Re: fix tablespace handling in pg_combinebackup - Mailing list pgsql-hackers

22.04.2024 00:49, Thomas Munro wrote:
> On Mon, Apr 22, 2024 at 12:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
>>   From what I can see, the following condition (namely, -l):
>>                   if ($path =~ /^pg_tblspc\/(\d+)$/ && -l "$backup_path/$path")
>>                   {
>>                       push @tsoids, $1;
>>                       return 0;
>>                   }
>>
>> is false for junction points on Windows (cf [1]), but the target path is:
> Ah, yes, right, -l doesn't like junction points.  Well, we're already
> using the Win32API::File package (it looks like a CPAN library, but I
> guess the Windows perl distros like Strawberry are all including it
> for free?).  See PostgreSQL::Test::Utils::is_symlink(), attached.
> That seems to work as expected, but someone who actually knows perl
> can surely make it better.  Then I hit the next problem:
>
> readlink
C:\cirrus\build/testrun/pg_combinebackup/002_compare_backups\data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/16415:
> Inappropriate I/O control operation at
> C:/cirrus/src/test/perl/PostgreSQL/Test/Cluster.pm line 927.
>
> https://cirrus-ci.com/task/5162332353986560
>
> I don't know where exactly that error message is coming from, but
> assuming that Strawberry Perl contains this code:
>
> https://github.com/Perl/perl5/blob/f936cd91ee430786a1bb6068a4a7c8362610dd5f/win32/win32.c#L2041
> https://github.com/Perl/perl5/blob/f936cd91ee430786a1bb6068a4a7c8362610dd5f/win32/win32.c#L1976
>
> ... then it's *very* similar to what we're doing in our own
> pgreadlink() code.  I wondered if the buffer size might be too small
> for our path, but it doesn't seem so:
>
> https://github.com/Perl/perl5/blob/f936cd91ee430786a1bb6068a4a7c8362610dd5f/win32/win32.c#L1581C1-L1581C35
>
> (I think MAX_PATH is 256 on Windows.)
>
> If there is some unfixable problem with what they're doing in their
> readlink(), then I guess it should be possible to read the junction
> point directly in Perl using Win32API::File::DeviceIoControl()... but
> I can't see what's wrong with it!  Maybe it's not the same code?

I wonder whether the target path (\??\) of that junction point is fully correct.
I tried:
 > mklink /j 

"C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/test"

\\?\C:\t1
Junction created for 

C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/test

<<===>> \\?\C:\t1
and
my $path = 

'C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/test';
my $result = readlink($path);
works for me:
result: \\?\C:\t1

Whilst with:
my $path = 

'C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/16415';
readlink() fails with "Invalid argument".

 > dir 

"C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc"
04/22/2024  08:16 AM    <DIR>          .
04/22/2024  08:16 AM    <DIR>          ..
04/22/2024  06:52 AM    <JUNCTION>     16415 [\??\C:\Users\ADMINI~1\AppData\Local\Temp\1zznr8FW5N\ts1backup]
04/22/2024  08:16 AM    <JUNCTION>     test [\\?\C:\t1]

 > dir "\??\C:\Users\ADMINI~1\AppData\Local\Temp\1zznr8FW5N\ts1backup"
  Directory of C:\??\C:\Users\ADMINI~1\AppData\Local\Temp\1zznr8FW5N\ts1backup

File Not Found

 > dir "\\?\C:\t1"
  Directory of \\?\C:\t1

04/22/2024  08:06 AM    <DIR>          .
04/22/2024  08:06 AM    <DIR>          ..
                0 File(s)              0 bytes

Though
 > dir 

"C:/src/postgresql/build/testrun/pg_combinebackup/002_compare_backups/data/t_002_compare_backups_primary_data/backup/backup1/pg_tblspc/16415"
somehow really works:
  Directory of 

C:\src\postgresql\build\testrun\pg_combinebackup\002_compare_backups\data\t_002_compare_backups_primary_data\backup\backup1\pg_tblspc\16415

04/22/2024  06:52 AM    <DIR>          .
04/22/2024  06:52 AM    <DIR>          ..
04/22/2024  06:52 AM    <DIR>          PG_17_202404021

Best regards,
Alexander