[Bug]: Small positive logtail rpc-max-message-size passes validation and panics TN

[Ariznawlll]

Environment

• Branch: main
• Commit: 118bb865d037c0c11113786db98999284945d1c4
• OS: macOS arm64

Description

The TN and TAE logtail configuration validators accept any positive rpc-max-message-size, but NewLogtailServer requires the value to be larger than the encoded LogtailResponseSegment header. A small positive value therefore passes configuration validation and later panics while a TN replica creates its TAE/logtail storage.

For example, rpc-max-message-size = 1 passes both validation paths and deterministically reaches:

if s.maxChunkSize <= 0 {
	panic("rpc max message size isn't enough")
}

White-box reproduction

func TestValidatedSmallRPCMessageSizePanics(t *testing.T) {
	cfg := &options.LogtailServerCfg{RpcMaxMessageSize: 1}
	cfg.Validate()
	require.Equal(t, int64(1), cfg.RpcMaxMessageSize)

	require.PanicsWithValue(t, "rpc max message size isn't enough", func() {
		_, _ = NewLogtailServer("127.0.0.1:0", cfg, nil, mockRuntime(), nil)
	})
}

Run:

go test ./pkg/vm/engine/tae/logtail/service \
  -run '^TestCodexValidatedSmallRPCMessageSizePanics$' -count=3 -v

Result: reproduced 3/3 on the latest main.

Production path

TN Config.Validate
  -> accepts every RpcMaxMessageSize > 0
TN create replica
  -> taestorage.NewTAEStorage
    -> service.NewLogtailServer
      -> NewLogtailServerSegmentPool(configured size)
      -> LeastEffectiveCapacity() <= 0
      -> panic("rpc max message size isn't enough")

The relevant validators only replace values <= 0; they do not enforce the minimum protocol-header size.

Expected behavior

Invalid positive values should be rejected by configuration validation with a descriptive error, or NewLogtailServer should return an error. User configuration must not cause a TN process panic during replica creation.

Actual behavior

The configuration is accepted and the TN panics later when the logtail server is constructed.

Suggested direction

• Define and validate a minimum RpcMaxMessageSize that can hold the maximum segment header plus at least one payload byte.
• Replace the constructor panic with a returned error as defense in depth.