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Extend the abilities of libpq's target_session_attrs parameter.

Browse Source 
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
Download Patch File Download Diff File Expand all files Collapse all files

366
src/interfaces/libpq/fe-connect.c
View File

@ -356,7 +356,7 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
{"target_session_attrs", "PGTARGETSESSIONATTRS",
DefaultTargetSessionAttrs, NULL,
"Target-Session-Attrs", "", 11, /* sizeof("read-write") = 11 */
"Target-Session-Attrs", "", 15, /* sizeof("prefer-standby") = 15 */
offsetof(struct pg_conn, target_session_attrs)},
/* Terminating entry --- MUST BE LAST */
@ -583,6 +583,8 @@ pqDropServerData(PGconn *conn)
conn->pstatus = NULL;
conn->client_encoding = PG_SQL_ASCII;
conn->std_strings = false;
conn->default_transaction_read_only = PG_BOOL_UNKNOWN;
conn->in_hot_standby = PG_BOOL_UNKNOWN;
conn->sversion = 0;
/* Drop large-object lookup data */
@ -1388,6 +1390,36 @@ connectOptions2(PGconn *conn)
goto oom_error;
}
/*
* validate target_session_attrs option, and set target_server_type
*/
if (conn->target_session_attrs)
{
if (strcmp(conn->target_session_attrs, "any") == 0)
conn->target_server_type = SERVER_TYPE_ANY;
else if (strcmp(conn->target_session_attrs, "read-write") == 0)
conn->target_server_type = SERVER_TYPE_READ_WRITE;
else if (strcmp(conn->target_session_attrs, "read-only") == 0)
conn->target_server_type = SERVER_TYPE_READ_ONLY;
else if (strcmp(conn->target_session_attrs, "primary") == 0)
conn->target_server_type = SERVER_TYPE_PRIMARY;
else if (strcmp(conn->target_session_attrs, "standby") == 0)
conn->target_server_type = SERVER_TYPE_STANDBY;
else if (strcmp(conn->target_session_attrs, "prefer-standby") == 0)
conn->target_server_type = SERVER_TYPE_PREFER_STANDBY;
else
{
conn->status = CONNECTION_BAD;
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("invalid %s value: \"%s\"\n"),
"target_session_attrs",
conn->target_session_attrs);
return false;
}
}
else
conn->target_server_type = SERVER_TYPE_ANY;
/*
* Resolve special "auto" client_encoding from the locale
*/
@ -1400,23 +1432,6 @@ connectOptions2(PGconn *conn)
goto oom_error;
}
/*
* Validate target_session_attrs option.
*/
if (conn->target_session_attrs)
{
if (strcmp(conn->target_session_attrs, "any") != 0
&& strcmp(conn->target_session_attrs, "read-write") != 0)
{
conn->status = CONNECTION_BAD;
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("invalid %s value: \"%s\"\n"),
"target_settion_attrs",
conn->target_session_attrs);
return false;
}
}
/*
* Only if we get this far is it appropriate to try to connect. (We need a
* state flag, rather than just the boolean result of this function, in
@ -2057,6 +2072,10 @@ connectDBStart(PGconn *conn)
conn->try_next_host = true;
conn->status = CONNECTION_NEEDED;
/* Also reset the target_server_type state if needed */
if (conn->target_server_type == SERVER_TYPE_PREFER_STANDBY_PASS2)
conn->target_server_type = SERVER_TYPE_PREFER_STANDBY;
/*
* The code for processing CONNECTION_NEEDED state is in PQconnectPoll(),
* so that it can easily be re-executed if needed again during the
@ -2250,6 +2269,9 @@ PQconnectPoll(PGconn *conn)
/* These are reading states */
case CONNECTION_AWAITING_RESPONSE:
case CONNECTION_AUTH_OK:
case CONNECTION_CHECK_WRITABLE:
case CONNECTION_CONSUME:
case CONNECTION_CHECK_STANDBY:
{
/* Load waiting data */
int n = pqReadData(conn);
@ -2274,9 +2296,8 @@ PQconnectPoll(PGconn *conn)
/* Special cases: proceed without waiting. */
case CONNECTION_SSL_STARTUP:
case CONNECTION_NEEDED:
case CONNECTION_CHECK_WRITABLE:
case CONNECTION_CONSUME:
case CONNECTION_GSS_STARTUP:
case CONNECTION_CHECK_TARGET:
break;
default:
@ -2311,15 +2332,28 @@ keep_going: /* We will come back to here until there is
int ret;
char portstr[MAXPGPATH];
if (conn->whichhost + 1 >= conn->nconnhost)
if (conn->whichhost + 1 < conn->nconnhost)
conn->whichhost++;
else
{
/*
* Oops, no more hosts. An appropriate error message is already
* set up, so just set the right status.
