Wal recovery failure with encryption due to zero bytes WAL size. (#7924)

Summary:
Fixes issue https://github.com/facebook/rocksdb/issues/7410

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7924

Reviewed By: jay-zhuang

Differential Revision: D26216805

Pulled By: ajkr

fbshipit-source-id: 5720635ee06003269bfaf8c8616b3671974b08a8

env/env_encryption.cc

@@ -646,6 +646,15 @@ class EncryptedEnvImpl : public EnvWrapper {
     if (!status.ok()) {
       return status;
     }
+    uint64_t file_size;
+    status = EnvWrapper::GetFileSize(fname, &file_size);
+    if (!status.ok()) {
+      return status;
+    }
+    if (!file_size) {
+      *result = std::move(underlying);
+      return status;
+    }
     // Create cipher stream
     std::unique_ptr<BlockAccessCipherStream> stream;
     size_t prefix_length;
@@ -830,7 +839,7 @@ class EncryptedEnvImpl : public EnvWrapper {
   virtual Status GetFileSize(const std::string& fname,
                              uint64_t* file_size) override {
     auto status = EnvWrapper::GetFileSize(fname, file_size);
-    if (!status.ok()) {
+    if (!status.ok() || !(*file_size)) {
       return status;
     }
     EncryptionProvider* provider;