Env class that can randomly read and write

Summary: I have implemented basic simple use case that I need for External Value Store I'm working on. There is a potential for making this prettier by refactoring/combining WritableFile and RandomAccessFile, avoiding some copypasta. However, I decided to implement just the basic functionality, so I can continue working on the other diff.

Test Plan: Added a unittest

Reviewers: dhruba, haobo, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13365

include/rocksdb/env.h

@@ -28,6 +28,7 @@ class RandomAccessFile;
 class SequentialFile;
 class Slice;
 class WritableFile;
+class RandomRWFile;
 class Options;
 
 using std::unique_ptr;
@@ -109,6 +110,15 @@ class Env {
                                  unique_ptr<WritableFile>* result,
                                  const EnvOptions& options) = 0;
 
+  // Create an object that both reads and writes to a file on
+  // specified offsets (random access). If file already exists,
+  // does not overwrite it. On success, stores a pointer to the
+  // new file in *result and returns OK. On failure stores nullptr
+  // in *result and returns non-OK.
+  virtual Status NewRandomRWFile(const std::string& fname,
+                                 unique_ptr<RandomRWFile>* result,
+                                 const EnvOptions& options) = 0;
+
   // Returns true iff the named file exists.
   virtual bool FileExists(const std::string& fname) = 0;
 
@@ -329,7 +339,7 @@ class WritableFile {
 
   /*
    * Sync data and/or metadata as well.
-   * By default, sync only metadata.
+   * By default, sync only data.
    * Override this method for environments where we need to sync
    * metadata as well.
    */
@@ -418,6 +428,55 @@ class WritableFile {
   void operator=(const WritableFile&);
 };
 
+// A file abstraction for random reading and writing.
+class RandomRWFile {
+ public:
+  RandomRWFile() {}
+  virtual ~RandomRWFile() {}
+
+  // Write data from Slice data to file starting from offset
+  // Returns IOError on failure, but does not guarantee
+  // atomicity of a write.  Returns OK status on success.
+  //
+  // Safe for concurrent use.
+  virtual Status Write(uint64_t offset, const Slice& data) = 0;
+  // Read up to "n" bytes from the file starting at "offset".
+  // "scratch[0..n-1]" may be written by this routine.  Sets "*result"
+  // to the data that was read (including if fewer than "n" bytes were
+  // successfully read).  May set "*result" to point at data in
+  // "scratch[0..n-1]", so "scratch[0..n-1]" must be live when
+  // "*result" is used.  If an error was encountered, returns a non-OK
+  // status.
+  //
+  // Safe for concurrent use by multiple threads.
+  virtual Status Read(uint64_t offset, size_t n, Slice* result,
+                      char* scratch) const = 0;
+  virtual Status Close() = 0; // closes the file
+  virtual Status Sync() = 0; // sync data
+
+  /*
+   * Sync data and/or metadata as well.
+   * By default, sync only data.
+   * Override this method for environments where we need to sync
+   * metadata as well.
+   */
+  virtual Status Fsync() {
+    return Sync();
+  }
+
+  /*
+   * Pre-allocate space for a file.
+   */
+  virtual Status Allocate(off_t offset, off_t len) {
+    return Status::OK();
+  }
+
+ private:
+  // No copying allowed
+  RandomRWFile(const RandomRWFile&);
+  void operator=(const RandomRWFile&);
+};
+
 // An interface for writing log messages.
 class Logger {
  public:
@@ -497,6 +556,10 @@ class EnvWrapper : public Env {
                          const EnvOptions& options) {
     return target_->NewWritableFile(f, r, options);
   }
+  Status NewRandomRWFile(const std::string& f, unique_ptr<RandomRWFile>* r,
+                         const EnvOptions& options) {
+    return target_->NewRandomRWFile(f, r, options);
+  }
   bool FileExists(const std::string& f) { return target_->FileExists(f); }
   Status GetChildren(const std::string& dir, std::vector<std::string>* r) {
     return target_->GetChildren(dir, r);