Improve the granularity of PQsocketPoll's timeout parameter.

Commit f5e4dedfa exposed libpq's internal function PQsocketPoll
without a lot of thought about whether that was an API we really
wanted to chisel in stone.  The main problem with it is the use of
time_t to specify the timeout.  While we do want an absolute time
so that a loop around PQsocketPoll doesn't have problems with
timeout slippage, time_t has only 1-second resolution.  That's
already problematic for libpq's own internal usage --- for example,
pqConnectDBComplete has long had a kluge to treat "connect_timeout=1"
as 2 seconds so that it doesn't accidentally round to nearly zero.
And it's even less likely to be satisfactory for external callers.
Hence, let's change this while we still can.

The best idea seems to be to use an int64 count of microseconds since
the epoch --- basically the same thing as the backend's TimestampTz,
but let's use the standard Unix epoch (1970-01-01) since that's more
likely for clients to be easy to calculate.  Millisecond resolution
would be plenty for foreseeable uses, but maybe the day will come that
we're glad we used microseconds.

Also, since time(2) isn't especially helpful for computing timeouts
defined this way, introduce a new function PQgetCurrentTimeUSec
to get the current time in this form.

Remove the hack in pqConnectDBComplete, so that "connect_timeout=1"
now means what you'd expect.

We can also remove the "#include <time.h>" that f5e4dedfa added to
libpq-fe.h, since there's no longer a need for time_t in that header.
It seems better for v17 not to enlarge libpq-fe.h's include footprint
from what it's historically been, anyway.

I also failed to resist the temptation to do some wordsmithing
on PQsocketPoll's documentation.

Patch by me, per complaint from Dominique Devienne.

Discussion: https://postgr.es/m/913559.1718055575@sss.pgh.pa.us

src/interfaces/libpq/exports.txt

@@ -204,3 +204,4 @@ PQcancelReset             201
 PQcancelFinish            202
 PQsocketPoll              203
 PQsetChunkedRowsMode      204
+PQgetCurrentTimeUSec      205

src/interfaces/libpq/fe-connect.c

@@ -2470,7 +2470,7 @@ int
 pqConnectDBComplete(PGconn *conn)
 {
 	PostgresPollingStatusType flag = PGRES_POLLING_WRITING;
-	time_t		finish_time = ((time_t) -1);
+	pg_usec_time_t end_time = -1;
 	int			timeout = 0;
 	int			last_whichhost = -2;	/* certainly different from whichhost */
 	int			last_whichaddr = -2;	/* certainly different from whichaddr */
@@ -2479,7 +2479,7 @@ pqConnectDBComplete(PGconn *conn)
 		return 0;
 
 	/*
-	 * Set up a time limit, if connect_timeout isn't zero.
+	 * Set up a time limit, if connect_timeout is greater than zero.
 	 */
 	if (conn->connect_timeout != NULL)
 	{
@@ -2490,19 +2490,6 @@ pqConnectDBComplete(PGconn *conn)
 			conn->status = CONNECTION_BAD;
 			return 0;
 		}
-
-		if (timeout > 0)
-		{
-			/*
-			 * Rounding could cause connection to fail unexpectedly quickly;
-			 * to prevent possibly waiting hardly-at-all, insist on at least
-			 * two seconds.
-			 */
-			if (timeout < 2)
-				timeout = 2;
-		}
-		else					/* negative means 0 */
-			timeout = 0;
 	}
 
 	for (;;)
@@ -2519,7 +2506,7 @@ pqConnectDBComplete(PGconn *conn)
 			(conn->whichhost != last_whichhost ||
 			 conn->whichaddr != last_whichaddr))
 		{
-			finish_time = time(NULL) + timeout;
+			end_time = PQgetCurrentTimeUSec() + (pg_usec_time_t) timeout * 1000000;
 			last_whichhost = conn->whichhost;
 			last_whichaddr = conn->whichaddr;
 		}
@@ -2534,7 +2521,7 @@ pqConnectDBComplete(PGconn *conn)
 				return 1;		/* success! */
 
