BUG #15929: logical decoding can not write down the analyse result when the output file is touched.

The following bug has been logged on the website:

Bug reference:      15929
Logged by:          movead li
Email address:      movead.li@highgo.ca
PostgreSQL version: 12beta1
Operating system:   centos7
Description:

I was doing some testing of logical decoding and ran into a scenario that
may be a bug. The following steps will lead to reproducing the behaviour :


1.Start a logical decoding use below command:

pg_recvlogical -d postgres --slot=test2 --create-slot
pg_recvlogical -d postgres --slot=test2 --start -f ld.log

2. When I do a DML on the database it writes down the info into ld.log
file,
so far so good.

3. Next I open the log file. “Id.log” with vi and make some arbitrary
changes
in the log file.

4. If I run a DML command now the DML info will not appear in the ld.log
file. 
Refer to step 2, when I did a DML it was appearing in the log file.

5. If I do some manipulation with the log file using the OS commands below :


mv ld.log ld.log1; 
cp ld.log1 ld.log;

Then the DML info will start appearing in file ld.log1.

This seems like a bug to be because the user might accidentally some data
without noticing.

I have looked at the code to debug this behaviour and find a solution

In file ‘pg_recvlogical.c’ the ‘StreamLogicalLog()’ function seem to have
a
issue with handling of DML info. It uses open() to open the file and it does
not
check the file state when writing to the file after a long wait.