8d32717b6b
Previously, we did many conversions for Cyrillic and Central European single-byte encodings by converting to a related MULE_INTERNAL coding scheme before converting to the destination. This seems unnecessarily inefficient. Moreover, if the conversion encounters an untranslatable character, the error message will confusingly complain about failure to convert to or from MULE_INTERNAL, rather than the user-visible encodings. Worse still, this approach results in some completely unnecessary conversion failures; there are cases where the chosen MULE subset lacks characters that exist in both of the user-visible encodings, causing a conversion failure that need not occur. This patch fixes the first two of those deficiencies by introducing a new local2local() conversion support subroutine for direct conversion between any two single-byte character sets, and adding new conversion tables where needed. However, I generated the new conversion tables by testing PG 9.5's behavior, so that the actual conversion behavior is bug-compatible with previous releases; the only user-visible behavior change is that the error messages for conversion failures are saner. Changes in the conversion behavior will probably ensue after discussion. Interestingly, although this approach requires more tables, the .so files actually end up smaller (at least on my x86_64 machine); the tables are smaller than the management code needed for double conversion. Per a complaint from Albe Laurenz.
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