debuggerd: dump frozen Java processes

Test: stacks <pid>
Test: adb shell debuggerd -b <pid>
Bug: 319754740
Change-Id: Iae230ff34409bee4a325113aed65433292975427
diff --git a/debuggerd/debuggerd.cpp b/debuggerd/debuggerd.cpp
index 26726cf..0d4b91f 100644
--- a/debuggerd/debuggerd.cpp
+++ b/debuggerd/debuggerd.cpp
@@ -91,6 +91,15 @@
     }
   }
 
+  // unfreeze if pid is frozen.
+  const std::string freeze_file = android::base::StringPrintf(
+      "/sys/fs/cgroup/uid_%d/pid_%d/cgroup.freeze", proc_info.uid, proc_info.pid);
+  if (std::string freeze_status;
+      android::base::ReadFileToString(freeze_file, &freeze_status) && freeze_status[0] == '1') {
+    android::base::WriteStringToFile("0", freeze_file);
+    // we don't restore the frozen state as this is considered a benign change.
+  }
+
   unique_fd output_fd(fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 0));
   if (output_fd.get() == -1) {
     err(1, "failed to fcntl dup stdout");