pgsql: Fix handling of missing files when using pg_rewind with online s

Fix handling of missing files when using pg_rewind with online source

When working with an online source cluster, pg_rewind gets a list of all
the files in the source data directory using a WITH RECURSIVE query,
returning a NULL result for a file's metadata if it gets removed between
the moment it is listed in a directory and the moment its metadata is
obtained with pg_stat_file() (say a recycled WAL segment). The query
result was processed in such a way that for each tuple we checked only
that the first file's metadata was NULL. This could have two
consequences, both resulting in a failure of the rewind:
- If the first tuple referred to a removed file, all files from the
source would be ignored.
- Any file actually missing would not be considered as such.

While on it, rework slightly the code so as no values are saved if we
know that a file is going to be skipped.

Issue introduced by b36805f, so backpatch down to 9.5.

Author: Justin Pryzby, Michael Paquier
Reviewed-by: Daniel Gustafsson, Masahiko Sawada
Discussion: https://postgr.es/m/20200713061010.GC23581@telsasoft.com
Backpatch-through: 9.5

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/23924feca065aa75f0a9edcf78ad953a51405fe8

Modified Files
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src/bin/pg_rewind/libpq_fetch.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)