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.info lcov_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