[jira] [Resolved] (IMPALA-8561) ScanRanges with mtime=-1 can lead to inconsistent reads when using the file handle cache

["Joe McDonnell (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/IMPALA-8561?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Joe McDonnell resolved IMPALA-8561.
-----------------------------------
       Resolution: Fixed
    Fix Version/s: Impala 3.3.0

> ScanRanges with mtime=-1 can lead to inconsistent reads when using the file handle cache
> ----------------------------------------------------------------------------------------
>
>                 Key: IMPALA-8561
>                 URL: https://issues.apache.org/jira/browse/IMPALA-8561
>             Project: IMPALA
>          Issue Type: Bug
>          Components: Backend
>    Affects Versions: Impala 3.3.0
>            Reporter: Joe McDonnell
>            Assignee: Joe McDonnell
>            Priority: Blocker
>             Fix For: Impala 3.3.0
>
>
> {color:red}colored text{color}The file handle cache relies on the mtime to distinguish between different versions of a file. For example, if file X exists with mtime=1, then it is overwritten and the metadata is updated so that now it is at mtime=2, the file handle cache treats them as completely different things and can never use a single file handle to serve both. However, some codepaths generate ScanRanges with an mtime of -1. This removes the ability to distinguish these two versions of a file and can read to consistency problems.
> A specific example is the code that reads the parquet footer [HdfsParquetScanner::ProcessFooter()|https://github.com/apache/impala/blob/832c9de7810b47b5f782bccb761e07264e7548e5/be/src/exec/parquet/hdfs-parquet-scanner.cc#L1354]. We don't know ahead of time how big the Parquet footer is. So, we read 100KB (determined by [FOOTER_SIZE|https://github.com/apache/impala/blob/449fe73d2145bd22f0f857623c3652a097f06d73/be/src/exec/hdfs-scanner.h#L331]). If the footer size encoded in the last few bytes of the file indicates that the footer is larger than that [code here|https://github.com/apache/impala/blob/832c9de7810b47b5f782bccb761e07264e7548e5/be/src/exec/parquet/hdfs-parquet-scanner.cc#L1414], then we issue a separate read for the actual size of the footer. That separate read does not inherit the mtime of the original read and instead uses an mtime of -1. I verified this by adding tracing and issuing a select against functional_parquet.widetable_1000_cols.
> A failure scenario associated with this is that we read the last 100KB using a ScanRange with mtime=2, then we find that the footer is larger than 100KB and issue a ScanRange with mtime=-1. This uses a file handle that is from a previous version of the file equivalent to mtime=1. The data it is reading may not come from the end of the file, or it may be at the end of the file but the footer has a different length. (There is no validation on the new read to check the magic value or metadata size reported by the new buffer.) Either would result in a failure to deserialize the thrift for the footer. For example, a problem case could produce an error message like:
>  
> {noformat}
> File hdfs://test-warehouse/example_file.parq of length 1048576 bytes has invalid file metadata at file offset 462017. Error = couldn't deserialize thrift msg:
> TProtocolException: Invalid data
> .{noformat}
> To fix this, we should examine all locations that can result in ScanRanges with mtime=-1 and eliminate any that we can. For example, the HdfsParquetScanner::ProcessFooter() code should create a ScanRange that inherits the mtime from the original footer ScanRange. Also, the file handle cache should refuse to cache file handles with mtime=-1.
> The code in HdfsParquetScanner::ProcessFooter() should add validation for the magic value and metadata size when reading a footer larger than 100KB to verify that we are reading something valid. The thrift deserialize failure gives some information, but catching this case more specifically would provide a better error message.
>  



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