[clang] [clang][DependencyScanning] Logging Dependency Scanning Events (PR #195896)

[Qiongsi Wu via cfe-commits]

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@@ -0,0 +1,68 @@
+//===- AtomicLineLogger.h ---------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Defines a logger where each line is written atomically to the file. It is
+/// safe to share a logger instance across threads.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_ATOMICLINELOGGER_H
+#define LLVM_CLANG_BASIC_ATOMICLINELOGGER_H
+
+#include "clang/Basic/LLVM.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/raw_ostream.h"
+#include <memory>
+
+namespace clang {
+
+class LogLine {
+  SmallString<128> Buffer;
+  std::optional<llvm::raw_svector_ostream> FormattingOS;
+  raw_ostream *Dest = nullptr;
+
+public:
+  explicit LogLine(raw_ostream &Dest);
+  LogLine() {}
+  LogLine(LogLine &&Other);
+  LogLine(const LogLine &) = delete;
+  LogLine &operator=(const LogLine &) = delete;
+  LogLine &operator=(LogLine &&) = delete;
+
+  ~LogLine() {
+    if (Dest) {
+      assert(FormattingOS && "Cannot have uninitialized FormattingOS");
+      *FormattingOS << '\n';
+      Dest->write(Buffer.data(), Buffer.size());
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qiongsiwu wrote:

Good catch! TSan does get tripped at `pos += Size;` inside `write_impl`. The `OnDiskCASLogger` has exactly the same problem. 

I've rewritten the code that performs the write (https://github.com/llvm/llvm-project/pull/205395, with this PR rebased on top of it), and the unit tests pass with TSan. I will check with the CAS folks to see if we need to do something there. 

https://github.com/llvm/llvm-project/pull/195896