macOS: use SDK's libCurl.

[MaddTheSane]

Use the SDK's provided libcurl instead of relying on an outdated stub library.

[mikedld]

We currently support macOS 10.10 and up while require latest(-ish) Xcode to build. Using libcurl stud library and headers from SDK installed with Xcode 11/12 could lead to unintended use of features added in later libcurl versions than the one bundled with macOS 10.10 which is obviously bad. Using older SDK means either using older Xcode (due to how SDKs are provided by Apple) or extracting that older SDK from older Xcode and using it with newer Xcode (which may work, but we don't really need the full SDK).

IMHO existing approach is good enough.

[MaddTheSane]

One problem: the stub libraries have no ARM64 section, so compiling for Apple Silicon fails.

[mikedld]

The fact that arm64 is only supported from macOS 11 doesn't mean that Transmission has to drop support for earlier macOS versions in its x86_64 builds (at least not yet, I would imagine).

[MaddTheSane]

Agreed.

I know that Apple modifies the zlib headers to mark availability (when they were "added" to macOS), emitting a warning and weak importing of functions when targeting an older version of macOS. Maybe Apple does the same for libCurl?

See also #1528: libcurl isn't the only stub library that Transmission uses.

[livings124]

Can we test if a warning is emitted? Was there any new functionality added since the 10.10 version that we can quickly just check with a test compile?

If there is a warning, I'd prefer to just use the OS's version (and even if not, I'm not fully opposed).

[livings124]

Can we test if a warning is emitted? Was there any new functionality added since the 10.10 version that we can quickly just check with a test compile?

If there is a warning, I'd prefer to just use the OS's version (and even if not, I'm not fully opposed).

@MaddTheSane ping on this

[MaddTheSane]

No warning is emitted.

[livings124]

Oh that's not ideal.

Even so, I don't see it as a hard no to migrating to the SDK's libcurl. We would just need to be more aggressive when new OSs are released.

[ckerr]

Oh that's not ideal.

Even so, I don't see it as a hard no to migrating to the SDK's libcurl. We would just need to be more aggressive when new OSs are released.

@livings124 so what do you want to do with this PR? 🙂

[livings124]

Oh that's not ideal.

Even so, I don't see it as a hard no to migrating to the SDK's libcurl. We would just need to be more aggressive when new OSs are released.

@livings124 so what do you want to do with this PR? 🙂

I'm alright with merging this. I think we'd catch any api changes soon after an os beta is released.

[mikedld]

I'm going to look into possibly setting up an agent to test on target/minimal macOS version, which would hopefully be enough once we have some more unit tests written.