 			case PGRES_POLLING_READING:
-				ret = pqWaitTimed(1, 0, conn, finish_time);
+				ret = pqWaitTimed(1, 0, conn, end_time);
 				if (ret == -1)
 				{
 					/* hard failure, eg select() problem, aborts everything */
@@ -2544,7 +2531,7 @@ pqConnectDBComplete(PGconn *conn)
 				break;
 
 			case PGRES_POLLING_WRITING:
-				ret = pqWaitTimed(0, 1, conn, finish_time);
+				ret = pqWaitTimed(0, 1, conn, end_time);
 				if (ret == -1)
 				{
 					/* hard failure, eg select() problem, aborts everything */

src/interfaces/libpq/fe-misc.c

@@ -54,7 +54,7 @@
 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);
+						  pg_usec_time_t end_time);
 
 /*
  * PQlibVersion: return the libpq version number
@@ -977,22 +977,25 @@ pqFlush(PGconn *conn)
 int
 pqWait(int forRead, int forWrite, PGconn *conn)
 {
-	return pqWaitTimed(forRead, forWrite, conn, (time_t) -1);
+	return pqWaitTimed(forRead, forWrite, conn, -1);
 }
 
 /*
- * pqWaitTimed: wait, but not past finish_time.
- *
- * finish_time = ((time_t) -1) disables the wait limit.
+ * pqWaitTimed: wait, but not past end_time.
  *
  * Returns -1 on failure, 0 if the socket is readable/writable, 1 if it timed out.
+ *
+ * The timeout is specified by end_time, which is the int64 number of
+ * microseconds since the Unix epoch (that is, time_t times 1 million).
+ * Timeout is infinite if end_time is -1.  Timeout is immediate (no blocking)
+ * if end_time is 0 (or indeed, any time before now).
  */
 int
-pqWaitTimed(int forRead, int forWrite, PGconn *conn, time_t finish_time)
+pqWaitTimed(int forRead, int forWrite, PGconn *conn, pg_usec_time_t end_time)
 {
 	int			result;
 
-	result = pqSocketCheck(conn, forRead, forWrite, finish_time);
+	result = pqSocketCheck(conn, forRead, forWrite, end_time);
 
 	if (result < 0)
 		return -1;				/* errorMessage is already set */
@@ -1013,7 +1016,7 @@ pqWaitTimed(int forRead, int forWrite, PGconn *conn, time_t finish_time)
 int
 pqReadReady(PGconn *conn)
 {
-	return pqSocketCheck(conn, 1, 0, (time_t) 0);
+	return pqSocketCheck(conn, 1, 0, 0);
 }
 
 /*
@@ -1023,7 +1026,7 @@ pqReadReady(PGconn *conn)
 int
 pqWriteReady(PGconn *conn)
 {
-	return pqSocketCheck(conn, 0, 1, (time_t) 0);
+	return pqSocketCheck(conn, 0, 1, 0);
 }
 
 /*
@@ -1035,7 +1038,7 @@ pqWriteReady(PGconn *conn)
  * for read data directly.
  */
 static int
-pqSocketCheck(PGconn *conn, int forRead, int forWrite, time_t end_time)
+pqSocketCheck(PGconn *conn, int forRead, int forWrite, pg_usec_time_t end_time)
 {
 	int			result;
 
@@ -1079,11 +1082,13 @@ pqSocketCheck(PGconn *conn, int forRead, int forWrite, time_t end_time)
  * condition (without waiting).  Return >0 if condition is met, 0
  * if a timeout occurred, -1 if an error or interrupt occurred.
  *
+ * The timeout is specified by end_time, which is the int64 number of
+ * microseconds since the Unix epoch (that is, time_t times 1 million).
  * Timeout is infinite if end_time is -1.  Timeout is immediate (no blocking)
  * if end_time is 0 (or indeed, any time before now).
  */
 int
-PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time)
+PQsocketPoll(int sock, int forRead, int forWrite, pg_usec_time_t end_time)
 {
 	/* We use poll(2) if available, otherwise select(2) */
 #ifdef HAVE_POLL
@@ -1103,14 +1108,16 @@ PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time)
 		input_fd.events |= POLLOUT;
 
