MINOR: thread: add is_thread_harmless() to know if a thread already is harmless

The harmless status is not re-entrant, so sometimes for signal handling
it can be useful to know if we're already harmless or not. Let's add a
function doing that, and make the debugger use it instead of manipulating
the harmless mask.
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index 3ce436a..fe23127 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -116,6 +116,11 @@
 {
 }
 
+static inline int is_thread_harmless()
+{
+	return 1;
+}
+
 static inline void thread_harmless_end()
 {
 }
@@ -251,6 +256,12 @@
 	HA_ATOMIC_OR(&tg_ctx->threads_harmless, ti->ltid_bit);
 }
 
+/* Returns non-zero if the current thread is already harmless */
+static inline int is_thread_harmless()
+{
+	return !!(HA_ATOMIC_LOAD(&tg_ctx->threads_harmless) & ti->ltid_bit);
+}
+
 /* Ends the harmless period started by thread_harmless_now(). Usually this is
  * placed after the poll() call. If it is discovered that a job was running and
  * is relying on the thread still being harmless, the thread waits for the
diff --git a/src/debug.c b/src/debug.c
index 7b0711e..3abcc0f 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -1323,6 +1323,8 @@
 /* handles DEBUGSIG to dump the state of the thread it's working on */
 void debug_handler(int sig, siginfo_t *si, void *arg)
 {
+	int harmless = is_thread_harmless();
+
 	/* first, let's check it's really for us and that we didn't just get
 	 * a spurious DEBUGSIG.
 	 */
@@ -1366,7 +1368,7 @@
 	/* mark the current thread as stuck to detect it upon next invocation
 	 * if it didn't move.
 	 */
-	if (!(_HA_ATOMIC_LOAD(&tg_ctx->threads_harmless) & ti->ltid_bit) &&
+	if (!harmless &&
 	    !(_HA_ATOMIC_LOAD(&th_ctx->flags) & TH_FL_SLEEPING))
 		_HA_ATOMIC_OR(&th_ctx->flags, TH_FL_STUCK);
 }