Event loops: the C library's resolver run on the thread's stack

glibc's getaddrinfo() sizes what it takes with alloca(), IPv6 most, by
the thread's stack, not by the fiber's it may be called on: a client
moved to a new configuration on a reload goes on on its fiber, and the
new configuration's services may resolve with the system resolver,
which otherwise is never called on a loop (threadperclient). A fiber
yields to have it called on the loop thread's own stack
(onthreadstack()), and goes on with its answer; the loop waits for it
as the fiber would have.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/common.c

@@ -340,6 +340,8 @@ void (*fiberblockfunc)(SOCKET s, short events) = NULL;
 void (*fiberclosefunc)(SOCKET s) = NULL;
 /* set as well: whether this thread is a loop's, which is not to wait */
 int (*onloopfunc)(void) = NULL;
+/* and what runs func(arg) on the thread's own stack, not a fiber's */
+void (*onthreadstackfunc)(void (*func)(void *arg), void *arg) = NULL;
 #define BLOCKED(s, ev) do { if(fiberblockfunc && (errno == EAGAIN || errno == EWOULDBLOCK)) \
 	fiberblockfunc((s), (ev)); } while(0)
 
@@ -1174,12 +1176,45 @@ int afdetect(unsigned char *name){
 }
 
 /* A name resolved the way a mode says: a service's names with its own */
+#if !defined(NOIPV6) && !defined(NOSTDRESOLVE)
+/* What the C library's resolver is asked, and its answer */
+struct stdresolve {
+	unsigned char *name;
+	int family;
+	struct sockaddr *sa;
+	uint32_t res;
+};
+
+/* getaddrinfo(): run on a thread's own stack, which glibc may take much of,
+   for IPv6 most (alloca() sized by the thread's stack, not a fiber's) */
+static void stdresolve46(void *arg){
+	struct stdresolve *r = (struct stdresolve *)arg;
+	struct addrinfo *ai, hint;
+
+	r->res = 0;
+	memset(&hint, 0, sizeof(hint));
+	hint.ai_family = (r->family == 6 || r->family == 64)?AF_INET6:AF_INET;
+	if (getaddrinfo((char *)r->name, NULL, &hint, &ai)) {
+		if(r->family == 64 || r->family == 46){
+			hint.ai_family = (r->family == 64)?AF_INET:AF_INET6;
+			if (getaddrinfo((char *)r->name, NULL, &hint, &ai)) return;
+		}
+		else return;
+	}
+	if(ai){
+		if(ai->ai_addr->sa_family == AF_INET || ai->ai_addr->sa_family == AF_INET6){
+			*SAFAMILY(r->sa)=ai->ai_addr->sa_family;
+			memcpy(SAADDR(r->sa), SAADDR(ai->ai_addr), SAADDRLEN(ai->ai_addr));
+			r->res = *SAFAMILY(r->sa);
+		}
+		freeaddrinfo(ai);
+	}
+}
+#endif
+
 uint32_t getip46m(int mode, int family, unsigned char *name,  struct sockaddr *sa){
 #ifndef NOIPV6
 	int detect;
-#ifndef NOSTDRESOLVE
-	struct addrinfo *ai, hint;
-#endif
         RESOLVFUNC tmpresolv;
 
 	if(!sa) return 0;
@@ -1210,23 +1245,16 @@ uint32_t getip46m(int mode, int family, unsigned char *name,  struct sockaddr *s
 		return 0;
 	}
 #ifndef NOSTDRESOLVE
-	memset(&hint, 0, sizeof(hint));
-	hint.ai_family = (family == 6 || family == 64)?AF_INET6:AF_INET;
-	if (getaddrinfo((char *)name, NULL, &hint, &ai)) {
-		if(family == 64 || family == 46){
-			hint.ai_family = (family == 64)?AF_INET:AF_INET6;
-			if (getaddrinfo((char *)name, NULL, &hint, &ai)) return 0;
-		}
-		else return 0;
-	}
-	if(ai){
-		if(ai->ai_addr->sa_family == AF_INET || ai->ai_addr->sa_family == AF_INET6){
-			*SAFAMILY(sa)=ai->ai_addr->sa_family;
-			memcpy(SAADDR(sa), SAADDR(ai->ai_addr), SAADDRLEN(ai->ai_addr));
-			freeaddrinfo(ai);
-			return *SAFAMILY(sa);
-		}
-		freeaddrinfo(ai);
+	{
+		struct stdresolve r;
+
+		r.name = name;
+		r.family = family;
+		r.sa = sa;
+		/* a fiber's stack is not one to give it */
+		if(onthreadstackfunc) (*onthreadstackfunc)(stdresolve46, &r);
+		else stdresolve46(&r);
+		return r.res;
 	}
 #endif
 	return 0;

src/mainloop.c

@@ -81,6 +81,9 @@ struct looper {
 	unsigned nidle;
 	struct sockreg **regs;		/* by descriptor, the loop's thread only */
 	unsigned nregs;
+	/* what a fiber yielded to have run on the thread's stack */
+	void (*stackcall)(void *arg);
+	void *stackarg;
 };
 
 /* what srvspawn() posts to a loop */
@@ -141,8 +144,16 @@ static void fibergive(struct looper *lp, struct fiberslot *fs){
    client is done. */
 static void fiberrun(struct looper *lp, struct fiber *f){
 	struct fiberslot *fs = (struct fiberslot *)f;
+	int alive;
 
-	if(!fiberresume(f)) fibergive(lp, fs);
+	/* a call it asked for on the thread's stack made, and it goes on */
+	while((alive = fiberresume(f)) && lp->stackcall){
+		void (*call)(void *arg) = lp->stackcall;
+
+		lp->stackcall = NULL;
+		call(lp->stackarg);
+	}
+	if(!alive) fibergive(lp, fs);
 	/* in the middle of its client's code: started over from its stack's
 	   start for the next client, or freed where it cannot be */
 	else if(fs->abandoned){
@@ -1132,6 +1143,20 @@ static unsigned ncpus(void){
 #endif
 }
 
+/* onthreadstackfunc: func(arg) on the loop thread's own stack, for a fiber
+   calling what wants a big one: the loop waits as long as it takes. */
+static void onthreadstack(void (*func)(void *arg), void *arg){
+	struct looper *lp = curlooper;
+
+	if(!lp || !fibercurrent()){
+		func(arg);
+		return;
+	}
+	lp->stackcall = func;
+	lp->stackarg = arg;
+	fiberyield();
+}
+
 /* The loops, started the first time a client is to be served on one, for
    the life of the process: as many as loops said then. 0 if they run. */
 static int loopsstart(void){
@@ -1191,6 +1216,7 @@ static int loopsstart(void){
 #endif
 		fiberpollfunc = fiberpoll;
 		onloopfunc = onlooper;
+		onthreadstackfunc = onthreadstack;
 	}
 	_3proxy_mutex_unlock(&loops_mutex);
 	return !i;

src/proxy.h

@@ -340,6 +340,7 @@ extern int (*fiberpollfunc)(struct pollfd *fds, unsigned nfds, int timeout);
 extern void (*fiberblockfunc)(SOCKET s, short events);
 extern void (*fiberclosefunc)(SOCKET s);
 extern int (*onloopfunc)(void);
+extern void (*onthreadstackfunc)(void (*func)(void *arg), void *arg);
 int sockreadable(SOCKET s);
 #ifndef _WIN32
 int fdclose(int fd);