SERVER-34506 TLASIO test for isJustForContinuation

Adding an integration test for transport layer asio which uses a fail
point to induce single byte at a time reads and writes.  We use this,
along with a debug block in the future header, to ensure that the
continuation folding in futures is working properly with
opportunisticRead/Write chaining.

To test the other side of this, adding a new test command called "echo"
which returns the command object passed to it (to allow for a large
message body on the read and write path more easily).

1 files changed, 22 insertions, 0 deletions

diff --git a/src/mongo/util/future.h b/src/mongo/util/future.h
index 2b4d2c9c4f4..7a8fece88ab 100644
--- a/src/mongo/util/future.h
+++ b/src/mongo/util/future.h
@@ -41,6 +41,7 @@
 #include "mongo/stdx/mutex.h"
 #include "mongo/stdx/utility.h"
 #include "mongo/util/assert_util.h"
+#include "mongo/util/debug_util.h"
 #include "mongo/util/intrusive_counter.h"
 #include "mongo/util/scopeguard.h"
 
@@ -356,6 +357,27 @@ public:
 
         dassert(oldState == SSBState::kWaiting);
 
+        DEV {
+            // If you hit this limit one of two things has probably happened
+            //
+            // 1. The justForContinuation optimization isn't working.
+            // 2. You may be creating a variable length chain.
+            //
+            // If those statements don't mean anything to you, please ask an editor of this file.
+            // If they don't work here anymore, I'm sorry.
+            const size_t kMaxDepth = 32;
+
+            size_t depth = 0;
+            for (auto ssb = continuation.get(); ssb;
+                 ssb = ssb->state.load(std::memory_order_acquire) == SSBState::kWaiting
+                     ? ssb->continuation.get()
+                     : nullptr) {
+                depth++;
+
+                invariant(depth < kMaxDepth);
+            }
+        }
+
         if (callback) {
             callback(this);
         }