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Some quick notes about extending libpq for new protocol

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-interfaces(at)postgreSQL(dot)org, pgsql-hackers(at)postgreSQL(dot)org
Subject: Some quick notes about extending libpq for new protocol
Date: 2003-06-02 17:19:36
Message-ID: (view raw or flat)
Lists: pgsql-hackerspgsql-interfaces
Here are my notes about what to do to extend libpq's API to support the
new features of the 3.0 frontend/backend protocol.  Not very polished
but I trust you can follow it.  Any objections?

			regards, tom lane

LIBPQ updates for new protocol


* Access to individual fields of an error/notice result; also some setting to
determine how verbose PQerrorMessage is

	char *PQerrorField(conn, char fieldcode)
	char *PQresultErrorField(const PGresult *res, char fieldcode)

fieldcode is as per the protocol spec.  NULL is returned if no such field in
current result (this includes case where current result isn't an error).

Errors generated internally by libpq will have severity, primary message,
and SQLSTATE, but typically no other fields.

Seems there is no alternative for Notice except to invent a new callback
level :-(.  PQnoticeProcessor API doesn't get the job done.

	typedef PQnoticeReceiver (PGconn *conn, PGresult *res, void *arg);
	PQsetNoticeReceiver(conn, function, arg)

res will be a NONFATAL_ERROR result from which PQresultErrorField() calls
may be used to extract the individual fields.  Default version just gets
PQresultErrorMessage() and passes it to connection's noticeProcessor.


TERSE: single-line error (severity, primary text, and position only)
DEFAULT: above plus any detail, hint, or context fields (backwards compatible)
VERBOSE: all available data

* Transaction status inquiry; also access to ParameterStatus.  (Provide way to
get at server version unconditionally, so that apps do not need fallback code)

	enum PQtransactionStatus(conn)
	UNKNOWN is returned if on pre-3.0 protocol or bad connection

	const char *PQparameterStatus(conn, const char *paramName)
		returns NULL if param not available, sets errmsg

	const char *PQserverVersion(conn)
		uses paramstatus if possible, else does query

	int PQprotocolVersion(conn)
		returns protocol major version (2 or 3)

* Representation of binary results?

	seems to just need some notes?  Old way is not too incompatible
	except for endianness issues.

* Extended query support: for the moment I'm just going to provide an
extension to allow access to out-of-line parameters and binary format.
Later maybe add some stuff to support separate Parse, Bind, Execute steps.

	PQexecParams(conn, const char *query,
		     int nParams,
		     Oid *paramTypes, -- may be NULL to leave all unspecified
		     char **paramValues,
		     int *paramLengths, -- may be NULL if all text
		     int *paramFormats, -- NULL means all text
		     int resultFormat)  -- zero or one

a NULL value of param[i] means that param is NULL, else it is pointer
to either null-terminated C string (if text) or binary value of length
paramLengths[i] (if binary).  Note paramLengths is not examined for
text-format params.  Semantics similar to PQexec, but cannot put multiple
commands into query string.  Note we don't allow per-column result format
selection, since we don't have a good way to tell the number of result

For asynchronous processing, also add PQsendQueryParams, with same parameters
but behaves similarly to PQsendQuery; follow with PQgetResult.

Each of these sends Parse/Bind/DescribePortal/Execute/Sync using unnamed stmt
and portal.  Will fail on old-protocol connections.

* Better COPY API

	COPY OUT and COPY IN PGresults now carry #cols and format info.
	(only if new-protocol conn, else return "zero cols")

	PQexec will cope with getting out of pre-existing COPY state (by
	discarding data or sending CopyDone)

	PQputline/PQputnbytes reject if not in COPY IN state, else they
	wrap the data as a CopyData message and send it.  It is up to
	application to send valid data for the COPY operation.

	int PQputCopyData(conn, const char *buffer, int nbytes, bool async)
	Functionally similar to PQputnbytes,
	but preferred for new coding
	Returns 1: data sent or queued
		0: data not sent, try again later (only if async is true)
		-1: trouble, use PQerrorMessage to find out why
	Do NOT send "\." as terminator.

	int PQputCopyEnd(conn, const char *error, bool async)
	Ends a COPY IN successfully if error is NULL, else makes it fail
	with given null-terminated string as the error message.
	Same return conventions as PQputCopyData.  After successful
	completion, call PQgetResult to check success or failure of COPY
	command.  (If doing async COPY, can wait in the usual way for
	PQgetResult to be ready.)

	PQgetCopyData(conn, char **buffer, bool async)
		returns a malloc'd chunk of data if successful
		got some data
			return value is # bytes received
		data not yet avail (only in async case)
			return is 0, *buffer is NULL
		end of copy in operation (incl. error detected)
			return is -1, *buffer is NULL
			now call PQgetResult
	The returned buffer will be null-terminated, but this is only
	helpful for text copy.  The returned data always corresponds to
	one data row.

	PQendcopy: deprecated, but equivalent to PQputCopyEnd (if COPY IN
		state) followed by synchronous GetResult.

* Better FunctionCall API?

	nah, just deprecate in favor of invoking the function via

* Access to Notification param field

	add a third field to PGnotify struct

* Access to source table's OID and column number in RowDescription,
  also per-column format codes

	Oid PQftable(const PGresult *res, int col)
	int PQftablecol(const PGresult *res, int col)
	int PQfformat(const PGresult *res, int col)

PQbinaryTuples is now deprecated in favor of looking at PQfformat.
PQbinaryTuples will return true only if all columns of the result
are binary format.


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