Use SOCK_NONBLOCK to reduce system calls for outgoing connections (#293)

What this PR mainly does:

1. Refactor the `anetCreateSocket()` to make it more generic for more
   socket arguments, and use `SOCK_NONBLOCK` if available, which will
   reduce two system calls (`F_GETFL` and `F_SETFL`) of enabling the
   non-blocking mode on each newly created socket.
2. Clean up the unused `anetUnixGenericConnect()` that calls
   `anetCreateSocket()`.

`SOCK_NONBLOCK` for system call `socket()` is supported on most
UNIX-like platforms (`linux`, `dragonfly`, `freebsd`, `netbsd`,
`openbsd`, `solaris`, etc.). This improvement will significantly reduce
the system calls considering how massively `anetTcpGenericConnect()`
will be called when needed.

As for the cleanup, `anetUnixGenericConnect` was introduced in c61e692
and the only reference back then was from the `createClient()` in
`redis-benchmark.c` which had been removed in ec8f066 and made it the
dead code. Most of that dead code was also cleaned up in f657315e, and
it seems that the `anetUnixGenericConnect` got left out. Therefore, I
also cleaned it up, ***but I'm not so certain about doing this cleanup
in this PR. Maybe you would prefer to do it in a separate PR?***

References:

- [socket(2) on Linux](https://man7.org/linux/man-pages/man2/socket.2.html)
- [socket(2) on FreeBSD](https://man.freebsd.org/cgi/man.cgi?socket(2))
- [socket(2) on DragonFly](https://man.dragonflybsd.org/?command=socket&section=2)
- [socket(2) on NetBSD](https://man.netbsd.org/socket.2)
- [socket(2) on OpenBSD](https://man.openbsd.org/socket.2)
- [socket(3c) on Solaris](https://docs.oracle.com/cd/E88353_01/html/E37843/socket-3c.html)
- [socket(3socket) on illumos](https://illumos.org/man/3SOCKET/socket)

---------

Signed-off-by: Andy Pan <i@andypan.me>

src/anet.c

@@ -380,19 +380,51 @@ static int anetSetReuseAddr(char *err, int fd) {
     return ANET_OK;
 }
 
-static int anetCreateSocket(char *err, int domain) {
+/* In general, SOCK_CLOEXEC won't have noticeable effect
+ * except for cases which really need this flag.
+ * Otherwise, it is just a flag that is nice to have.
+ * Its absence shouldn't affect a common socket's functionality.
+ */
+#define ANET_SOCKET_CLOEXEC 1
+#define ANET_SOCKET_NONBLOCK 2
+#define ANET_SOCKET_REUSEADDR 4
+static int anetCreateSocket(char *err, int domain, int type, int protocol, int flags) {
     int s;
-    if ((s = socket(domain, SOCK_STREAM, 0)) == -1) {
+
+#ifdef SOCK_CLOEXEC
+    if (flags & ANET_SOCKET_CLOEXEC) {
+      type |= SOCK_CLOEXEC;
+      flags &= ~ANET_SOCKET_CLOEXEC;
+    }
+#endif
+
+#ifdef SOCK_NONBLOCK
+    if (flags & ANET_SOCKET_NONBLOCK) {
+      type |= SOCK_NONBLOCK;
+      flags &= ~ANET_SOCKET_NONBLOCK;
+    }
+#endif
+
+    if ((s = socket(domain, type, protocol)) == -1) {
         anetSetError(err, "creating socket: %s", strerror(errno));
         return ANET_ERR;
     }
 
-    /* Make sure connection-intensive things like the benchmark tool
-     * will be able to close/open sockets a zillion of times */
-    if (anetSetReuseAddr(err,s) == ANET_ERR) {
+    if (flags & ANET_SOCKET_CLOEXEC && anetCloexec(s) == ANET_ERR) {
         close(s);
         return ANET_ERR;
     }
+
+    if (flags & ANET_SOCKET_NONBLOCK && anetNonBlock(err, s) == ANET_ERR) {
+        close(s);
+        return ANET_ERR;
+    }
+
+    if (flags & ANET_SOCKET_REUSEADDR && anetSetReuseAddr(err,s) == ANET_ERR) {
+        close(s);
+        return ANET_ERR;
+    }
+
     return s;
 }
 
