Allow TestLib::slurp_file to skip contents, and use as needed

In order to avoid getting old logfile contents certain functions in
PostgresNode were doing one of two things. On Windows it rotated the
logfile and restarted the server, while elsewhere it truncated the log
file. Both of these are unnecessary. We borrow from the buildfarm which
does this instead: note the size of the logfile before we start, and
then when fetching the logfile skip to that position before accumulating
contents. This is spelled differently on Windows but the effect is the
same. This is largely centralized in TestLib's slurp_file function,
which has a new optional parameter, the offset to skip to before
starting to reading the file. Code in the client becomes much neater.

Backpatch to all live branches.

Michael Paquier, slightly modified by me.

Discussion: https://postgr.es/m/YHajnhcMAI3++pJL@paquier.xyz

1 files changed, 16 insertions, 5 deletions

diff --git a/src/test/perl/TestLib.pm b/src/test/perl/TestLib.pm
index a7490d2ce79..fecc6733da6 100644
--- a/src/test/perl/TestLib.pm
+++ b/src/test/perl/TestLib.pm
@@ -46,7 +46,7 @@ use warnings;
 use Config;
 use Cwd;
 use Exporter 'import';
-use Fcntl qw(:mode);
+use Fcntl qw(:mode :seek);
 use File::Basename;
 use File::Find;
 use File::Spec;
@@ -117,7 +117,7 @@ BEGIN
 	if ($windows_os)
 	{
 		require Win32API::File;
-		Win32API::File->import(qw(createFile OsFHandleOpen CloseHandle));
+		Win32API::File->import(qw(createFile OsFHandleOpen CloseHandle setFilePointer));
 	}
 
 	# Specifies whether to use Unix sockets for test setups.  On
@@ -402,21 +402,27 @@ sub slurp_dir
 
 =pod
 
-=item slurp_file(filename)
+=item slurp_file(filename [, $offset])
 
-Return the full contents of the specified file.
+Return the full contents of the specified file, beginning from an
+offset position if specified.
 
 =cut
 
 sub slurp_file
 {
-	my ($filename) = @_;
+	my ($filename, $offset) = @_;
 	local $/;
 	my $contents;
 	if ($Config{osname} ne 'MSWin32')
 	{
 		open(my $in, '<', $filename)
 		  or die "could not read \"$filename\": $!";
+		if (defined($offset))
+		{
+			seek($in, $offset, SEEK_SET)
+			  or die "could not seek \"$filename\": $!";
+		}
 		$contents = <$in>;
 		close $in;
 	}
@@ -426,6 +432,11 @@ sub slurp_file
 		  or die "could not open \"$filename\": $^E";
 		OsFHandleOpen(my $fh = IO::Handle->new(), $fHandle, 'r')
 		  or die "could not read \"$filename\": $^E\n";
+		if (defined($offset))
+		{
+			setFilePointer($fh, $offset, qw(FILE_BEGIN))
+			  or die "could not seek \"$filename\": $^E\n";
+		}
 		$contents = <$fh>;
 		CloseHandle($fHandle)
 		  or die "could not close \"$filename\": $^E\n";