Re: Bogus reports from coverage.postgresql.org

I wrote:
> I also notice an interesting warning with either version:

> $ make coverage-html
> /usr/bin/lcov --gcov-tool /usr/bin/gcov -q --no-external -c -i -d . -d . -o lcov_base.info
> geninfo: Note: --initial does not generate branch coverage data
> /usr/bin/lcov --gcov-tool /usr/bin/gcov -q --no-external -c -d . -d . -o lcov_test.info
> rm -rf coverage
> /usr/bin/genhtml -q --legend -o coverage --title='PostgreSQL 11devel' --num-spaces=4 --prefix='/home/tgl/pgsql'
lcov_base.infolcov_test.info 
> touch coverage-html-stamp

> No idea what "--initial" refers to, but it suggests that we're
> misusing the tool somehow with this configuration.

So I went and read the lcov man page, and according to it we're doing
this entirely wrong.  The correct process, saith lcov, is

       -i
       --initial
              Capture initial zero coverage data.

              Run  lcov  with -c and this option on the directories containing
              .bb, .bbg or .gcno files  before  running  any  test  case.  The
              result  is  a  "baseline"  coverage data file that contains zero
              coverage for every instrumented line.  Combine  this  data  file
              (using  lcov  -a) with coverage data files captured after a test
              run to ensure that the percentage of total lines covered is cor-
              rect  even when not all source code files were loaded during the
              test.

              Recommended procedure when capturing data for a test case:

              1. create baseline coverage data file
                     # lcov -c -i -d appdir -o app_base.info

              2. perform test
                     # appdir/test

              3. create test coverage data file
                     # lcov -c -d appdir -o app_test.info

              4. combine baseline and test coverage data
                     # lcov -a app_base.info -a app_test.info -o
                     app_total.info

Our process swaps steps 1 and 2.  What I now suspect is that
we accidentally get away with that for basic mode, but it
doesn't work with lcov_branch_coverage.

            regards, tom lane