Re: refactoring basebackup.c

Thanks for the patch, Dipesh.
I had a look at the patch and also tried to take the backup. I have
following suggestions and observations:

I get following error at my end:

$ pg_basebackup -D /tmp/zstd_bk -Ft -Xfetch --compress=server-zstd:7@4
pg_basebackup: error: could not initiate base backup: ERROR:  could not compress data: Unsupported parameter
pg_basebackup: removing data directory "/tmp/zstd_bk"

This is mostly because I have the zstd library version v1.4.4, which
does not have default support for parallel workers. Maybe we should
have a better error, something that is hinting that the parallelism is
not supported by the particular build.

The regression for pg_verifybackup test 008_untar.pl also fails with a
similar error. Here, I think we should have some logic in regression to
skip the test if the parameter is not supported?

+   if (ZSTD_isError(ret))                                                    
+       elog(ERROR,                                                            
+            "could not compress data: %s",                                    
+            ZSTD_getErrorName(ret));    

I think all of this can go on one line, but anyhow we have to improve
the error message here.

Also, just a thought, for the versions where parallelism is not
supported, should we instead just throw a warning and fall back to
non-parallel behavior?

Regards,
Jeevan Ladhe