Add support for PG_DIAG_*_NAME error fields

[ged]

Original report by Joe Van Dyk (Bitbucket: joevandyk, GitHub: joevandyk).

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See related discussion here: http://psycopg.lighthouseapp.com/projects/62710/tickets/149

http://www.postgresql.org/docs/devel/static/libpq-exec.html#LIBPQ-PQRESULTERRORFIELD

[ged]

Original comment by Lars Kanis (Bitbucket: larskanis, GitHub: larskanis).

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Thanks for the pointer. I'll add the new constants.

[ged]

Original comment by Michael Granger (Bitbucket: ged, GitHub: ged).

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This looks awesome. I wonder if we could use the new constants in conjunction with the implementation of #5 to do something like they did with Psycopg?

[ged]

Original comment by Joe Van Dyk (Bitbucket: joevandyk, GitHub: joevandyk).

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http://www.depesz.com/2013/03/07/waiting-for-9-3-provide-database-object-names-as-separate-fields-in-error-messages/ has more btw.

[ged]

Original comment by masklinn (Bitbucket: masklinn, GitHub: masklinn).

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@ged psycopg doesn't really use the #5 thing for diagnostics, they added a new attribute to the root Psycopg exception, and that attribute links to an object bearing all diagnostic attributes. It's not limited to any specific kind of error.

Although it is related to having a root exception type for the adapter, I don't know if pg has one.

[ged]

Original comment by Lars Kanis (Bitbucket: larskanis, GitHub: larskanis).

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The new PG_DIAG_*_NAME error fields IMHO don't help in regards to issue #5. But psycopg does already proper subclassing of exceptions that is described in #5. It's realized through errorcodes.py which is parsed and autogenerated from the postgresql.org documentation. We might adopt some ideas (although I would like parsing from postgresql/src/backend/utils/errorcodes.txt more).

Regarding to the PG_DIAG_*_NAME error fields, no further changes are needed. We already have this link to the result object, which can be used to retrieve the PG_DIAG_* fields.

[ged]

Original comment by Michael Granger (Bitbucket: ged, GitHub: ged).

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Add support for PG_DIAG_*_NAME error fields.

This fixes issue #161 .