fix: test: Use virtualenv's Python interpreter when running tests from a venv

Merge branch 'nicki/pytest-venv-python' into 'main'

See merge request isc-projects/bind9!11854

bin/tests/system/conftest.py

@@ -402,6 +402,11 @@ def system_test_dir(request, system_test_name, expected_artifacts):
     # Log which binaries are used for the test(s)
     isctest.log.info("testing binaries from: %s", os.environ.get("TOP_BUILDDIR"))
 
+    # Log what Python interpreter is used to run the test(s)
+    isctest.log.info(
+        "using Python interpreter at: %s to run the test(s)", os.environ.get("PYTHON")
+    )
+
     # System tests are meant to be executed from their directory - switch to it.
     old_cwd = os.getcwd()
     os.chdir(testdir)

bin/tests/system/isctest/vars/build.py

@@ -11,6 +11,8 @@
 
 from pathlib import Path
 
+import sys
+
 SYSTEM_TEST_DIR_GIT_PATH = "bin/tests/system"
 
 
@@ -53,6 +55,17 @@ def load_vars_from_build_files() -> dict[str, str]:
         if var_file.exists():
             build_vars[var] = var_file.read_text(encoding="utf-8").strip()
 
+    # When running inside a virtualenv, prefer the virtualenv's interpreter
+    # over the path baked in by meson.
+    # This is needed because meson is configured to prefer specific versions
+    # (e.g. python3.12 and python3.11) by default to properly detect the
+    # right python interpreter in CI - this is undesirable when running
+    # in a virtualenv as it can lead to the system tests using the system
+    # versions of Python dependencies defeating the purpose of the virtual
+    # environment.
+    if sys.prefix != sys.base_prefix:
+        build_vars["PYTHON"] = sys.executable
+
     return build_vars