[llvm] Avoid a race condition in opt-viewer/optrecord (PR #131214)

[Miro Hrončok via llvm-commits]

================
@@ -64,17 +64,19 @@ class Remark(yaml.YAMLObject):
 
     default_demangler = "c++filt -n"
     demangler_proc = None
+    demangler_lock = Lock()
 
     @classmethod
     def set_demangler(cls, demangler):
         cls.demangler_proc = subprocess.Popen(
             demangler.split(), stdin=subprocess.PIPE, stdout=subprocess.PIPE
         )
-        cls.demangler_lock = Lock()
 
     @classmethod
     def demangle(cls, name):
         with cls.demangler_lock:
+            if not cls.demangler_proc:
----------------
hroncok wrote:

When it's outside of the lock, two workers could attempt to create the process.

 1. worker A sees demangler_proc is None
 2. worker B sees demangler_proc is None
 3. worked A assigns a new process to demangler_proc
 4. worker B assigns a new process to demangler_proc

https://github.com/llvm/llvm-project/pull/131214