Stackless issue python#282: Refactor slp_eval_frame

Decouple the cleanup logic for initial stubs of sub-interpreters from
the stack handling. Previously this logic depended on the value of
SLP_CSTACK_GOODGAP being positive.

Stackless/core/stacklesseval.c

@@ -317,6 +317,7 @@ slp_eval_frame(PyFrameObject *f)
     PyThreadState *ts = _PyThreadState_GET();
     PyFrameObject *fprev = f->f_back;
     intptr_t * stackref;
+    int set_cstack_base;
     PyObject *retval;
 
     if (fprev == NULL && ts->st.main == NULL) {
@@ -345,51 +346,58 @@ slp_eval_frame(PyFrameObject *f)
 
         /* mark the stack base */
         stackref = SLP_STACK_REFPLUS + (intptr_t *) &f;
-        if (ts->st.cstack_base == NULL)
+        set_cstack_base = ts->st.cstack_base == NULL;
+        if (set_cstack_base)
             ts->st.cstack_base = stackref - SLP_CSTACK_GOODGAP;
         if (stackref > ts->st.cstack_base) {
-            PyCStackObject *initial_stub;
-            retval = climb_stack_and_eval_frame(f);
-            initial_stub = ts->st.initial_stub;
-            /* cst might be NULL in OOM conditions */
-            if (ts->interp != _PyRuntime.interpreters.main && initial_stub != NULL) {
-                PyCStackObject *cst;
-                register int found = 0;
-                assert(initial_stub->startaddr == ts->st.cstack_base);
-                for (cst = initial_stub->next; cst != initial_stub; cst = cst->next) {
-                    if (Py_SIZE(cst) != 0 && cst->task != NULL &&
-                            cst->tstate == ts) {
-                        assert(cst->startaddr == ts->st.cstack_base);
-                        found = 1;
-                        break;
-                    }
-                }
-                if (!found) {
-                    Py_CLEAR(ts->st.initial_stub);
-                    ts->st.cstack_base = NULL;
-                    ts->st.cstack_root = NULL;
-                }
+            retval = climb_stack_and_eval_frame(f);  /* recursively calls slp_eval_frame(f) */
+        } else {
+            assert(SLP_CURRENT_FRAME(ts) == NULL);  /* else we would change the current frame */
+            SLP_STORE_NEXT_FRAME(ts, f);
+            returning = make_initial_stub();
+            if (returning < 0)
+                return NULL;
+            /* returning will be 1 if we "switched back" to this stub, and 0
+             * if this is the original call that just created the stub.
+             * If the stub is being reused, the argument, i.e. the frame,
+             * is in ts->frame
+             */
+            SLP_ASSERT_FRAME_IN_TRANSFER(ts);
+            if (returning != 1) {
+                assert(f == SLP_PEEK_NEXT_FRAME(ts));
             }
-            return retval;
+            assert(ts->interp->st.initial_tstate != NULL);
+            assert(ts->interp->st.mem_bomb != NULL);
+
+            retval = slp_run_tasklet();
         }
 
-        assert(SLP_CURRENT_FRAME(ts) == NULL);  /* else we would change the current frame */
-        SLP_STORE_NEXT_FRAME(ts, f);
-        returning = make_initial_stub();
-        if (returning < 0)
-            return NULL;
-        /* returning will be 1 if we "switched back" to this stub, and 0
-         * if this is the original call that just created the stub.
-         * If the stub is being reused, the argument, i.e. the frame,
-         * is in ts->frame
-         */
-        SLP_ASSERT_FRAME_IN_TRANSFER(ts);
-        if (returning != 1) {
-            assert(f == SLP_PEEK_NEXT_FRAME(ts));
+        /* Clear the initial stub, except for the main interpreter.
+         * Without this code, terminating sub-interpreters would leak references. */
+        if (set_cstack_base  /* only for the outermost call */
+                && ts->interp != _PyRuntime.interpreters.main  /* not for the main interpreter */
+                && ts->st.initial_stub != NULL) /* not yet cleared */
+        {
+            PyCStackObject *initial_stub = ts->st.initial_stub;
+            PyCStackObject *cst;
+            register int found = 0;
+            assert(initial_stub->startaddr == ts->st.cstack_base);
+            for (cst = initial_stub->next; cst != initial_stub; cst = cst->next) {
+                if (Py_SIZE(cst) != 0 && cst->task != NULL &&
+                        cst->tstate == ts) {
+                    assert(cst->startaddr == ts->st.cstack_base);
+                    found = 1;  /* Initial stub is still in use. */
+                    break;
+                }
+            }
+            if (!found) {
+                /* This initial stub is no longer in use, free it */
+                Py_CLEAR(ts->st.initial_stub);
+                ts->st.cstack_base = NULL;
+                ts->st.cstack_root = NULL;
+            }
         }
-        assert(ts->interp->st.initial_tstate != NULL);
-        assert(ts->interp->st.mem_bomb != NULL);
-        return slp_run_tasklet();
+        return retval;
     }
     assert(ts->interp->st.initial_tstate != NULL);
     assert(ts->interp->st.mem_bomb != NULL); /* see above */