On Thu, Nov 2, 2023 at 11:52 AM Kyotaro Horiguchi
<horikyota.ntt@gmail.com> wrote:
>
> At Tue, 31 Oct 2023 18:11:48 +0530, vignesh C <vignesh21@gmail.com> wrote in
> > Few others are also facing this problem with similar code like in:
> > https://stackoverflow.com/questions/15882799/fgets-returning-error-for-file-returned-by-popen
>
> I'm inclined to believe that the pipe won't enter the EOF state until
> the target command terminates (then the top-level cmd.exe). The target
> command likely terminated prematurely due to an error before priting
> any output.
>
> If we append "2>&1" to the command line, we can capture the error
> message through the returned pipe if any. Such error messages will
> cause the subsequent code to fail with an error such as "unexpected
> string: 'the output'". I'm not sure, but if this is permissive, the
> returned error messages could potentially provide insight into the
> underlying issue, paving the way for a potential solution.
>
Appending '2>&1 test:
The command still results in NULL and ends up failing as no data is
returned. Which means even no error message is returned. The error log
with appended '2>$1' is -
no data was returned by command
""D:/Project/pg1/postgres/tmp_install/bin/pg_resetwal" -V 2>&1"
check for "D:/Project/pg1/postgres/tmp_install/bin/pg_resetwal"
failed: cannot execute
Failure, exiting
Further observations:
1. To make sure the forked process completes before fgets(), I tested
with Sleep(100) before fgets() call.
...
...
if ((pgver = popen(cmd, "r")) == NULL)
{
perror("popen failure");
return NULL;
}
errno = 0;
Sleep(100);
if (fgets(line, maxsize, pgver) == NULL)
{
if (feof(pgver))
fprintf(stderr, "no data was returned by command \"%s\"\n", cmd);
...
...
This also doesn't resolve the issue, the error is still seen intermittently.
2. Even though fgets() fails, the output is still getting captured in
'line' string.
Tested with printing the 'line' in case of failure:
...
...
if ((pgver = popen(cmd, "r")) == NULL)
{
perror("popen failure");
return NULL;
}
errno = 0;
if (fgets(line, maxsize, pgver) == NULL)
{
if (line)
fprintf(stderr, "cmd output - %s\n", line);
if (feof(pgver))
fprintf(stderr, "no data was returned by command \"%s\"\n", cmd);
…
…
And the log looks like -
cmd output - postgres (PostgreSQL) 17devel
no data was returned by command
""D:/Project/pg1/postgres/tmp_install/bin/pg_controldata" -V"
check for "D:/Project/pg1/postgres/tmp_install/bin/pg_controldata"
failed: cannot execute
Failure, exiting
Attached test result log for the same - "regress_log_003_logical_slots".
Thanks,
Nisha Moond
