anet.c: new API anetTcpNonBlockBestEffortBindConnect()

This performs a best effort source address binding attempt. If it is
possible to bind the local address and still have a successful
connect(), then this socket is returned. Otherwise the call is retried
without source address binding attempt.

Related to issues #2609 and #2612.

src/anet.c

@@ -264,6 +264,7 @@ static int anetCreateSocket(char *err, int domain) {
 
 #define ANET_CONNECT_NONE 0
 #define ANET_CONNECT_NONBLOCK 1
+#define ANET_CONNECT_BE_BINDING 2 /* Best effort binding. */
 static int anetTcpGenericConnect(char *err, char *addr, int port,
                                  char *source_addr, int flags)
 {
@@ -331,9 +332,17 @@ static int anetTcpGenericConnect(char *err, char *addr, int port,
         close(s);
         s = ANET_ERR;
     }
+
 end:
     freeaddrinfo(servinfo);
-    return s;
+
+    /* Handle best effort binding: if a binding address was used, but it is
+     * not possible to create a socket, try again without a binding address. */
+    if (s == ANET_ERR && source_addr && (flags & ANET_CONNECT_BE_BINDING)) {
+        return anetTcpGenericConnect(err,addr,port,NULL,flags);
+    } else {
+        return s;
+    }
 }
 
 int anetTcpConnect(char *err, char *addr, int port)
@@ -346,9 +355,18 @@ int anetTcpNonBlockConnect(char *err, char *addr, int port)
     return anetTcpGenericConnect(err,addr,port,NULL,ANET_CONNECT_NONBLOCK);
 }
 
-int anetTcpNonBlockBindConnect(char *err, char *addr, int port, char *source_addr)
+int anetTcpNonBlockBindConnect(char *err, char *addr, int port,
+                               char *source_addr)
+{
+    return anetTcpGenericConnect(err,addr,port,source_addr,
+            ANET_CONNECT_NONBLOCK);
+}
+
+int anetTcpNonBlockBestEffortBindConnect(char *err, char *addr, int port,
+                                         char *source_addr)
 {
-    return anetTcpGenericConnect(err,addr,port,source_addr,ANET_CONNECT_NONBLOCK);
+    return anetTcpGenericConnect(err,addr,port,source_addr,
+            ANET_CONNECT_NONBLOCK|ANET_CONNECT_BE_BINDING);
 }
 
 int anetUnixGenericConnect(char *err, char *path, int flags)

src/anet.h

@@ -50,6 +50,7 @@
 int anetTcpConnect(char *err, char *addr, int port);
 int anetTcpNonBlockConnect(char *err, char *addr, int port);
 int anetTcpNonBlockBindConnect(char *err, char *addr, int port, char *source_addr);
+int anetTcpNonBlockBestEffortBindConnect(char *err, char *addr, int port, char *source_addr);
 int anetUnixConnect(char *err, char *path);
 int anetUnixNonBlockConnect(char *err, char *path);
 int anetRead(int fd, char *buf, int count);