build: document why we check for `std::system` #32491

It's probably debatable if we support targets like iOS, but for now, document why we are checking for this standard library feature.

Trying to use std::system for a aarch64-darwin-ios target results in:

test.cpp:7:10: error: 'system' is unavailable: not available on iOS
    7 |     std::system("some_command");
      |          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_stdlib.h:203:6: note: 'system' has been explicitly marked unavailable here
  203 | int      system(const char *) __DARWIN_ALIAS_C(system);
      |          ^
1 error generated.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32491.

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ACK 8f4ba90b8ff47c7f90fe65d3ed37f486f9fe3a74

My understanding is that QML based QT will run fine on iOs, though I haven't tried myself.

Back when this was introduced #15457 (comment) it was observered that there's also more obscure platforms out there that don't have and/or sandbox std::system. Though nowadays we would probably direct them towards the kernel project.

it was observered that there's also more obscure platforms out there that don't have and/or sandbox std::system. @laanwj you might remember / know of such systems?

@laanwj you might remember / know of such systems?

Yes. At the time there was cloudabi, which was a sandboxed version of POSIX that removed all contextual access and ambient authority, so processes only get capability handles passed in to what they need. So there was no shell and hence no std::system. But that project was abandoned years ago.

i'm not sure any other such platforms exist right now, i'm fine with removing the check. But adding a comment makes sense nevertheless.

Aside: i would like to eventually convert usage of std::system to subprocess, especially now (after #32343) that it doesn't leak file descriptors anymore.

Aside: i would like to eventually convert usage of std::system to subprocess, especially now (after #32343) that it doesn't leak file descriptors anymore.

:+1:

i'm not sure any other such platforms exist right now, i'm fine with removing the check. But adding a comment makes sense nevertheless.

Yea. Wanting to remove the check is what led me to end up documenting it, given it's unclear wether we support these builds or not.