Fix nginx worker 100% cpu usage (spinning on write returning EAGAIN)

Bump the pipe capacity, because running out of buffer space may cause
a write to spin indefinitely on EAGAIN.
Bumping the pipe capacity should eliminate the problem in practice,
though in theory the module could still be subject to it.
For now, leaving behind a todo with a suggested solution (should
the problem ever show up again).

Fixes #1380

Author: oschaaf

src/ngx_event_connection.cc

@@ -52,6 +52,12 @@ bool NgxEventConnection::Init(ngx_cycle_t* cycle) {
   } else {
     pipe_read_fd_ = file_descriptors[0];
     pipe_write_fd_ = file_descriptors[1];
+    // Attempt to bump the pipe capacity, because running out of buffer space
+    // can potentially lead up to writes spinning on EAGAIN.
+    // See https://github.com/pagespeed/ngx_pagespeed/issues/1380
+    // TODO(oschaaf): Consider implementing a queueing mechanism for retrying
+    // failed writes.
+    fcntl(pipe_write_fd_, F_SETPIPE_SZ, 200*1024 /* minimal amount of bytes */);
     return true;
   }
   close(file_descriptors[0]);