Introduce MinidumpWritable, its dependencies, and their tests

util/file/string_file_writer.h

Index: util/file/string_file_writer.h
diff --git a/util/file/string_file_writer.h b/util/file/string_file_writer.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_FILE_STRING_FILE_WRITER_H_
+#define CRASHPAD_UTIL_FILE_STRING_FILE_WRITER_H_
+
+#include <sys/types.h>
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/numerics/safe_math.h"
+#include "util/file/file_writer.h"
+
+namespace crashpad {
+
+//! \brief A file writer backed by a virtual file, as opposed to a file on disk
+//!     or other operating system file descriptor-based file.
+//!
+//! The virtual file is a buffer in memory. This class is convenient for use
+//! with other code that normally expects to write files, when it is impractical
+//! or inconvenient to write a file. It is expected that tests, in particular,
+//! will benefit from using this class.
+class StringFileWriter : public FileWriterInterface {
+ public:
+  StringFileWriter();
+  ~StringFileWriter();
+
+  //! \brief Returns a string containing the virtual file’s contents.
+  const std::string& string() const { return string_; }
+
+  //! \brief Resets the virtual file’s contents to be empty, and resets its file
+  //!     position to `0`.
+  void Reset();
+
+  // FileWriterInterface:
+  virtual bool Write(const void* data, size_t size) override;
+  virtual bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
+  virtual off_t Seek(off_t offset, int whence) override;
+
+ private:
+  //! \brief The virtual file’s contents.
+  std::string string_;
+
+  //! \brief The file offset of the virtual file.
+  //!
+  //! \note This is stored in a `size_t` to match the characteristics of
+  //!     #string_, the `std::string` used to store the virtual file’s contents.
+  //!     This type will have different characteristics than the `off_t` used to
+  //!     report file offsets. The implementation must take care when converting
+  //!     between these distinct types.
+  base::CheckedNumeric<size_t> offset_;
+
+  DISALLOW_COPY_AND_ASSIGN(StringFileWriter);
+};
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_FILE_STRING_FILE_WRITER_H_