unix: don't write more than IOV_MAX iovecs

Write no more than `IOV_MAX` chunks with `writev()` at
once, otherwise `writev()` returns EINVAL.

src/unix/stream.c

@@ -33,6 +33,7 @@
 #include <sys/uio.h>
 #include <sys/un.h>
 #include <unistd.h>
+#include <limits.h> /* IOV_MAX */
 
 #if defined(__APPLE__)
 # include <sys/event.h>
@@ -742,6 +743,10 @@ start:
   iov = (struct iovec*) &(req->bufs[req->write_index]);
   iovcnt = req->bufcnt - req->write_index;
 
+  /* Limit iov count to avoid EINVALs from writev() */
+  if (iovcnt > IOV_MAX)
+    iovcnt = IOV_MAX;
+
   /*
    * Now do the actual writev. Note that we've been updating the pointers
    * inside the iov each time we write. So there is no need to offset it.

test/test-tcp-writealot.c

@@ -26,8 +26,8 @@
 
 
 #define WRITES            3
-#define CHUNKS_PER_WRITE  3
-#define CHUNK_SIZE        10485760 /* 10 MB */
+#define CHUNKS_PER_WRITE  4096
+#define CHUNK_SIZE        10024 /* 10 kb */
 
 #define TOTAL_BYTES       (WRITES * CHUNKS_PER_WRITE * CHUNK_SIZE)