@@ -418,12 +450,15 @@ static int anetTcpGenericConnect(char *err, const char *addr, int port,
     for (p = servinfo; p != NULL; p = p->ai_next) {
         /* Try to create the socket and to connect it.
          * If we fail in the socket() call, or on connect(), we retry with
-         * the next entry in servinfo. */
-        if ((s = socket(p->ai_family,p->ai_socktype,p->ai_protocol)) == -1)
+         * the next entry in servinfo.
+         *
+         * Make sure connection-intensive things like the benchmark tool
+         * will be able to close/open sockets a zillion of times.
+         */
+        int sockflags = ANET_SOCKET_CLOEXEC | ANET_SOCKET_REUSEADDR;
+        if (flags & ANET_CONNECT_NONBLOCK) sockflags |= ANET_SOCKET_NONBLOCK;
+        if ((s = anetCreateSocket(err,p->ai_family,p->ai_socktype,p->ai_protocol,sockflags)) == ANET_ERR)
             continue;
-        if (anetSetReuseAddr(err,s) == ANET_ERR) goto error;
-        if (flags & ANET_CONNECT_NONBLOCK && anetNonBlock(err,s) != ANET_OK)
-            goto error;
         if (source_addr) {
             int bound = 0;
             /* Using getaddrinfo saves us from self-determining IPv4 vs IPv6 */
@@ -491,34 +526,6 @@ int anetTcpNonBlockBestEffortBindConnect(char *err, const char *addr, int port,
             ANET_CONNECT_NONBLOCK|ANET_CONNECT_BE_BINDING);
 }
 
-int anetUnixGenericConnect(char *err, const char *path, int flags)
-{
-    int s;
-    struct sockaddr_un sa;
-
-    if ((s = anetCreateSocket(err,AF_LOCAL)) == ANET_ERR)
-        return ANET_ERR;
-
-    sa.sun_family = AF_LOCAL;
-    valkey_strlcpy(sa.sun_path,path,sizeof(sa.sun_path));
-    if (flags & ANET_CONNECT_NONBLOCK) {
-        if (anetNonBlock(err,s) != ANET_OK) {
-            close(s);
-            return ANET_ERR;
-        }
-    }
-    if (connect(s,(struct sockaddr*)&sa,sizeof(sa)) == -1) {
-        if (errno == EINPROGRESS &&
-            flags & ANET_CONNECT_NONBLOCK)
-            return s;
-
-        anetSetError(err, "connect: %s", strerror(errno));
-        close(s);
-        return ANET_ERR;
-    }
-    return s;
-}
-
 static int anetListen(char *err, int s, struct sockaddr *sa, socklen_t len, int backlog, mode_t perm) {
     if (bind(s,sa,len) == -1) {
         anetSetError(err, "bind: %s", strerror(errno));
@@ -607,7 +614,10 @@ int anetUnixServer(char *err, char *path, mode_t perm, int backlog)
         anetSetError(err,"unix socket path too long (%zu), must be under %zu", strlen(path), sizeof(sa.sun_path));
         return ANET_ERR;
     }
-    if ((s = anetCreateSocket(err,AF_LOCAL)) == ANET_ERR)
+
+    int type = SOCK_STREAM;
+    int flags = ANET_SOCKET_CLOEXEC | ANET_SOCKET_NONBLOCK | ANET_SOCKET_REUSEADDR;
+    if ((s = anetCreateSocket(err,AF_LOCAL,type,0,flags)) == ANET_ERR)
         return ANET_ERR;
 
     memset(&sa,0,sizeof(sa));

src/unix.c

@@ -66,8 +66,6 @@ static int connUnixListen(connListener *listener) {
             serverLog(LL_WARNING, "Failed opening Unix socket: %s", server.neterr);
             exit(1);
         }
-        anetNonBlock(NULL, fd);
-        anetCloexec(fd);
         listener->fd[listener->count++] = fd;
     }