tests: Exercise fallback to /proc/PID/mem

Add an environment variable to force reads from `/proc/PID/mem` instead
of using `process_vm_readv` in order to give us a way to exercise both
paths on the same kernel.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

Author: godlygeek

src/pystack/_pystack/mem.cpp

@@ -228,7 +228,7 @@ ProcessMemoryManager::ProcessMemoryManager(pid_t pid)
 ssize_t
 ProcessMemoryManager::readChunk(remote_addr_t addr, size_t len, char* dst) const
 {
-    if (d_memfile) {
+    if (d_memfile || getenv("_PYSTACK_NO_PROCESS_VM_READV") != nullptr) {
         return readChunkThroughMemFile(addr, len, dst);
     } else {
         return readChunkDirect(addr, len, dst);

tests/integration/test_smoke.py

@@ -38,13 +38,23 @@
 
 @pytest.mark.parametrize("method", STACK_METHODS)
 @pytest.mark.parametrize("blocking", [True, False], ids=["blocking", "non-blocking"])
-def test_simple_execution(method, blocking, tmpdir):
+@pytest.mark.parametrize(
+    "use_process_vm_readv", [True, False], ids=["process_vm_readv", "/proc/PID/mem"]
+)
+def test_simple_execution(method, blocking, use_process_vm_readv, tmpdir, monkeypatch):
     """Test that we can retrieve the thread state of a single process.
 
     This test is specially useful to run under valgrind or similar tools and to
     quickly check that the very basic functionality works as expected.
     """
 
+    # GIVEN
+    env_var = "_PYSTACK_NO_PROCESS_VM_READV"
+    if use_process_vm_readv:
+        monkeypatch.delenv(env_var, raising=False)
+    else:
+        monkeypatch.setenv(env_var, "1")
+
     # WHEN
     with spawn_child_process(
         sys.executable, TEST_SINGLE_THREAD_FILE, tmpdir