Expose PQsocketPoll via libpq

This is useful when connecting to a database asynchronously via
PQconnectStart(), since it handles deciding between poll() and
select(), and some of the required boilerplate.

Tristan Partin, reviewed by Gurjeet Singh, Heikki Linnakangas, Jelte
Fennema-Nio, and me.

Discussion: http://postgr.es/m/D08WWCPVHKHN.3QELIKZJ2D9RZ@neon.tech

doc/src/sgml/libpq.sgml

@@ -262,6 +262,41 @@ PGconn *PQsetdb(char *pghost,
      </listitem>
     </varlistentry>
 
+    <varlistentry id="libpq-PQsocketPoll">
+     <term><function>PQsocketPoll</function><indexterm><primary>PQsocketPoll</primary></indexterm></term>
+     <listitem>
+      <para>
+       <indexterm><primary>nonblocking connection</primary></indexterm>
+       Poll a connection&apos;s underlying socket descriptor retrieved with <xref linkend="libpq-PQsocket"/>.
+<synopsis>
+int PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time);
+</synopsis>
+      </para>
+
+      <para>
+       This function sets up polling of a file descriptor. The underlying function is either
+       <function>poll(2)</function> or <function>select(2)</function>, depending on platform
+       support. The primary use of this function is iterating through the connection sequence
+       described in the documentation of <xref linkend="libpq-PQconnectStartParams"/>. If
+       <parameter>forRead</parameter> is specified, the function waits for the socket to be ready
+       for reading. If <parameter>forWrite</parameter> is specified, the function waits for the
+       socket to be ready for write. See <literal>POLLIN</literal> and <literal>POLLOUT</literal>
+       from <function>poll(2)</function>, or <parameter>readfds</parameter> and
+       <parameter>writefds</parameter> from <function>select(2)</function> for more information. If
+       <parameter>end_time</parameter> is not <literal>-1</literal>, it specifies the time at which
+       this function should stop waiting for the condition to be met.
+      </para>
+
+      <para>
+       The function returns a value greater than <literal>0</literal> if the specified condition
+       is met, <literal>0</literal> if a timeout occurred, or <literal>-1</literal> if an error
+       occurred. The error can be retrieved by checking the <literal>errno(3)</literal> value. In
+       the event <literal>forRead</literal> and <literal>forWrite</literal> are not set, the
+       function immediately returns a timeout condition.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry id="libpq-PQconnectStartParams">
      <term><function>PQconnectStartParams</function><indexterm><primary>PQconnectStartParams</primary></indexterm></term>
      <term><function>PQconnectStart</function><indexterm><primary>PQconnectStart</primary></indexterm></term>
@@ -358,7 +393,10 @@ PostgresPollingStatusType PQconnectPoll(PGconn *conn);
        Loop thus: If <function>PQconnectPoll(conn)</function> last returned
        <symbol>PGRES_POLLING_READING</symbol>, wait until the socket is ready to
        read (as indicated by <function>select()</function>, <function>poll()</function>, or
-       similar system function).
+       similar system function).  Note that <function>PQsocketPoll</function>
+       can help reduce boilerplate by abstracting the setup of
+       <function>select(2)</function> or <function>poll(2)</function> if it is
+       available on your system.
        Then call <function>PQconnectPoll(conn)</function> again.
        Conversely, if <function>PQconnectPoll(conn)</function> last returned
        <symbol>PGRES_POLLING_WRITING</symbol>, wait until the socket is ready

src/interfaces/libpq/exports.txt

@@ -202,3 +202,4 @@ PQcancelSocket            199
 PQcancelErrorMessage      200
 PQcancelReset             201
 PQcancelFinish            202
+PQsocketPoll              203

src/interfaces/libpq/fe-misc.c

@@ -55,7 +55,6 @@ static int	pqPutMsgBytes(const void *buf, size_t len, PGconn *conn);
 static int	pqSendSome(PGconn *conn, int len);
 static int	pqSocketCheck(PGconn *conn, int forRead, int forWrite,
 						  time_t end_time);
-static int	pqSocketPoll(int sock, int forRead, int forWrite, time_t end_time);
 
 /*
  * PQlibVersion: return the libpq version number
@@ -1059,7 +1058,7 @@ pqSocketCheck(PGconn *conn, int forRead, int forWrite, time_t end_time)
 
 	/* We will retry as long as we get EINTR */
 	do
-		result = pqSocketPoll(conn->sock, forRead, forWrite, end_time);
+		result = PQsocketPoll(conn->sock, forRead, forWrite, end_time);
 	while (result < 0 && SOCK_ERRNO == EINTR);
 
 	if (result < 0)
@@ -1083,8 +1082,8 @@ pqSocketCheck(PGconn *conn, int forRead, int forWrite, time_t end_time)
  * Timeout is infinite if end_time is -1.  Timeout is immediate (no blocking)
  * if end_time is 0 (or indeed, any time before now).
  */
-static int
-pqSocketPoll(int sock, int forRead, int forWrite, time_t end_time)
+int
+PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time)
 {
 	/* We use poll(2) if available, otherwise select(2) */
 #ifdef HAVE_POLL

src/interfaces/libpq/libpq-fe.h

@@ -21,6 +21,7 @@ extern "C"
 #endif
 
 #include <stdio.h>
+#include <time.h>
 
 /*
  * postgres_ext.h defines the backend's externally visible types,
@@ -670,6 +671,9 @@ extern int	lo_export(PGconn *conn, Oid lobjId, const char *filename);
 /* Get the version of the libpq library in use */
 extern int	PQlibVersion(void);
 
+/* Poll a socket for reading and/or writing with an optional timeout */
+extern int	PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time);
+
 /* Determine length of multibyte encoded char at *s */
 extern int	PQmblen(const char *s, int encoding);