Fix EncryptedEnv assert (#5735)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/5734. By reading the code the assert don't quite make sense to me, since `dataSize` and `fileOffset` has no correlation. But my knowledge about `EncryptedEnv` is very limited.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5735

Test Plan:
run `ENCRYPTED_ENV=1 ./db_encryption_test`

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Differential Revision: D17133849

fbshipit-source-id: bb7262d308e5b2503c400b180edc252668df0ef0

db/db_encryption_test.cc

@@ -83,6 +83,34 @@ TEST_F(DBEncryptionTest, CheckEncrypted) {
   }
 }
 
+TEST_F(DBEncryptionTest, ReadEmptyFile) {
+  auto defaultEnv = Env::Default();
+
+  // create empty file for reading it back in later
+  auto envOptions = EnvOptions(CurrentOptions());
+  auto filePath = dbname_ + "/empty.empty";
+
+  Status status;
+  {
+    std::unique_ptr<WritableFile> writableFile;
+    status = defaultEnv->NewWritableFile(filePath, &writableFile, envOptions);
+    ASSERT_OK(status);
+  }
+
+  std::unique_ptr<SequentialFile> seqFile;
+  status = defaultEnv->NewSequentialFile(filePath, &seqFile, envOptions);
+  ASSERT_OK(status);
+
+  std::string scratch;
+  Slice data;
+  // reading back 16 bytes from the empty file shouldn't trigger an assertion.
+  // it should just work and return an empty string
+  status = seqFile->Read(16, &data, (char*)scratch.data());
+  ASSERT_OK(status);
+
+  ASSERT_TRUE(data.empty());
+}
+
 #endif // ROCKSDB_LITE
 
 }  // namespace rocksdb

env/env_encryption.cc

@@ -745,8 +745,6 @@ Status BlockAccessCipherStream::Decrypt(uint64_t fileOffset, char *data, size_t
   std::string scratch;
   AllocateScratch(scratch);
 
-  assert(fileOffset < dataSize);
-
   // Decrypt individual blocks.
   while (1) {
     char *block = data;