[lldb] Fix unsafe map mutation in ProcessElfCore::FindModuleUUID (llvm#159444)

The `ProcessElfCore::FindModuleUUID` function can be called by multiple
threads at the same time when `target.parallel-module-load` is true. We
were using the `operator[]` to lookup the UUID which will mutate the map
when the key is not present. This is unsafe in a multi-threaded contex
so we now use a read-only `find` operation and explicitly return an
invalid UUID when the key is not present.

The `m_uuids` map can follow a create-then-query pattern. It is
populated in the `DoLoadCore` function which looks like it is only
called in a single-threaded context so we do not need extra locking as
long as we keep the other accesses read-only.

Other fixes I considered

* Use a lock to protect access - We don't need to modify the map after
creation so we can allow concurrent read-only access.
* Store the map in a const pointer container to prevent accidental
mutation in other places.
     - Only accessed in one place currently so just added a comment.
* Store the UUID in the NT_FILE_Entry after building the mapping
correctly in `UpdateBuildIdForNTFileEntries`. - The map lets us avoid a
linear search in `FindModuleUUID`.

This commit also reverts the temporary workaround from llvm#159395 which
disabled parallel module loading to avoid the test failure.

Fixes llvm#159377

Author: dmpots

lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp

@@ -308,7 +308,13 @@ llvm::StringRef ProcessElfCore::GetMainExecutablePath() {
 }
 
 UUID ProcessElfCore::FindModuleUUID(const llvm::StringRef path) {
-  return m_uuids[std::string(path)];
+  // Lookup the UUID for the given path in the map.
+  // Note that this could be called by multiple threads so make sure
+  // we access the map in a thread safe way (i.e. don't use operator[]).
+  auto it = m_uuids.find(std::string(path));
+  if (it != m_uuids.end())
+    return it->second;
+  return UUID();
 }
 
 lldb_private::DynamicLoader *ProcessElfCore::GetDynamicLoader() {

lldb/test/API/functionalities/postmortem/netbsd-core/TestNetBSDCore.py

@@ -117,8 +117,6 @@ def check_backtrace(self, thread, filename, backtrace):
                 )
 
     def do_test(self, filename, pid, region_count):
-        # Temporary workaround for https://github.com/llvm/llvm-project/issues/159377
-        self.runCmd("settings set target.parallel-module-load false")
         target = self.dbg.CreateTarget(filename)
         process = target.LoadCore(filename + ".core")