc

@@ -31,15 +31,27 @@ struct lockfile_valid_hostname {
 
 bool
 Lockfile::is_stale() {
-  process_type process = locked_by();
+  const auto [hostname, pid] = locked_by();
 
   char buf[256];
 
-  if (process.second <= 0 || ::gethostname(buf, 255) != 0 ||
-      buf != process.first)
+  if (pid <= 0) {
+    // um... pid is not supposed to be negative
     return false;
+  }
+
+  if (::gethostname(buf, 255) != 0 || buf != hostname) {
+    // it is probably locked by another machine
+    // maybe the directory is shared or the hostname has changed
+    return false;
+  }
+
+  if (::getpid() == pid) {
+    // this happens a lot with controlled environments like containers
+    return true;
+  }
 
-  return ::kill(process.second, 0) != 0 && errno != EPERM;
+  return ::kill(pid, 0) != 0 && errno != EPERM;
 }
 
 bool