[Support] Class to facilitate file locking

This change define RAII class `FileLocker` and methods `lock` and
`tryLockFor` of the class `raw_fd_stream` to facilitate using file locks.

Differential Revision: https://reviews.llvm.org/D79066

include/llvm/Support/FileSystem.h

@@ -1179,6 +1179,35 @@ std::error_code unlockFile(int FD);
 /// means that the filesystem may have failed to perform some buffered writes.
 std::error_code closeFile(file_t &F);
 
+/// RAII class that facilitates file locking.
+class FileLocker {
+  int FD; ///< Locked file handle.
+  FileLocker(int FD) : FD(FD) {}
+  friend class llvm::raw_fd_ostream;
+
+public:
+  FileLocker(const FileLocker &L) = delete;
+  FileLocker(FileLocker &&L) : FD(L.FD) { L.FD = -1; }
+  ~FileLocker() {
+    if (FD != -1)
+      unlockFile(FD);
+  }
+  FileLocker &operator=(FileLocker &&L) {
+    FD = L.FD;
+    L.FD = -1;
+    return *this;
+  }
+  FileLocker &operator=(const FileLocker &L) = delete;
+  std::error_code unlock() {
+    if (FD != -1) {
+      std::error_code Result = unlockFile(FD);
+      FD = -1;
+      return Result;
+    }
+    return std::error_code();
+  }
+};
+
 std::error_code getUniqueID(const Twine Path, UniqueID &Result);
 
 /// Get disk space usage information.

include/llvm/Support/raw_ostream.h

@@ -16,6 +16,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include <cassert>
+#include <chrono>
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
@@ -30,12 +31,14 @@ class format_object_base;
 class FormattedString;
 class FormattedNumber;
 class FormattedBytes;
+template <class T> class LLVM_NODISCARD Expected;
 
 namespace sys {
 namespace fs {
 enum FileAccess : unsigned;
 enum OpenFlags : unsigned;
 enum CreationDisposition : unsigned;
+class FileLocker;
 } // end namespace fs
 } // end namespace sys
 
@@ -468,7 +471,7 @@ public:
   /// fsync.
   void close();
 
-  bool supportsSeeking() { return SupportsSeeking; }
+  bool supportsSeeking() const { return SupportsSeeking; }
 
   /// Flushes the stream and repositions the underlying file descriptor position
   /// to the offset specified from the beginning of the file.
@@ -496,6 +499,38 @@ public:
   ///      - from The Zen of Python, by Tim Peters
   ///
   void clear_error() { EC = std::error_code(); }
+
+  /// Locks the underlying file.
+  ///
+  /// @returns RAII object that releases the lock upon leaving the scope, if the
+  ///          locking was successful. Otherwise returns corresponding
+  ///          error code.
+  ///
+  /// The function blocks the current thread until the lock become available or
+  /// error occurs.
+  ///
+  /// Possible use of this function may be as follows:
+  ///
+  ///   @code{.cpp}
+  ///   if (auto L = stream.lock()) {
+  ///     // ... do action that require file to be locked.
+  ///   } else {
+  ///     handleAllErrors(std::move(L.takeError()), [&](ErrorInfoBase &EIB) {
+  ///       // ... handle lock error.
+  ///     });
+  ///   }
+  ///   @endcode
+  LLVM_NODISCARD Expected<sys::fs::FileLocker> lock();
+
+  /// Tries to lock the underlying file within the specified period.
+  ///
+  /// @returns RAII object that releases the lock upon leaving the scope, if the
+  ///          locking was successful. Otherwise returns corresponding
+  ///          error code.
+  ///
+  /// It is used as @ref lock.
+  LLVM_NODISCARD
+  Expected<sys::fs::FileLocker> tryLockFor(std::chrono::milliseconds Timeout);
 };
 
 /// This returns a reference to a raw_fd_ostream for standard output. Use it

lib/Support/raw_ostream.cpp

@@ -861,6 +861,21 @@ bool raw_fd_ostream::has_colors() const {
   return sys::Process::FileDescriptorHasColors(FD);
 }
 
+Expected<sys::fs::FileLocker> raw_fd_ostream::lock() {
+  std::error_code EC = sys::fs::lockFile(FD);
+  if (!EC)
+    return sys::fs::FileLocker(FD);
+  return errorCodeToError(EC);
+}
+
+Expected<sys::fs::FileLocker>
+raw_fd_ostream::tryLockFor(std::chrono::milliseconds Timeout) {
+  std::error_code EC = sys::fs::tryLockFile(FD, Timeout);
+  if (!EC)
+    return sys::fs::FileLocker(FD);
+  return errorCodeToError(EC);
+}
+
 void raw_fd_ostream::anchor() {}
 
 //===----------------------------------------------------------------------===//

unittests/Support/Path.cpp

@@ -2179,4 +2179,51 @@ TEST_F(FileSystemTest, widenPath) {
 }
 #endif
 
+#ifdef _WIN32
+// Windows refuses lock request if file region is already locked by the same
+// process. POSIX system in this case updates the existing lock.
+TEST_F(FileSystemTest, FileLocker) {
+  using namespace std::chrono;
+  int FD;
+  std::error_code EC;
+  SmallString<64> TempPath;
+  EC = fs::createTemporaryFile("test", "temp", FD, TempPath);
+  ASSERT_NO_ERROR(EC);
+  FileRemover Cleanup(TempPath);
+  raw_fd_ostream Stream(TempPath, EC);
+
+  EC = fs::tryLockFile(FD);
+  ASSERT_NO_ERROR(EC);
+  EC = fs::unlockFile(FD);
+  ASSERT_NO_ERROR(EC);
+
+  if (auto L = Stream.lock()) {
+    ASSERT_ERROR(fs::tryLockFile(FD));
+    ASSERT_NO_ERROR(L->unlock());
+    ASSERT_NO_ERROR(fs::tryLockFile(FD));
+    ASSERT_NO_ERROR(fs::unlockFile(FD));
+  } else {
+    ADD_FAILURE();
+    handleAllErrors(L.takeError(), [&](ErrorInfoBase &EIB) {});
+  }
+
+  ASSERT_NO_ERROR(fs::tryLockFile(FD));
+  ASSERT_NO_ERROR(fs::unlockFile(FD));
+
+  {
+    Expected<fs::FileLocker> L1 = Stream.lock();
+    ASSERT_THAT_EXPECTED(L1, Succeeded());
+    raw_fd_ostream Stream2(FD, false);
+    Expected<fs::FileLocker> L2 = Stream2.tryLockFor(250ms);
+    ASSERT_THAT_EXPECTED(L2, Failed());
+    ASSERT_NO_ERROR(L1->unlock());
+    Expected<fs::FileLocker> L3 = Stream.tryLockFor(0ms);
+    ASSERT_THAT_EXPECTED(L3, Succeeded());
+  }
+
+  ASSERT_NO_ERROR(fs::tryLockFile(FD));
+  ASSERT_NO_ERROR(fs::unlockFile(FD));
+}
+#endif
+
 } // anonymous namespace