Change the cvd directory location to /var/tmp/cvd

It was at /tmp/cvd, but that directory is typically mounted with a tmpfs
system not suitable for the large ammounts of storage the cvd directory
requires.

To avoid leaving running instances behind after an update the old path
is still used when it already exists. Since it's under the /tmp
directory it will be deleted on the next system reboot and, of course,
won't exists after a clean re-image.

Bug: b/458388172

Author: jemoreira

base/cvd/cuttlefish/host/commands/cvd/utils/common.cpp

@@ -38,6 +38,9 @@ namespace cuttlefish {
 
 namespace {
 
+constexpr char kDefaultCvdDir[] = "/var/tmp/cvd";
+constexpr char kPreviousDefaultCvdDir[] = "/tmp/cvd";
+
 std::string DefaultBaseDir() {
   auto time = std::chrono::system_clock::now().time_since_epoch().count();
   return fmt::format("{}/{}", PerUserDir(), time);
@@ -54,6 +57,29 @@ Result<void> LinkOrMakeDir(const std::string& path,
   return {};
 }
 
+bool ShouldUsePreviousCvdLocation() {
+  // Don't use the old one if the new one already exists
+  if (DirectoryExists(kDefaultCvdDir)) {
+    return false;
+  }
+  // Check the locks directory. Even if /tmp/cvd exists it would only be a
+  // problem if an instance exists, but existing instances would have their
+  // corresponding lock file.
+  // It's still possible for a long running cvd create command to have started
+  // before cvd was updated and not yet gotten to create the instance lock file,
+  // in which case it will wrongfully create it under /tmp. If that happens it
+  // can only be addressed by manually killing the running instance and deleting
+  // /tmp/cvd, or just rebooting. This is hopefully unlikely to occur since cvd
+  // would have to be updated at the same time the first cvd load command after
+  // the last boot runs.
+  const std::string old_locks_path =
+      fmt::format("{}/{}", kPreviousDefaultCvdDir, "lock");
+  if (!DirectoryExists(old_locks_path)) {
+    return false;
+  }
+  Result<bool> is_empty_res = IsDirectoryEmpty(old_locks_path);
+  return is_empty_res.ok() && !is_empty_res.value();
+}
 }  // namespace
 
 /*
@@ -155,7 +181,12 @@ android::base::LogSeverity GetMinimumVerbosity() {
   return android::base::GetMinimumLogSeverity();
 }
 
-std::string CvdDir() { return "/tmp/cvd"; }
+std::string CvdDir() {
+  if (ShouldUsePreviousCvdLocation()) {
+    return kPreviousDefaultCvdDir;
+  }
+  return kDefaultCvdDir;
+}
 
 std::string PerUserDir() { return fmt::format("{}/{}", CvdDir(), getuid()); }