Here is my sample code and I had to execute PQreset twice to connect to the database after asynchronous command ( PQsendQueryParams ).
#include <libpq-fe.h>
#include <iostream>
#include <stdlib.h>
using namespace std;
bool checkStatus(PGconn* conn, bool readable, bool writable, int second, int
microsecond)
{
int sock = PQsocket(conn);
fd_set read_mask, write_mask;
struct timeval timer;
if (sock > 0) {
timer.tv_sec = second;
timer.tv_usec = microsecond;
FD_ZERO(&read_mask);
FD_ZERO(&write_mask);
if (readable) {
FD_SET(sock, &read_mask);
}
if (writable) {
FD_SET(sock, &write_mask);
}
if (select(sock + 1, (readable) ? &read_mask: NULL, (writable) ? &write_mask : NULL, NULL, &timer) == 0) {
} else if ( readable && FD_ISSET(sock, &read_mask)) {
return true;
} else if ( writable && FD_ISSET(sock, &write_mask)) {
return true;
}
}
return false;
}
main(int argc, char* argv[])
{
bool success;
PGconn* conn;
const char* connInfo = argv[1];
conn = PQconnectdb(connInfo);
ConnStatusType type = PQstatus(conn);
if (type == CONNECTION_BAD) {
cout << PQerrorMessage(conn) << endl;
exit(1);
} else {
cout << "connected" << endl;
}
Retry:
const char* sql = "select * from pg_sleep(20)";
int nResult = PQsendQueryParams(conn,
sql, 0, NULL, NULL, NULL, NULL, 0);
cout << "Sent sleepquery...." << endl;
if (nResult == 1) {
PQflush(conn);
while (PQisBusy(conn) == 1) {
success = checkStatus(conn, true, false, 1, 0);
if (success) {
PQconsumeInput(conn);
} else {
if (PQstatus(conn) == CONNECTION_BAD) {
PQreset(conn);
if (PQstatus(conn) == CONNECTION_BAD) {
cout << "still invalid connection after PQreset" << endl;
sleep(1);
PQreset(conn);
}
goto Retry;
} else {
cout << ".";
cout.flush();
}
}
}
PGresult* rst = NULL;
while ((rst = PQgetResult(conn)) != NULL) {
int r, nRows = PQntuples(rst);
int c, nColumns = PQnfields(rst);
for (r = 0; r < nRows; r++) {
for (c = 0; c < nColumns; c++) {
cout << PQgetvalue(rst, r, c) << " ";
}
cout << endl;
}
PQclear(rst);
}
} else {
cout << PQerrorMessage(conn) << endl;
}
}
Once you run this program, you will see "." being printed on the console.
If you intentionally kill the postgres process instance, you will see the following output.
connected
Sent sleepquery....
........still invalid connection after PQreset
Sent sleepquery....
You will see "still invalid connection after PQreset".
For synchronous command (PQExec), PQreset works fine. However for asynchronous command like PQsendQueryParams, PQreset doesn't seem to work. I had to sleep a few seconds and then run PQreset again.
Anyone running into this problem?