feat: Add new tunable --shm-metadata-msg-size

The shm metadata msg will be right-padded to the given size. This
tunable may be used to saturate the kernel msg buffers more quickly with
the effect that the ZeroMQ message queue size - on which the FairMQ
shmem transport relies upon - behaves more accurately for very small
queue sizes.

This introduces a change for the meta msg format in the multipart case:
old: | MetaHeader 1 | ... | MetaHeader n |
new: | n | MetaHeader 1 | ... | MetaHeader n | padded to fMetadataMsgSize |
where `n` is a `size_t` and contains the number of following meta headers.
Previously, this number was infered from the msg buffer size itself which is
no longer possible due to the potential padding.

Implements #432

fairmq/shmem/Manager.h

@@ -29,7 +29,7 @@
 #include <algorithm> // max
 #include <chrono>
 #include <condition_variable>
-#include <cstddef> // max_align_t
+#include <cstddef> // max_align_t, std::size_t
 #include <cstdlib> // getenv
 #include <cstring> // memcpy
 #include <memory> // make_unique
@@ -151,6 +151,7 @@ class Manager
         , fBadAllocMaxAttempts(1)
         , fBadAllocAttemptIntervalInMs(config ? config->GetProperty<int>("bad-alloc-attempt-interval", 50) : 50)
         , fNoCleanup(config ? config->GetProperty<bool>("shm-no-cleanup", false) : false)
+        , fMetadataMsgSize(config ? config->GetProperty<std::size_t>("shm-metadata-msg-size", 0) : 0)
     {
         using namespace boost::interprocess;
 
@@ -828,6 +829,8 @@ class Manager
         }
     }
 
+    auto GetMetadataMsgSize() const noexcept { return fMetadataMsgSize; }
+
     ~Manager()
     {
         fRegionsGen += 1; // signal TL cache invalidation
@@ -884,6 +887,8 @@ class Manager
     int fBadAllocMaxAttempts;
     int fBadAllocAttemptIntervalInMs;
     bool fNoCleanup;
+
+    std::size_t fMetadataMsgSize;
 };
 
 } // namespace fair::mq::shmem