Fix coverage: generate separate reports and combine with fastcov -C

Coverage data comes from two sources that may overlap:
  /gcov     — ProxySQL daemon execution (via GCOV_PREFIX)
  WORKSPACE — TAP test execution (.gcda next to binaries)

Both may cover the same code (e.g. libproxysql.a exercised by both
ProxySQL runtime and TAP test harness). Previously we copied .gcda
files between directories, which either overwrote (cp -f) or kept
only the first version (cp -n) instead of combining counters.

Now generates two separate .info files (proxysql.info + tap_tests.info)
and combines them with fastcov -C, which correctly sums execution
counters for overlapping source files.

test/infra/control/run-tests-isolated.bash

@@ -269,31 +269,46 @@ docker run \
                     echo \">>> Coverage generation log: \${coverage_log}\"
                     local nproc_val=\$(nproc)
 
-                    # Copy .gcno files to /gcov so fastcov can find them.
-                    # For .gcda files: ProxySQL's execution data is already in /gcov (via GCOV_PREFIX).
-                    # Only copy .gcda files that DON'T already exist in /gcov (TAP test coverage).
-                    # Using cp -n (no-clobber) prevents overwriting ProxySQL's real execution data
-                    # with stale .gcda files from the source tree.
+                    # Coverage data comes from two sources:
+                    #   /gcov     — ProxySQL daemon execution (via GCOV_PREFIX)
+                    #   WORKSPACE — TAP test binary execution (.gcda next to source)
+                    # Both may cover the same code (e.g. libproxysql.a exercised by
+                    # both ProxySQL runtime and TAP test harness). We generate separate
+                    # .info files and combine them with fastcov -C.
                     if [ -d \"/gcov\" ] && [ \"\$(ls -A /gcov 2>/dev/null)\" ]; then
                         echo \">>> Preparing coverage data directory...\" >> \"\${coverage_log}\" 2>&1
+
+                        # Copy .gcno files to /gcov (needed by fastcov, compile-time only)
                         cd \"\${WORKSPACE}\" && find . -path './ci_infra_logs' -prune -o -name '*.gcno' -type f -print | while read gcfile; do
                             target=\"/gcov/\${gcfile#./}\"
                             target_dir=\"\$(dirname \"\$target\")\"
                             mkdir -p \"\$target_dir\"
                             cp -f \"\$gcfile\" \"\$target\"
                         done
-                        # Copy TAP test .gcda files (don't overwrite ProxySQL's existing .gcda)
-                        cd \"\${WORKSPACE}\" && find . -path './ci_infra_logs' -prune -o -name '*.gcda' -type f -print | while read gcfile; do
-                            target=\"/gcov/\${gcfile#./}\"
-                            target_dir=\"\$(dirname \"\$target\")\"
-                            mkdir -p \"\$target_dir\"
-                            cp -n \"\$gcfile\" \"\$target\"
-                        done
-                        echo \">>> Running fastcov on /gcov...\"
+                        # Generate coverage from ProxySQL daemon execution (/gcov)
+                        echo \">>> Running fastcov on /gcov (ProxySQL daemon)...\"
+                        local proxysql_info=\"\${COVERAGE_REPORT_DIR}/proxysql.info\"
                         cd /gcov
                         fastcov -b -j\"\${nproc_val}\" --process-gcno -l \
                             --include \"\${WORKSPACE}/include/\" \"\${WORKSPACE}/lib/\" \"\${WORKSPACE}/src/\" \"\${WORKSPACE}/test/\" \
-                            -d . -o \"\${coverage_file}\" >> \"\${coverage_log}\" 2>&1 || echo \">>> WARNING: Coverage generation failed (see \${coverage_log})\"
+                            -d . -o \"\${proxysql_info}\" >> \"\${coverage_log}\" 2>&1 || echo \">>> WARNING: ProxySQL coverage generation failed\" >> \"\${coverage_log}\"
+
+                        # Generate coverage from TAP test execution (workspace)
+                        echo \">>> Running fastcov on workspace (TAP tests)...\"
+                        local tap_info=\"\${COVERAGE_REPORT_DIR}/tap_tests.info\"
+                        cd \"\${WORKSPACE}\"
+                        fastcov -b -j\"\${nproc_val}\" --process-gcno -l \
+                            --include \"\${WORKSPACE}/include/\" \"\${WORKSPACE}/lib/\" \"\${WORKSPACE}/src/\" \"\${WORKSPACE}/test/\" \
+                            -d . -o \"\${tap_info}\" >> \"\${coverage_log}\" 2>&1 || echo \">>> WARNING: TAP test coverage generation failed\" >> \"\${coverage_log}\"
+
+                        # Combine both coverage reports
+                        echo \">>> Combining ProxySQL + TAP test coverage...\"
+                        local info_files=\"\"
+                        [ -f \"\${proxysql_info}\" ] && info_files=\"\${proxysql_info}\"
+                        [ -f \"\${tap_info}\" ] && info_files=\"\${info_files} \${tap_info}\"
+                        if [ -n \"\${info_files}\" ]; then
+                            fastcov -C \${info_files} -o \"\${coverage_file}\" >> \"\${coverage_log}\" 2>&1 || echo \">>> WARNING: Coverage combination failed (see \${coverage_log})\"
+                        fi
                     else
                         echo \">>> WARNING: /gcov directory is empty or missing, skipping coverage\"
                     fi