In-memory environment read beyond EOF

Summary:
Made it consistent with posix Env, which uses pread() that returns 0
(success) when an offset is given beyond EOF. The purpose of making these Envs
behave consistently is I am repurposing the in-memory Envs' tests for the basic
Env tests in D58635.

Test Plan: ran mock_env_test and memenv_test

Reviewers: IslamAbdelRahman, lightmark, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D58845

util/memenv.cc

@@ -2,14 +2,17 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 
+#include <string.h>
+
+#include <algorithm>
+#include <map>
+#include <string>
+#include <vector>
+
 #include "rocksdb/env.h"
 #include "rocksdb/status.h"
 #include "port/port.h"
 #include "util/mutexlock.h"
-#include <map>
-#include <string.h>
-#include <string>
-#include <vector>
 
 namespace rocksdb {
 
@@ -70,10 +73,7 @@ class FileState {
   uint64_t Size() const { return size_; }
 
   Status Read(uint64_t offset, size_t n, Slice* result, char* scratch) const {
-    if (offset > size_) {
-      return Status::IOError("Offset greater than file size.");
-    }
-    const uint64_t available = size_ - offset;
+    const uint64_t available = size_ - std::min(size_, offset);
     if (n > available) {
       n = available;
     }

util/memenv_test.cc

@@ -124,7 +124,7 @@ TEST_F(MemEnvTest, ReadWrite) {
   ASSERT_EQ(0, result.compare("d"));
 
   // Too high offset.
-  ASSERT_TRUE(!rand_file->Read(1000, 5, &result, scratch).ok());
+  ASSERT_TRUE(rand_file->Read(1000, 5, &result, scratch).ok());
 }
 
 TEST_F(MemEnvTest, Locks) {

util/mock_env.cc

@@ -99,10 +99,7 @@ class MemFile {
 
   Status Read(uint64_t offset, size_t n, Slice* result, char* scratch) const {
     MutexLock lock(&mutex_);
-    if (offset > Size()) {
-      return Status::IOError("Offset greater than file size.");
-    }
-    const uint64_t available = Size() - offset;
+    const uint64_t available = Size() - std::min(Size(), offset);
     if (n > available) {
       n = available;
     }

util/mock_env_test.cc

@@ -123,7 +123,7 @@ TEST_F(MockEnvTest, ReadWrite) {
   ASSERT_EQ(0, result.compare("d"));
 
   // Too high offset.
-  ASSERT_TRUE(!rand_file->Read(1000, 5, &result, scratch).ok());
+  ASSERT_TRUE(rand_file->Read(1000, 5, &result, scratch).ok());
 }
 
 TEST_F(MockEnvTest, Locks) {