[BUG]: Entity delta desync causes invalid beam model error

[0Ky]

What happened?

When connected to HLTV (live or demo playback from it), the GoldSrc client intermittently crashes during entity linking with:

Sys_Error: Bad model on beam ( not sprite )

^{* This issue does not reproduce in direct server connection, only when client connected to HLTV or playing back HLTV recorded demo.}

The root cause appears to be "entity state desynchronization in HLTV delta compression/decompression", causing a BEAM entity (ENTITY_BEAM) to reference a non-sprite model (e.g. models/v_tripmine.mdl).

Example observed state:

ent->prevstate.modelindex = 76
cl.model_precache[76].name = "sprites/laserbeam.spr"

ent->curstate.modelindex = 57
cl.model_precache[57].name = "models/v_tripmine.mdl"

ent->prevstate.entityType = 2 (ENTITY_BEAM)
ent->curstate.entityType = 2 (ENTITY_BEAM)
ent->baseline.entityType = 1 (ENTITY_NORMAL)

In this state, the entity appears to retain beam classification while its model changes from a sprite to a studio model. GoldSrc expects beam entities to reference sprite models, so this invalid combination triggers the Sys_Error on the client.

The gdb backtrace shows the crash manifesting in CL_LinkCustomEntity at ../engine/cl_ents.c:1422, but that's just where the invalid state finally gets checked and rejected, it's the symptom, not the actual cause. The bad data was assembled earlier, likely upstream in the delta/baseline reconstruction and simply carried forward through packet/entity state structures until the client validated the beam model and aborted.

The issue most likely lies within HLTV entity state handling, especially delta compression or decompression somewhere in:

ReHLDS/rehlds/HLTV/Core/src/World.cpp

Lines 1547 to 1556 in 0124d56

	// Insert new the entity to frame
	entity[newindex].entityType = custom ? ENTITY_BEAM : ENTITY_NORMAL;
	delta_t *delta = GetDeltaEncoder(newnum, custom);
	m_Delta.ParseDelta(stream, (byte *)&deltaEntity[oldindex], (byte *)&entity[newindex], delta);
	entity[newindex].number = newnum;
	newindex++;
	oldindex++;

Environment

[OS]
- Lubuntu 26.04 LTS

[HLDS Base]
- Protocol version 48
- Exe version 1.1.2.2/Stdio (valve)
- Exe build: 19:52:19 Aug  3 2020 (8684)

[ReHLDS]
- Protocol version 48
- Exe version 1.1.2.2/Stdio (valve)
- ReHLDS version: 3.15.0.898-dev
- Build date: 01:14:53 Jun 11 2026 (4443)
- Build from: https://github.com/rehlds/ReHLDS/commit/0124d56

Relevant log output

#0  0xf663364b in ?? () from target:/usr/lib/pressure-vessel/overrides/lib/i386-linux-gnu/libc.so.6
#1  0xf662682c in ?? () from target:/usr/lib/pressure-vessel/overrides/lib/i386-linux-gnu/libc.so.6
#2  0xf66268c9 in ?? () from target:/usr/lib/pressure-vessel/overrides/lib/i386-linux-gnu/libc.so.6
#3  0xf66a59b3 in __wait4_time64 () from target:/usr/lib/pressure-vessel/overrides/lib/i386-linux-gnu/libc.so.6
#4  0xf66a5b2a in waitpid () from target:/usr/lib/pressure-vessel/overrides/lib/i386-linux-gnu/libc.so.6
#5  0xf2eb111e in ?? () from target:/lib/i386-linux-gnu/libSDL2-2.0.so.0
#6  0xf2e85a66 in ?? () from target:/lib/i386-linux-gnu/libSDL2-2.0.so.0
#7  0xf2e85b7a in ?? () from target:/lib/i386-linux-gnu/libSDL2-2.0.so.0
#8  0xf34b6840 in Sys_Error (error=error@entry=0xf363544c "Bad model on beam ( not sprite ) ") at ../engine/sys_dll.c:582
#9  0xf3506fe5 in CL_LinkCustomEntity (state=0xa81ff40, ent=0xbec161f8) at ../engine/cl_ents.c:1422
#10 CL_LinkPacketEntities () at ../engine/cl_ents.c:1597
#11 0xf3508be3 in CL_EmitEntities () at ../engine/cl_ents.c:2348
#12 0xf348b038 in _Host_Frame (time=time@entry=3.09979987e-05) at ../engine/host.c:1468
#13 0xf348b2a8 in Host_Frame (time=3.09979987e-05, iState=1, stateInfo=stateInfo@entry=0xff89c5dc) at ../engine/host.c:1549
#14 0xf34bc404 in CEngine::Frame (this=0xf371ef40 <g_Engine>) at ../engine/sys_engine.cpp:245
#15 0xf34b9464 in RunListenServer (instance=0x0, basedir=0x804d120 <szBaseDir> "/home/user/.steam/debian-installation/steamapps/common/Half-Life", 
    cmdline=0x8ffeda0 "/home/user/.steam/debian-installation/steamapps/common/Half-Life/hl_linux -steam -dev -debug -noipx -nojoy -nohdmodels -noontop -noforcemparms -nofbo -novid -insecure", postRestartCmdLineArgs=0x804f260 <main::szNewCommandParams> "", 
    launcherFactory=0x80497f0 <CreateInterface(char const*, int*)>, filesystemFactory=0xf47bf450 <CreateInterface(char const*, int*)>)
    at ../engine/sys_dll2.cpp:968
#16 0x0804905b in main (argc=12, argv=0xff89c854) at ../launcher/launcher.cpp:469