`R__memcompress` fails with limited stack size

[linev]

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Description

R__memcompress crashed from civetweb threads.
Main reason - size of struct bits_internal_state structure approximately 128K which is bigger than default stack size of civetweb thread (approx 100K).

Actually there is compile definition DYN_ALLOC which allows to allocate this memory dynamically,
but it is never used/defined. And even worse. If one enable such definition - allocated memory never will be released while R__lm_free() never called.

Proposed solution - implement R__memcompress_dynamic() with dynamically allocated struct bits_internal_state.
No change of API, no extra work - just new function with same functionality. And of course add everywhere R__lm_free() calls

Reproducer

Compile ROOT with modified flags for RHTTP library.

Add to net/http/CMakeLists.txt, line 75 following statement:

  target_compile_definitions(RHTTP PRIVATE -DUSE_STACK_SIZE=102400)

Compile root and run:

[shell] net/http/test/testHttpServer

Problem appears when civetweb compiled in #22731

ROOT version

master, all previous versions as well

Installation method

build from source

Operating system

OpenSUSE

Additional context

No response

[Roaimkhan]

@linev From the description, it looks like the stack overflow stems from bits_internal_state being allocated on the thread stack, which exceeds the default CivetWeb thread stack size. I'll investigate introducing a dynamic allocation path (without changing the public API) and ensure all corresponding R__lm_free() calls are handled correctly before opening a PR.

[linev]

@Roaimkhan

Looks like this functionality is no longer in production use in ROOT. So in my mind change to dynamic allocation of complete bits_internal_state structure should be most simple solution. And I see no reason to activate DYN_ALLOC defines for that - rather completely dismiss this code. But may be Philippe @pcanal has other ideas.