fix(callgrind): drop BBCCs whose top context fn is skip-flagged

When the (cxt == 0) clause in setup_bbcc force-pushes a skipped fn —
e.g. the first BB after CALLGRIND_START_INSTRUMENTATION lives in a
skipped object — the BBCC ends up with cxt->fn[0]->skip == True.
Without filtering, those BBCCs emit a top-level fn= block and the
skipped fn leaks into the dump.

Filter them out at dump time in print_bbccs_of_thread, right before
print_fn_pos would emit the ob=/fl=/fn= header. The call edges from
non-skipped callers into skipped fns (cfn=) are unaffected because
they're emitted from the caller's BBCC, whose context is not skipped.

Also broadens the runtime_obj_skip_c post-check to grep for any
fn=skipme*, since the actual leaked fn in the repro is skipme_run
(the one calling START_INSTRUMENTATION), not skipme_func.

Author: not-matthias

callgrind/dump.c

@@ -1553,7 +1553,17 @@ static void print_bbccs_of_thread(thread_info* ti)
     }
 
     if (*p == 0) break;
-    
+
+    /* Don't emit BBCCs whose top context fn is flagged for obj-skip.
+     * This happens when the (cxt == 0) clause in setup_bbcc force-
+     * pushes a skipped fn (first BB after instrumentation start that
+     * landed in a skipped object). Without this filter the skipped fn
+     * leaks into the dump as a top-level fn= block. */
+    if ((*p)->cxt->fn[0]->skip) {
+      p++;
+      continue;
+    }
+
     if (print_fn_pos(print_fp, &lastFnPos, *p)) {
 
       /* new function */

callgrind/tests/runtime_obj_skip_c.vgtest

@@ -1,5 +1,5 @@
 prereq: test -f runtime_obj_skip_c && test -f runtime_obj_skip_c_lib.so
 prog-asis: ./runtime_obj_skip_c
 vgopts: --instr-atstart=no --compress-strings=no --callgrind-out-file=callgrind.out.runtime_obj_skip_c
-post: sh -c 'if grep -q "^fn=skipme_func" callgrind.out.runtime_obj_skip_c; then echo "FAIL: skipme_func leaked into top-level fn= block"; else echo OK; fi'
+post: sh -c 'if grep -q "^fn=skipme" callgrind.out.runtime_obj_skip_c; then echo "FAIL: skipme_* leaked into top-level fn= block"; else echo OK; fi'
 cleanup: rm -f callgrind.out.runtime_obj_skip_c