 	/* Compute appropriate timeout interval */
-	if (end_time == ((time_t) -1))
+	if (end_time == -1)
 		timeout_ms = -1;
+	else if (end_time == 0)
+		timeout_ms = 0;
 	else
 	{
-		time_t		now = time(NULL);
+		pg_usec_time_t now = PQgetCurrentTimeUSec();
 
 		if (end_time > now)
-			timeout_ms = (end_time - now) * 1000;
+			timeout_ms = (end_time - now) / 1000;
 		else
 			timeout_ms = 0;
 	}
@@ -1138,17 +1145,28 @@ PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time)
 	FD_SET(sock, &except_mask);
 
 	/* Compute appropriate timeout interval */
-	if (end_time == ((time_t) -1))
+	if (end_time == -1)
 		ptr_timeout = NULL;
+	else if (end_time == 0)
+	{
+		timeout.tv_sec = 0;
+		timeout.tv_usec = 0;
+		ptr_timeout = &timeout;
+	}
 	else
 	{
-		time_t		now = time(NULL);
+		pg_usec_time_t now = PQgetCurrentTimeUSec();
 
 		if (end_time > now)
-			timeout.tv_sec = end_time - now;
+		{
+			timeout.tv_sec = (end_time - now) / 1000000;
+			timeout.tv_usec = (end_time - now) % 1000000;
+		}
 		else
+		{
 			timeout.tv_sec = 0;
-		timeout.tv_usec = 0;
+			timeout.tv_usec = 0;
+		}
 		ptr_timeout = &timeout;
 	}
 
@@ -1157,6 +1175,21 @@ PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time)
 #endif							/* HAVE_POLL */
 }
 
+/*
+ * PQgetCurrentTimeUSec: get current time with microsecond precision
+ *
+ * This provides a platform-independent way of producing a reference
+ * value for PQsocketPoll's timeout parameter.
+ */
+pg_usec_time_t
+PQgetCurrentTimeUSec(void)
+{
+	struct timeval tval;
+
+	gettimeofday(&tval, NULL);
+	return (pg_usec_time_t) tval.tv_sec * 1000000 + tval.tv_usec;
+}
+
 
 /*
  * A couple of "miscellaneous" multibyte related functions. They used

src/interfaces/libpq/libpq-fe.h

@@ -21,7 +21,6 @@ extern "C"
 #endif
 
 #include <stdio.h>
-#include <time.h>
 
 /*
  * postgres_ext.h defines the backend's externally visible types,
@@ -202,6 +201,9 @@ typedef struct pgNotify
 	struct pgNotify *next;		/* list link */
 } PGnotify;
 
+/* pg_usec_time_t is like time_t, but with microsecond resolution */
+typedef pg_int64 pg_usec_time_t;
+
 /* Function types for notice-handling callbacks */
 typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res);
 typedef void (*PQnoticeProcessor) (void *arg, const char *message);
@@ -673,7 +675,11 @@ extern int	lo_export(PGconn *conn, Oid lobjId, const char *filename);
 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);
+extern int	PQsocketPoll(int sock, int forRead, int forWrite,
+						 pg_usec_time_t end_time);
+
+/* Get current time in the form PQsocketPoll wants */
+extern pg_usec_time_t PQgetCurrentTimeUSec(void);
 
 /* Determine length of multibyte encoded char at *s */
 extern int	PQmblen(const char *s, int encoding);

src/interfaces/libpq/libpq-int.h

@@ -755,7 +755,7 @@ extern int	pqReadData(PGconn *conn);
 extern int	pqFlush(PGconn *conn);
 extern int	pqWait(int forRead, int forWrite, PGconn *conn);
 extern int	pqWaitTimed(int forRead, int forWrite, PGconn *conn,
-						time_t finish_time);
+						pg_usec_time_t end_time);
 extern int	pqReadReady(PGconn *conn);
 extern int	pqWriteReady(PGconn *conn);