* Oops, no more hosts.
*
* If we are trying to connect in "prefer-standby" mode, then drop
* the standby requirement and start over.
*
* Otherwise, an appropriate error message is already set up, so
* we just need to set the right status.
*/
goto error_return;
if (conn->target_server_type == SERVER_TYPE_PREFER_STANDBY &&
conn->nconnhost > 0)
{
conn->target_server_type = SERVER_TYPE_PREFER_STANDBY_PASS2;
conn->whichhost = 0;
}
else
goto error_return;
}
conn->whichhost++;
/* Drop any address info for previous host */
release_conn_addrinfo(conn);
@ -3550,28 +3584,131 @@ keep_going: /* We will come back to here until there is
case CONNECTION_CHECK_TARGET:
{
/*
* If a read-write connection is required, see if we have one.
*
* Servers before 7.4 lack the transaction_read_only GUC, but
* by the same token they don't have any read-only mode, so we
* may just skip the test in that case.
* If a read-write, read-only, primary, or standby connection
* is required, see if we have one.
*/
if (conn->sversion >= 70400 &&
conn->target_session_attrs != NULL &&
strcmp(conn->target_session_attrs, "read-write") == 0)
if (conn->target_server_type == SERVER_TYPE_READ_WRITE ||
conn->target_server_type == SERVER_TYPE_READ_ONLY)
{
bool read_only_server;
/*
* If the server didn't report
* "default_transaction_read_only" or "in_hot_standby" at
* startup, we must determine its state by sending the
* query "SHOW transaction_read_only". Servers before 7.4
* lack the transaction_read_only GUC, but by the same
* token they don't have any read-only mode, so we may
* just assume the results.
*/
if (conn->sversion < 70400)
{
conn->default_transaction_read_only = PG_BOOL_NO;
conn->in_hot_standby = PG_BOOL_NO;
}
if (conn->default_transaction_read_only == PG_BOOL_UNKNOWN ||
conn->in_hot_standby == PG_BOOL_UNKNOWN)
{
/*
* We use PQsendQueryContinue so that
* conn->errorMessage does not get cleared. We need
* to preserve any error messages related to previous
* hosts we have tried and failed to connect to.
*/
conn->status = CONNECTION_OK;
if (!PQsendQueryContinue(conn,
"SHOW transaction_read_only"))
goto error_return;
/* We'll return to this state when we have the answer */
conn->status = CONNECTION_CHECK_WRITABLE;
return PGRES_POLLING_READING;
}
/* OK, we can make the test */
read_only_server =
(conn->default_transaction_read_only == PG_BOOL_YES ||
conn->in_hot_standby == PG_BOOL_YES);
if ((conn->target_server_type == SERVER_TYPE_READ_WRITE) ?
read_only_server : !read_only_server)
{
/* Wrong server state, reject and try the next host */
if (conn->target_server_type == SERVER_TYPE_READ_WRITE)
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("session is read-only\n"));
else
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("session is not read-only\n"));
/* Close connection politely. */
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
/*
* Try next host if any, but we don't want to consider
* additional addresses for this host.
