Apparently writev() imposes a limit on the vector size. Work around that. Not…

Apparently writev() imposes a limit on the vector size.  Work around that.  Not sure why libc doesn't already do this.

c++/src/kj/io.c++

@@ -279,6 +279,17 @@ void FdOutputStream::write(const void* buffer, size_t size) {
 }
 
 void FdOutputStream::write(ArrayPtr<const ArrayPtr<const byte>> pieces) {
+  // Apparently, there is a maximum number of iovecs allowed per call.  I don't understand why.
+  // Also, most platforms define IOV_MAX but Linux defines only UIO_MAXIOV.  Unfortunately, Solaris
+  // defines a constant UIO_MAXIOV with a different meaning, so we check for IOV_MAX first.
+#if !defined(IOV_MAX) && defined(UIO_MAXIOV)
+#define IOV_MAX UIO_MAXIOV
+#endif
+  while (pieces.size() > IOV_MAX) {
+    write(pieces.slice(0, IOV_MAX));
+    pieces = pieces.slice(IOV_MAX, pieces.size());
+  }
+
   KJ_STACK_ARRAY(struct iovec, iov, pieces.size(), 16, 128);
 
   for (uint i = 0; i < pieces.size(); i++) {