Steve Loughran commented on HADOOP-13712:

OutputStream open(path) {
  if (!path.endsWith."/") {
    try {
     return GET path
   } catch (404) {
     // not a file
  // either no file or looking for "/", fallback for exception 
  // this will raise the FNFE if the path isn't there
  S3AFileStatus status = getFileStatus(path);
  if (status.isDirectory()) {
    throw new PathIsDirectoryException(path);
  } else {
    // we could only get here if there wasn't a file before, but there is now. 
    // Race condition? And how to react? Recursion?

> S3A open to avoid needless HEAD on the successful execution path
> ----------------------------------------------------------------
>                 Key: HADOOP-13712
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13712
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.7.3
>            Reporter: Steve Loughran
> S3A's open() operation does a {{getFileStatus()}} check to see if a file is 
> not a directory before opening with a GET. That initial check will take up at 
> least one HEAD request if the file is present, more if it isn't.
> As the GET itself performs the existence check, it is needless. A successful 
> GET of a path which doesn't end in "/" means a file was there. The only 
> reason a getFileStatus call is needed is to choose which error message to 
> display if the path isn't there: is it an FNFE or is it path-is-directory.
> Proposed: reorder the code to do the GET; only if that fails fallback to 
> getFileStatus()

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