fiber: mark stack slab leak on mprotect fail as a non-leak

With new ASAN-friendly small implementation unit/fiber_stack.c test
start to fail. The issue is leak sanitizer reports a leak. This is an
expected leak of test for mprotect failure on fiber stack destruction.
Let's tell sanitizer to ignore this case.

By the way let's drop test code for temporary redirecting stderr. It is
outdated as test is TAP-compatible. It was a PITA as due to this
redirection there was no leak report only error exit code.

Part of #7327

NO_CHANGELOG=internal
NO_DOC=internal

src/lib/core/fiber.c

@@ -1309,6 +1309,14 @@ fiber_stack_destroy(struct fiber *fiber, struct slab_cache *slabc)
 			 */
 			say_syserror("fiber: Can't put guard page to slab. "
 				     "Leak %zu bytes", (size_t)fiber->stack_size);
+			/*
+			 * Suppress memory leak report for this object.
+			 *
+			 * Works even though it is not a beginning of
+			 * allocation (there is ASAN slab cache allocation
+			 * header).
+			 */
+			LSAN_IGNORE_OBJECT(fiber->stack_slab);
 		} else {
 			slab_put(slabc, fiber->stack_slab);
 		}

src/trivia/util.h

@@ -827,6 +827,13 @@ illegal_instruction(void)
 }
 #endif
 
+#ifdef ENABLE_ASAN
+# include <sanitizer/lsan_interface.h>
+# define LSAN_IGNORE_OBJECT(ptr) __lsan_ignore_object(ptr)
+#else
+# define LSAN_IGNORE_OBJECT(ptr) ((void)ptr)
+#endif
+
 #if defined(__cplusplus)
 } /* extern "C" */
 #endif /* defined(__cplusplus) */

test/unit/fiber_stack.c

@@ -44,10 +44,6 @@ main_f(va_list ap)
 	 * gh-9026. Stack size crafted to be close to 64k so we should
 	 * hit red zone around stack when writing watermark if bug is not
 	 * fixed.
-	 *
-	 * The test is placed at the beginning because stderr is redirected
-	 * to /dev/null at the end of the test and ASAN diagnostic will
-	 * not be visible if the test will be placed at the end.
 	 */
 	fiber_attr = fiber_attr_new();
 	fiber_attr_setstacksize(fiber_attr, (64 << 10) - 128);
@@ -130,17 +126,9 @@ main_f(va_list ap)
 	inj = errinj(ERRINJ_FIBER_MPROTECT, ERRINJ_INT);
 	inj->iparam = PROT_READ | PROT_WRITE;
 
-	/* On fiber_mprotect() fail we are logging number of bytes to be
-	 * leaked. However, it depends on system page_size (_SC_PAGESIZE).
-	 * On different OS's this parameter may vary. So let's temporary
-	 * redirect stderr to dev/null to make this test stable regardless
-	 * of OS.
-	 */
-	freopen("/dev/null", "w", stderr);
 	fiber_start(fiber);
 	fiber_join(fiber);
 	inj->iparam = -1;
-	freopen("/dev/stderr", "w", stderr);
 
 	used_after = slabc->allocated.stats.used;
 	ok(used_after > used_before, "expected leak detected");