THRIFT-6183: Use the library-wide default frame size in TNonblockingServer

[Jens-G]

TNonblockingServer caps the frame it will accept from a connection at its own MAX_FRAME_SIZE, which has been 256 * 1024 * 1024 since THRIFT-1337 landed it in 2011 — years before TConfiguration existed. The library-wide default is TConfiguration::DEFAULT_MAX_FRAME_SIZE = 16384000, whose own comment reads "this value is used consistently across all Thrift libraries".

The two disagree by a factor of 16:

TNonblockingServer::getMaxFrameSize()     268435456
TConfiguration::DEFAULT_MAX_FRAME_SIZE     16384000

TConfiguration appears zero times in TNonblockingServer.h and TNonblockingServer.cpp, so the server neither reads a configured value nor inherits the default. It starts from its own constant, and only setMaxFrameSize() moves it. That constant is what sizes the read buffer, since TConnection::transition() resets the buffer to the length the peer declared before any payload byte has arrived.

Java's AbstractNonblockingServer has no such gap: its per-frame limit comes from trans_.getMaxFrameSize(), that is from TConfiguration, and it keeps a separate aggregate read-buffer budget on top of it.

The change

MAX_FRAME_SIZE now points at TConfiguration::DEFAULT_MAX_FRAME_SIZE. The constant is private, so this is not an API change, and setMaxFrameSize() still overrides it.

Compatibility — worth a reviewer's attention

This lowers a shipped default. A deployment that today accepts frames between 16,384,000 and 268,435,456 bytes on TNonblockingServer, and does not call setMaxFrameSize(), will start closing those connections. Nothing in the tree relies on the old value — there is no setMaxFrameSize() caller outside the new test, and the largest payload anywhere near this path is StressTestNonBlocking's 2 MB chunk size — but out-of-tree users may. Release note needed.

Test

One case added to the existing lib/cpp/test/TNonblockingServerTest.cpp. It fails before the change:

error: in "TNonblockingServerTest/default_max_frame_size_matches_configuration":
check server.getMaxFrameSize() == static_cast<size_t>(TConfiguration::DEFAULT_MAX_FRAME_SIZE)
has failed [268435456 != 16384000]

and also pins that setMaxFrameSize() still wins, so the escape hatch cannot quietly go away.

ctest on this branch: 32 of 35. The three failures — UnitTests, TInterruptTest, TServerIntegrationTest (SEGFAULT) — are all connect() failed: Connection refused and reproduce identically on pristine master in the same container, so they are environmental and pre-existing.

Not in this PR

• TNonblockingServer still does not consult TConfiguration at all. Wiring it up is a larger design change and belongs in its own ticket.
• lib/cpp/src/thrift/transport/TBufferTransports.h:349 still carries TFramedTransport::DEFAULT_MAX_FRAME_SIZE = 256 * 1024 * 1024, which nothing in the tree reads any more — all three constructors take configuration_->getMaxFrameSize(). That one is public, so correcting or removing it is an API question, unlike this change.

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