[lldb] Prevent race condition when fetching /proc/cpuinfo

@clayborg found a potential race condition when setting a static
variable. The fix seems simply to use call_once.

All relevant tests pass.

Differential Revision: https://reviews.llvm.org/D131081

Author: Walter Erquinigo

lldb/source/Plugins/Process/Linux/Procfs.cpp

@@ -7,27 +7,24 @@
 //===----------------------------------------------------------------------===//
 
 #include "Procfs.h"
-
 #include "lldb/Host/linux/Support.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Threading.h"
 
 using namespace lldb;
 using namespace lldb_private;
 using namespace process_linux;
 using namespace llvm;
 
 Expected<ArrayRef<uint8_t>> lldb_private::process_linux::GetProcfsCpuInfo() {
-  static Optional<std::vector<uint8_t>> cpu_info;
-  if (!cpu_info) {
-    auto buffer_or_error = errorOrToExpected(getProcFile("cpuinfo"));
-    if (!buffer_or_error)
-      return buffer_or_error.takeError();
-    MemoryBuffer &buffer = **buffer_or_error;
-    cpu_info = std::vector<uint8_t>(
-        reinterpret_cast<const uint8_t *>(buffer.getBufferStart()),
-        reinterpret_cast<const uint8_t *>(buffer.getBufferEnd()));
-  }
-  return *cpu_info;
+  static ErrorOr<std::unique_ptr<MemoryBuffer>> cpu_info_or_err =
+      getProcFile("cpuinfo");
+
+  if (!*cpu_info_or_err)
+    cpu_info_or_err.getError();
+
+  MemoryBuffer &buffer = **cpu_info_or_err;
+  return arrayRefFromStringRef(buffer.getBuffer());
 }
 
 Expected<std::vector<cpu_id_t>>