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libpq: Add target_session_attrs parameter. 

Commit 274bb2b385 made it possible to
specify multiple IPs in a connection string, but that's not good
enough for the case where you have a read-write master and a bunch of
read-only standbys and want to connect to whichever server is the
master at the current time.  This commit allows that, by making it
possible to specify target_session_attrs=read-write as a connection
parameter.

There was extensive discussion of the best name for the connection
parameter and its values as well as the best way to distinguish master
and standbys.  For now, adopt the same solution as JDBC: if the user
wants a read-write connection, issue 'show transaction_read_only' and
rejection the connection if the result is 'on'.  In the future, we
could add additional values of this new target_session_attrs parameter
that issue different queries; or we might have some way of
distinguishing the server type without resorting to an SQL query; but
right now, we have this, and that's (hopefully) a good start.

Victor Wagner and Mithun Cy.  Design review by Álvaro Herrera, Catalin
Iacob, Takayuki Tsunakawa, and Craig Ringer; code review by me.  I
changed Mithun's patch to skip all remaining IPs for a host if we
reject a connection based on this new parameter, rewrote the
documentation, and did some other cosmetic cleanup.

Discussion: http://postgr.es/m/CAD__OuhqPRGpcsfwPHz_PDqAGkoqS1UvnUnOnAB-LBWBW=wu4A@mail.gmail.com
This commit is contained in:
Robert Haas 2016-11-29 12:18:31 -05:00
parent 4fafa579b0
commit 721f7bd3cb
4 changed files with 232 additions and 56 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

29
doc/src/sgml/libpq.sgml
View File

@ -811,7 +811,7 @@ postgresql://localhost/mydb
postgresql://user@localhost
postgresql://user:secret@localhost
postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp
postgresql://host1:123,host2:456/somedb
postgresql://host1:123,host2:456/somedb?target_session_attrs=any&application_name=myapp
</programlisting>
Components of the hierarchical part of the <acronym>URI</acronym> can also
be given as parameters. For example:
@ -1386,6 +1386,23 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-target-session-attrs" xreflabel="target_session_attrs">
<term><literal>target_session_attrs</literal></term>
<listitem>
<para>
If this parameter is set to <literal>read-write</literal>, only a
connection in which read-write transactions are accepted by default
is considered acceptable. The query
<literal>show transaction_read_only</literal> will be sent upon any
successful connection; if it returns <literal>on</>, the connection
will be closed. If multiple hosts were specified in the connection
string, any remaining servers will be tried just as if the connection
attempt had failed. The default value of this parameter,
<literal>any</>, regards all connections as acceptable.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
@ -7069,6 +7086,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
linkend="libpq-connect-client-encoding"> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGTARGETSESSIONATTRS</envar></primary>
</indexterm>
<envar>PGTARGETSESSIONATTRS</envar> behaves the same as the <xref
linkend="libpq-connect-target-session-attrs"> connection parameter.
</para>
</listitem>
</itemizedlist>
</para>

