[gpsd-commit-watch] [SCM] GPSD branch, master, updated. release-3.15-68-g3b66e6c

[Eric S. Raymond]

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- Log -----------------------------------------------------------------
commit 3b66e6cb486b157aad6a65b22caae65e10b8b4e5
Author: Fred Wright <address@hidden>
Date:   Mon Jan 4 14:55:29 2016 -0500

    Address bug #46802: AIVDM to CSV is broken in some weird cases
    
    While trying the regression tests on a MacBook (PowerPC), I ran across
    some failures in the "Testing AIVDM decoding w/ CSV format" test. The
    failing type/dac/fid cases were as follows:
    
    6,1,14
    6,1,18
    6,1,30
    6,1,32
    8,200,10
    
    The proximate cause is endian dependence in the CSV output, but the
    real problem turns out to be that the code is broken on all platforms,
    but only fails on big-endian platforms because the
    incorrect-but-presumed-correct output was captured on a little-endian
    processor.
    
    The problem arises when the type 6 or type 8 case is supported by the
    AIVDM parser but not by the CSV formatter. When the parser doesn't
    support the case, it falls back to capturing the raw binary
    bitdata. When the formatter doesn't support the case, it falls back to
    dumping the raw binary bitdata. But in the offending cases, the raw
    data was never correctly captured, and is instead an overlaid view of
    the structured data, which is incorrect (as raw data) in all cases as
    well as being endian-sensitive due to the presence of multibyte
    quantities in host byte order.
    
    The only way to make the CSV fallback work as intended would be to
    change the struct ais_t type6 and type8 substructure definitions to
    move the bitdata field out of the union and populate it
    unconditionally. But this would increase the memory footprint and add
    to the core code for the sole purpose of providing more useful output
    in these cases of missing CSV formatting support, which doesn't seem
    very justifiable.
    
    The alternative is simply not to report the unavailable raw bitdata at
    all in these cases, but instead to report something indicating that
    the raw data is unknown. I've implemented a version of this where the
    formatting remains the same (including the bit count) but the hex data
    is replaced by "x" characters (which seemed safer than using "?",
    given the absence of the latter in any existing cases.
    
    After implementing the code change and before updating the test data,
    I saw one additional miscompare that I'd not seen on the PPC. This was
    for the 6,1,12 case, where the existing test data is too incomplete to
    populate the endian-sensitive fields in the decoded form, but which
    would have failed with a better version of the test case.
    
    After updating the test data as well as the code, the test passes on
    both x86_64 and PPC.
    
    P.S.:
    
    There are three additional uses of gpsd_hexdump in the CSV output, for
    types 17, 25, and 26, but those all seem to populate the bidata field
    unconditionally (and type 17 doesn't even have a "structured" flag),
    so I left them alone.
    
    Clearly the right fix would be to implement the missing CSV formatting
    cases, which would render the fallback code unreachable, but it could
    become reachable again if new cases were implemented in the parser and
    not in the CSV formatter, so having a well-behaved fallback seems like
    a good idea.

-----------------------------------------------------------------------

Summary of changes:
 gpsdecode.c           |   34 ++++++++++++++++++++++++++++------
 test/sample.aivdm.chk |   18 +++++++++---------
 2 files changed, 37 insertions(+), 15 deletions(-)


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