Extend the abilities of libpq's target_session_attrs parameter.

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

1 files changed, 16 insertions, 5 deletions

diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index e7307533876..a5507538555 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -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;
+	}
 }