Presumably it doesn't happen without the use of the pipe notation.
Indeed, it only happens when a pipe is involved.
I suspect that what's happening is that stdout isn't getting put into
binary mode, so that Microsoft's CR/NL translation corrupts the data.
If that's true, though, the resulting backup file should be corrupt;
is it valid according to "pg_restore backupfile"?
Yes pg_restore restores it without problems. However I did notice that it is larger then normal.
I have also tested what happens on freebsd when using a pipe and there are no warnings and backup is expected size.
I dug into the source code of pg_dump and found that the warning is given by _getFilePos(ArchiveHandle *AH, lclContext *ctx) in pg_backup_custom.c but it won't if ctx->hasSeek is set to false. Which is initialized by a call to checkSeek in pg_backup_archiver.c. Unfortunatly I'm not yet able to build and debug ton windows to verify if a correction to this function would help.
regards, Eelke