Fix branch coverage for yield statements. #308 #324

Turns out the "call" and "return" trace events are really "start frame" and
"end frame".  They happen not only when functions are entered and left, but
when generators yield and resume.  We aren't interested in arcs into and out
of yield statements, so the trace functions look more closely to see what's
really happening, and record an arc in human-friendly terms.

Thanks for Mickie Betz for pushing on this bug, although her code is no
longer here. :(

1 files changed, 13 insertions, 4 deletions

diff --git a/coverage/tracer.c b/coverage/tracer.c
index 43ecd188..d532dcce 100644
--- a/coverage/tracer.c
+++ b/coverage/tracer.c
@@ -3,6 +3,7 @@
 #include "Python.h"
 #include "structmember.h"
 #include "frameobject.h"
+#include "opcode.h"
 
 /* Compile-time debugging helpers */
 #undef WHAT_LOG         /* Define to log the WHAT params in the trace function. */
@@ -596,7 +597,11 @@ CTracer_handle_call(CTracer *self, PyFrameObject *frame)
         SHOWLOG(self->pdata_stack->depth, frame->f_lineno, filename, "skipped");
     }
 
-    self->cur_entry.last_line = -1;
+    /* A call event is really a "start frame" event, and can happen for
+     * re-entering a generator also.  f_lasti is -1 for a true call, and a
+     * real byte offset for a generator re-entry.
+     */
+    self->cur_entry.last_line = (frame->f_lasti < 0) ? -1 : frame->f_lineno;
 
     ret = RET_OK;
 
@@ -685,9 +690,13 @@ CTracer_handle_return(CTracer *self, PyFrameObject *frame)
     }
     if (self->pdata_stack->depth >= 0) {
         if (self->tracing_arcs && self->cur_entry.file_data) {
-            int first = frame->f_code->co_firstlineno;
-            if (CTracer_record_pair(self, self->cur_entry.last_line, -first) < 0) {
-                goto error;
+            /* Need to distinguish between RETURN_VALUE and YIELD_VALUE. */
+            int bytecode = MyText_AS_STRING(frame->f_code->co_code)[frame->f_lasti];
+            if (bytecode != YIELD_VALUE) {
+                int first = frame->f_code->co_firstlineno;
+                if (CTracer_record_pair(self, self->cur_entry.last_line, -first) < 0) {
+                    goto error;
+                }
             }
         }