THRIFT-6281: Hold the frames TFramedTransport writes to the configured frame size

[Jens-G]

JIRA: THRIFT-6281
Client: go

TFramedTransport.Flush checked a frame against math.MaxUint32 only. readFrame refuses a frame larger than the configured MaxFrameSize (default 16384000), but Flush wrote such frames. Measured on master with MaxFrameSize: 1024: Flush of a 2048-byte frame returned nil and wrote 2052 bytes, and a TFramedTransport with the same configuration refused that frame with "Incorrect frame size (2048)".

This is the plain framed counterpart of #3860 (THRIFT-6262), which does the same for THeaderTransport.

Change

• Flush applies GetMaxFrameSize(). A larger frame is refused with a TTransportException, like the existing checks in this file, and nothing of it is written.
• GetMaxFrameSize() never exceeds math.MaxInt32, so the new check also covers the old MaxUint32 one.
• The defer that returns the write buffer now runs before the check, so a refused frame is dropped. The next Write/Flush starts from an empty buffer rather than failing again on the same oversized one.
• lib/go/README.md gets a note for clients that send frames larger than the default. The note sits before the TLS section, so it does not collide with the one THRIFT-6262: Hold the frames THeaderTransport writes to the configured frame size #3860 appends.

Compatibility

A client that writes frames larger than its own MaxFrameSize now gets an error from Flush instead of sending a frame that a peer with the same configuration refuses.

Tests

New TestTFramedTransportFlushFrameSizeLimit:

• At the limit: a frame of exactly the limit is written and read back.
• One byte over: Flush returns a TTransportException and writes 0 bytes. The next small frame then goes out on its own, 4 + 4 bytes.

Results:

• Unmodified library: the over-limit case fails, because Flush returned nil.
• Mutations: each fails one assertion:
  • > → >=;
  • removing the check;
  • moving the buffer release back behind the check.
• Checks: go vet -stdmethods=false and go test -race ./thrift pass with Go 1.26 and 1.27, and GOARCH=386 vet and tests pass as well.

🤖 Generated with Claude Code

[fishy]

should we reuse the same helper in THeader implementation? in that helper we also checked against MaxUint32 which we didn't do here.