*/
conn->try_next_host = true;
goto keep_going;
}
}
else if (conn->target_server_type == SERVER_TYPE_PRIMARY ||
conn->target_server_type == SERVER_TYPE_STANDBY ||
conn->target_server_type == SERVER_TYPE_PREFER_STANDBY)
{
/*
* We use PQsendQueryContinue so that conn->errorMessage
* does not get cleared. We need to preserve any error
* messages related to previous hosts we have tried and
* failed to connect to.
* If the server didn't report "in_hot_standby" at
* startup, we must determine its state by sending the
* query "SELECT pg_catalog.pg_is_in_recovery()". Servers
* before 9.0 don't have that function, but by the same
* token they don't have any standby mode, so we may just
* assume the result.
*/
conn->status = CONNECTION_OK;
if (!PQsendQueryContinue(conn,
"SHOW transaction_read_only"))
goto error_return;
conn->status = CONNECTION_CHECK_WRITABLE;
return PGRES_POLLING_READING;
if (conn->sversion < 90000)
conn->in_hot_standby = PG_BOOL_NO;
if (conn->in_hot_standby == PG_BOOL_UNKNOWN)
{
/*
* We use PQsendQueryContinue so that
* conn->errorMessage does not get cleared. We need
* to preserve any error messages related to previous
* hosts we have tried and failed to connect to.
*/
conn->status = CONNECTION_OK;
if (!PQsendQueryContinue(conn,
"SELECT pg_catalog.pg_is_in_recovery()"))
goto error_return;
/* We'll return to this state when we have the answer */
conn->status = CONNECTION_CHECK_STANDBY;
return PGRES_POLLING_READING;
}
/* OK, we can make the test */
if ((conn->target_server_type == SERVER_TYPE_PRIMARY) ?
(conn->in_hot_standby == PG_BOOL_YES) :
(conn->in_hot_standby == PG_BOOL_NO))
{
/* Wrong server state, reject and try the next host */
if (conn->target_server_type == SERVER_TYPE_PRIMARY)
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("server is in hot standby mode\n"));
else
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("server is not in hot standby mode\n"));
/* Close connection politely. */
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
/*
* Try next host if any, but we don't want to consider
* additional addresses for this host.
*/
conn->try_next_host = true;
goto keep_going;
}
}
/* We can release the address list now. */
@ -3617,6 +3754,14 @@ keep_going: /* We will come back to here until there is
case CONNECTION_CONSUME:
{
/*
* This state just makes sure the connection is idle after
* we've obtained the result of a SHOW or SELECT query. Once
* we're clear, return to CONNECTION_CHECK_TARGET state to
* decide what to do next. We must transiently set status =
* CONNECTION_OK in order to use the result-consuming
* subroutines.
*/
conn->status = CONNECTION_OK;
if (!PQconsumeInput(conn))
goto error_return;
@ -3627,26 +3772,26 @@ keep_going: /* We will come back to here until there is
return PGRES_POLLING_READING;
}
/*
* Call PQgetResult() again to consume NULL result.
*/
/* Call PQgetResult() again until we get a NULL result */
res = PQgetResult(conn);
if (res != NULL)
{
PQclear(res);
conn->status = CONNECTION_CONSUME;
goto keep_going;
return PGRES_POLLING_READING;
}
/* We can release the address list now. */
release_conn_addrinfo(conn);
/* We are open for business! */
conn->status = CONNECTION_OK;
return PGRES_POLLING_OK;
conn->status = CONNECTION_CHECK_TARGET;
goto keep_going;
}
case CONNECTION_CHECK_WRITABLE:
{
/*
* Waiting for result of "SHOW transaction_read_only". We
* must transiently set status = CONNECTION_OK in order to use
* the result-consuming subroutines.