252
src/interfaces/libpq/fe-connect.c
View File

@ -108,6 +108,7 @@ static int ldapServiceLookup(const char *purl, PQconninfoOption *options,
#define DefaultOption ""
#define DefaultAuthtype ""
#define DefaultPassword ""
#define DefaultTargetSessionAttrs "any"
#ifdef USE_SSL
#define DefaultSSLMode "prefer"
#else
@ -300,6 +301,11 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
"Replication", "D", 5,
offsetof(struct pg_conn, replication)},
{"target_session_attrs", "PGTARGETSESSIONATTRS",
DefaultTargetSessionAttrs, NULL,
"Target-Session-Attrs", "", 11, /* sizeof("read-write") = 11 */
offsetof(struct pg_conn, target_session_attrs)},
/* Terminating entry --- MUST BE LAST */
{NULL, NULL, NULL, NULL,
NULL, NULL, 0}
@ -336,6 +342,8 @@ static PGconn *makeEmptyPGconn(void);
static bool fillPGconn(PGconn *conn, PQconninfoOption *connOptions);
static void freePGconn(PGconn *conn);
static void closePGconn(PGconn *conn);
static void release_all_addrinfo(PGconn *conn);
static void sendTerminateConn(PGconn *conn);
static PQconninfoOption *conninfo_init(PQExpBuffer errorMessage);
static PQconninfoOption *parse_connection_string(const char *conninfo,
PQExpBuffer errorMessage, bool use_defaults);
@ -1025,6 +1033,22 @@ 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;
printfPQExpBuffer(&conn->errorMessage,
libpq_gettext("invalid target_session_attrs value: \"%s\"\n"),
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
@ -1814,6 +1838,7 @@ PQconnectPoll(PGconn *conn)
/* Special cases: proceed without waiting. */
case CONNECTION_SSL_STARTUP:
case CONNECTION_NEEDED:
case CONNECTION_CHECK_WRITABLE:
break;
default:
@ -2752,27 +2777,6 @@ keep_going: /* We will come back to here until there is
goto error_return;
}
/* We can release the address lists now. */
if (conn->connhost != NULL)
{
int i;
for (i = 0; i < conn->nconnhost; ++i)
{
int family = AF_UNSPEC;
#ifdef HAVE_UNIX_SOCKETS
if (conn->connhost[i].type == CHT_UNIX_SOCKET)
family = AF_UNIX;
#endif
pg_freeaddrinfo_all(family,
conn->connhost[i].addrlist);
conn->connhost[i].addrlist = NULL;
}
}
conn->addr_cur = NULL;
/* Fire up post-connection housekeeping if needed */
if (PG_PROTOCOL_MAJOR(conn->pversion) < 3)
{
@ -2782,7 +2786,24 @@ keep_going: /* We will come back to here until there is
return PGRES_POLLING_WRITING;
}
/* Otherwise, we are open for business! */
/*
* If a read-write connection is required, see if we have one.
*/
if (conn->target_session_attrs != NULL &&
strcmp(conn->target_session_attrs, "read-write") == 0)
{
conn->status = CONNECTION_OK;
if (!PQsendQuery(conn,
"show transaction_read_only"))
goto error_return;
conn->status = CONNECTION_CHECK_WRITABLE;
return PGRES_POLLING_READING;
}
/* We can release the address lists now. */
release_all_addrinfo(conn);
/* We are open for business! */
conn->status = CONNECTION_OK;
return PGRES_POLLING_OK;
}
@ -2814,10 +2835,109 @@ keep_going: /* We will come back to here until there is
goto error_return;
}
/*
* If a read-write connection is requisted check for same.
*/
if (conn->target_session_attrs != NULL &&
strcmp(conn->target_session_attrs, "read-write") == 0)
{
conn->status = CONNECTION_OK;
if (!PQsendQuery(conn,
"show transaction_read_only"))
goto error_return;
conn->status = CONNECTION_CHECK_WRITABLE;
return PGRES_POLLING_READING;
}
/* We can release the address lists now. */
release_all_addrinfo(conn);
/* We are open for business! */
conn->status = CONNECTION_OK;
return PGRES_POLLING_OK;
case CONNECTION_CHECK_WRITABLE:
{
conn->status = CONNECTION_OK;
if (!PQconsumeInput(conn))
goto error_return;
if (PQisBusy(conn))
{
conn->status = CONNECTION_CHECK_WRITABLE;
return PGRES_POLLING_READING;
}
res = PQgetResult(conn);
if (res && (PQresultStatus(res) == PGRES_TUPLES_OK) &&
PQntuples(res) == 1)
{
char *val;
val = PQgetvalue(res, 0, 0);
if (strncmp(val, "on", 2) == 0)
{
PQclear(res);
/* Not writable; close connection. */
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("could not make a writable "
"connection to server "
"\"%s:%s\"\n"),
conn->connhost[conn->whichhost].host,
conn->connhost[conn->whichhost].port);
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
pqDropConnection(conn, true);
