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Extend the abilities of libpq's target_session_attrs parameter.

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In addition to the existing options of "any" and "read-write", we
now support "read-only", "primary", "standby", and "prefer-standby".
"read-write" retains its previous meaning of "transactions are
read-write by default", and "read-only" inverts that.  The other
three modes test specifically for hot-standby status, which is not
quite the same thing.  (Setting default_transaction_read_only on
a primary server renders it read-only to this logic, but not a
standby.)

Furthermore, if talking to a v14 or later server, no extra network
round trip is needed to detect the session's status; the GUC_REPORT
variables delivered by the server are enough.  When talking to an
older server, a SHOW or SELECT query is issued to detect session
read-only-ness or server hot-standby state, as needed.

Haribabu Kommi, Greg Nancarrow, Vignesh C, Tom Lane; reviewed at
various times by Laurenz Albe, Takayuki Tsunakawa, Peter Smith.

Discussion: https://postgr.es/m/CAF3+xM+8-ztOkaV9gHiJ3wfgENTq97QcjXQt+rbFQ6F7oNzt9A@mail.gmail.com
This commit is contained in:
Tom Lane
2021-03-02 20:17:45 -05:00
parent 57e6db706e
commit ee28cacf61
6 changed files with 463 additions and 121 deletions
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21
src/interfaces/libpq/fe-exec.c
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@ -1008,11 +1008,11 @@ pqSaveParameterStatus(PGconn *conn, const char *name, const char *value)
}
/*
* Special hacks: remember client_encoding and
* standard_conforming_strings, and convert server version to a numeric
* form. We keep the first two of these in static variables as well, so
* that PQescapeString and PQescapeBytea can behave somewhat sanely (at
* least in single-connection-using programs).
* Save values of settings that are of interest to libpq in fields of the
* PGconn object. We keep client_encoding and standard_conforming_strings
* in static variables as well, so that PQescapeString and PQescapeBytea
* can behave somewhat sanely (at least in single-connection-using
* programs).
*/
if (strcmp(name, "client_encoding") == 0)
{
@ -1029,6 +1029,7 @@ pqSaveParameterStatus(PGconn *conn, const char *name, const char *value)
}
else if (strcmp(name, "server_version") == 0)
{
/* We convert the server version to numeric form. */
int cnt;
int vmaj,
vmin,
@ -1062,6 +1063,16 @@ pqSaveParameterStatus(PGconn *conn, const char *name, const char *value)
else
conn->sversion = 0; /* unknown */
}
else if (strcmp(name, "default_transaction_read_only") == 0)
{
conn->default_transaction_read_only =
(strcmp(value, "on") == 0) ? PG_BOOL_YES : PG_BOOL_NO;
}
else if (strcmp(name, "in_hot_standby") == 0)
{
conn->in_hot_standby =
(strcmp(value, "on") == 0) ? PG_BOOL_YES : PG_BOOL_NO;
}
}