Re: File truncation within PostgresNode::issues_sql_like() wrong on Windows

On Wed, Apr 14, 2021 at 05:10:41PM -0400, Andrew Dunstan wrote:
> That seems rather heavy-handed. The buildfarm's approach is a bit
> different. Essentially it seeks to the previous position of the log file
> before reading contents. Here is its equivalent of slurp_file:
>
> use Fcntl qw(:seek);
> sub file_lines
> {
>     my $filename = shift;
>     my $filepos  = shift;
>     my $handle;
>     open($handle, '<', $filename) || croak "opening $filename: $!";
>     seek($handle, $filepos, SEEK_SET) if $filepos;
>     my @lines = <$handle>;
>     close $handle;
>     return @lines;
> }

That's a bit surprising to see that you can safely open a file handle
with perl like that without using Win32API::File, and I would have
assumed that this would have conflicted with the backend redirecting
its output to stderr the same way as a truncation on Windows.

> A client wanting what's done in PostgresNode would do something like:
>
> my $logpos  = -s $logfile;
> do_some_stuff();
> my @lines = file_lines($logfile, $logpos);
>
> This has the benefit of working the same on all platforms, and no
> truncation, rotation, or restart is required.

Jacob has suggested something like that a couple of days ago, but all
this code was not centralized yet in a single place.

For this code, the cleanest approach would be to extend slurp_file()
with an extra argument to seek the file before fetching its contents
based on a location given by the caller? Looking at the docs of
Win32API::File, we'd need to use SetFilePointer() instead of seek().
--
Michael