Add support for LinkFile() in AwsEnv

This helps avoid copying the file when ingesting external file with
move_files set to true.

Author: igorcanadi

cloud/aws/aws_env.cc

@@ -1440,6 +1440,14 @@ Status AwsEnv::RenameFile(const std::string& logical_src,
   return st;
 }
 
+Status AwsEnv::LinkFile(const std::string& src, const std::string& target) {
+  // We only know how to link file if both src and dest buckets are empty
+  if (has_dest_bucket_ || has_src_bucket_) {
+    return Status::NotSupported();
+  }
+  return base_env_->LinkFile(src, target);
+}
+
 //
 // Copy my IDENTITY file to cloud storage. Update dbid registry.
 //

cloud/aws/aws_env.h

@@ -155,10 +155,8 @@ class AwsEnv : public CloudEnvImpl {
   virtual Status RenameFile(const std::string& src,
                             const std::string& target) override;
 
-  virtual Status LinkFile(const std::string& /*src*/,
-                          const std::string& /*target*/) override {
-    return Status::NotSupported();  // not supported
-  }
+  virtual Status LinkFile(const std::string& src,
+                          const std::string& target) override;
 
   virtual Status LockFile(const std::string& fname, FileLock** lock) override;