*/
conn->status = CONNECTION_OK;
if (!PQconsumeInput(conn))
goto error_return;
@ -3658,61 +3803,102 @@ keep_going: /* We will come back to here until there is
}
res = PQgetResult(conn);
if (res && (PQresultStatus(res) == PGRES_TUPLES_OK) &&
if (res && PQresultStatus(res) == PGRES_TUPLES_OK &&
PQntuples(res) == 1)
{
char *val;
val = PQgetvalue(res, 0, 0);
if (strncmp(val, "on", 2) == 0)
{
/* Not writable; fail this connection. */
PQclear(res);
/* Append error report to conn->errorMessage. */
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("session is read-only\n"));
/* Close connection politely. */
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
/*
* Try next host if any, but we don't want to consider
* additional addresses for this host.
*/
conn->try_next_host = true;
goto keep_going;
}
/* Session is read-write, so we're good. */
PQclear(res);
char *val = PQgetvalue(res, 0, 0);
/*
* Finish reading any remaining messages before being
* considered as ready.
* "transaction_read_only = on" proves that at least one
* of default_transaction_read_only and in_hot_standby is
* on, but we don't actually know which. We don't care
* though for the purpose of identifying a read-only
* session, so satisfy the CONNECTION_CHECK_TARGET code by
* claiming they are both on. On the other hand, if it's
* a read-write session, they are certainly both off.
*/
if (strncmp(val, "on", 2) == 0)
{
conn->default_transaction_read_only = PG_BOOL_YES;
conn->in_hot_standby = PG_BOOL_YES;
}
else
{
conn->default_transaction_read_only = PG_BOOL_NO;
conn->in_hot_standby = PG_BOOL_NO;
}
PQclear(res);
/* Finish reading messages before continuing */
conn->status = CONNECTION_CONSUME;
goto keep_going;
}
/*
* Something went wrong with "SHOW transaction_read_only". We
* should try next addresses.
*/
/* Something went wrong with "SHOW transaction_read_only". */
if (res)
PQclear(res);
/* Append error report to conn->errorMessage. */
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("test \"SHOW transaction_read_only\" failed\n"));
libpq_gettext("\"SHOW transaction_read_only\" failed\n"));
/* Close connection politely. */
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
/* Try next address */
conn->try_next_addr = true;
/* Try next host. */
conn->try_next_host = true;
goto keep_going;
}
case CONNECTION_CHECK_STANDBY:
{
/*
* Waiting for result of "SELECT pg_is_in_recovery()". We
* must transiently set status = CONNECTION_OK in order to use
* the result-consuming subroutines.
*/
conn->status = CONNECTION_OK;
if (!PQconsumeInput(conn))
goto error_return;
if (PQisBusy(conn))
{
conn->status = CONNECTION_CHECK_STANDBY;
return PGRES_POLLING_READING;
}
res = PQgetResult(conn);
if (res && PQresultStatus(res) == PGRES_TUPLES_OK &&
PQntuples(res) == 1)
{
char *val = PQgetvalue(res, 0, 0);
if (strncmp(val, "t", 1) == 0)
conn->in_hot_standby = PG_BOOL_YES;
else
conn->in_hot_standby = PG_BOOL_NO;
PQclear(res);
/* Finish reading messages before continuing */
conn->status = CONNECTION_CONSUME;
goto keep_going;
}
/* Something went wrong with "SELECT pg_is_in_recovery()". */
if (res)
PQclear(res);
/* Append error report to conn->errorMessage. */
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("\"SELECT pg_is_in_recovery()\" failed\n"));
/* Close connection politely. */
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
/* Try next host. */
conn->try_next_host = true;
goto keep_going;
}
@ -3859,6 +4045,8 @@ makeEmptyPGconn(void)
conn->setenv_state = SETENV_STATE_IDLE;
conn->client_encoding = PG_SQL_ASCII;
conn->std_strings = false; /* unless server says differently */
conn->default_transaction_read_only = PG_BOOL_UNKNOWN;
conn->in_hot_standby = PG_BOOL_UNKNOWN;
conn->verbosity = PQERRORS_DEFAULT;
conn->show_context = PQSHOW_CONTEXT_ERRORS;
conn->sock = PGINVALID_SOCKET;