new framework to inhibit thread cancellation when needed

with these small changes, libc functions which need to call functions
which are cancellation points, but which themselves must not be
cancellation points, can use the CANCELPT_INHIBIT and CANCELPT_RESUME
macros to temporarily inhibit all cancellation.

src/internal/libc.h

@@ -45,6 +45,8 @@ void __lockfile(FILE *);
 #define CANCELPT_BEGIN CANCELPT(1)
 #define CANCELPT_TRY CANCELPT(0)
 #define CANCELPT_END CANCELPT(-1)
+#define CANCELPT_INHIBIT CANCELPT(2)
+#define CANCELPT_RESUME CANCELPT(-2)
 
 extern char **__environ;
 #define environ __environ

src/thread/pthread_create.c

@@ -57,9 +57,19 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
 static void cancelpt(int x)
 {
 	struct pthread *self = __pthread_self();
-	if (self->canceldisable) return;
-	if ((self->cancelpoint+=x)==1 && x>=0 && self->cancel)
-		docancel(self);
+	switch (x) {
+	case 1:
+		self->cancelpoint++;
+	case 0:
+		if (self->cancel && self->cancelpoint==1 && !self->canceldisable)
+			docancel(self);
+		break;
+	case -1:
+		self->cancelpoint--;
+		break;
+	default:
+		self->canceldisable += x;
+	}
 }
 
 /* "rsyscall" is a mechanism by which a thread can synchronously force all

src/thread/pthread_setcancelstate.c

@@ -3,8 +3,8 @@
 int pthread_setcancelstate(int new, int *old)
 {
 	struct pthread *self = pthread_self();
-	if (old) *old = self->canceldisable;
+	if (old) *old = self->canceldisable & 1;
 	if ((unsigned)new > 1) return EINVAL;
-	self->canceldisable = new;
+	self->canceldisable = (self->canceldisable & ~1) | new;
 	return 0;
 }