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https://issues.apache.org/jira/browse/IMPALA-8561?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16841805#comment-16841805
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Michael Ho commented on IMPALA-8561:
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Nice debugging ! This will probably affect the data cache too as it relies on
mtime as part of cache key.
> 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
>
> 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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