/* Skip any remaining addresses for this host. */
conn->addr_cur = NULL;
if (conn->whichhost + 1 < conn->nconnhost)
{
conn->status = CONNECTION_NEEDED;
goto keep_going;
}
/* No more addresses to try. So we fail. */
goto error_return;
}
PQclear(res);
/* We can release the address lists now. */
release_all_addrinfo(conn);
/* We are open for business! */
conn->status = CONNECTION_OK;
return PGRES_POLLING_OK;
}
/*
* Something went wrong with "show transaction_read_only". We
* should try next addresses.
*/
if (res)
PQclear(res);
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("test \"show transaction_read_only\" failed "
" on \"%s:%s\" \n"),
conn->connhost[conn->whichhost].host,
conn->connhost[conn->whichhost].port);
conn->status = CONNECTION_OK;
sendTerminateConn(conn);
pqDropConnection(conn, true);
if (conn->addr_cur->ai_next != NULL ||
conn->whichhost + 1 < conn->nconnhost)
{
conn->addr_cur = conn->addr_cur->ai_next;
conn->status = CONNECTION_NEEDED;
goto keep_going;
}
/* No more addresses to try. So we fail. */
goto error_return;
}
default:
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("invalid connection state %d, "
@ -3109,6 +3229,8 @@ freePGconn(PGconn *conn)
free(conn->outBuffer);
if (conn->rowBuf)
free(conn->rowBuf);
if (conn->target_session_attrs)
free(conn->target_session_attrs);
termPQExpBuffer(&conn->errorMessage);
termPQExpBuffer(&conn->workBuffer);
@ -3119,6 +3241,58 @@ freePGconn(PGconn *conn)
#endif
}
/*
* release_all_addrinfo
* - free addrinfo of all hostconn elements.
*/
static void
release_all_addrinfo(PGconn *conn)
{
if (conn->connhost != NULL)
{
int i;
for (i = 0; i < conn->nconnhost; ++i)
{
int family = AF_UNSPEC;
#ifdef HAVE_UNIX_SOCKETS
if (conn->connhost[i].type == CHT_UNIX_SOCKET)
family = AF_UNIX;
#endif
pg_freeaddrinfo_all(family,
conn->connhost[i].addrlist);
conn->connhost[i].addrlist = NULL;
}
}
conn->addr_cur = NULL;
}
/*
* sendTerminateConn
* - Send a terminate message to backend.
*/
static void
sendTerminateConn(PGconn *conn)
{
/*
* Note that the protocol doesn't allow us to send Terminate messages
* during the startup phase.
*/
if (conn->sock != PGINVALID_SOCKET && conn->status == CONNECTION_OK)
{
/*
* Try to send "close connection" message to backend. Ignore any
* error.
*/
pqPutMsgStart('X', false, conn);
pqPutMsgEnd(conn);
(void) pqFlush(conn);
}
}
/*
* closePGconn
* - properly close a connection to the backend
@ -3133,20 +3307,7 @@ closePGconn(PGconn *conn)
PGnotify *notify;
pgParameterStatus *pstatus;
/*
* Note that the protocol doesn't allow us to send Terminate messages
* during the startup phase.
*/
if (conn->sock != PGINVALID_SOCKET && conn->status == CONNECTION_OK)
{
/*
* Try to send "close connection" message to backend. Ignore any
* error.
*/
pqPutMsgStart('X', false, conn);
pqPutMsgEnd(conn);
(void) pqFlush(conn);
}
sendTerminateConn(conn);
/*
* Must reset the blocking status so a possible reconnect will work.
@ -3165,25 +3326,8 @@ closePGconn(PGconn *conn)
conn->asyncStatus = PGASYNC_IDLE;
pqClearAsyncResult(conn); /* deallocate result */
resetPQExpBuffer(&conn->errorMessage);
if (conn->connhost != NULL)
{
int i;
release_all_addrinfo(conn);
for (i = 0; i < conn->nconnhost; ++i)
{
int family = AF_UNSPEC;
#ifdef HAVE_UNIX_SOCKETS
if (conn->connhost[i].type == CHT_UNIX_SOCKET)
family = AF_UNIX;
#endif
pg_freeaddrinfo_all(family,
conn->connhost[i].addrlist);
conn->connhost[i].addrlist = NULL;
}
}
conn->addr_cur = NULL;
notify = conn->notifyHead;
while (notify != NULL)
{

4
src/interfaces/libpq/libpq-fe.h
View File

@ -62,7 +62,9 @@ typedef enum
* backend startup. */
CONNECTION_SETENV, /* Negotiating environment. */
CONNECTION_SSL_STARTUP, /* Negotiating SSL. */
CONNECTION_NEEDED /* Internal state: connect() needed */
CONNECTION_NEEDED, /* Internal state: connect() needed */
CONNECTION_CHECK_WRITABLE /* Check if we could make a writable
* connection. */
} ConnStatusType;
typedef enum

3
src/interfaces/libpq/libpq-int.h
View File

@ -361,6 +361,9 @@ struct pg_conn
char *krbsrvname; /* Kerberos service name */
#endif
char *target_session_attrs; /* Type of connection to make
* Possible values any, read-write. */
/* Optional file to write trace info to */
FILE *Pfdebug;