libutp build issues

[jpalus]

What is the issue?

When updating to transmission 4.0.0 I've hit a few issues with libutp:

• trying to build embedded libutp with gcc 12.2.0 fails with:

/home/users/builder/rpm/BUILD/transmission-4.0.0/build/third-party/utp/src/utp/utp_internal.cpp:3394:21: error: this statement may fall through [-Werror=implicit-fallthrough=]

• if I disable libutp with -DENABLE_UTP=OFF I get:

CMake Error at libtransmission/CMakeLists.txt:267 (target_link_libraries):
  Target "transmission" links to:

    libutp::libutp

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

• trying to use system libutp from https://github.com/bittorrent/libutp gives another build failure:

/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc: In lambda function:
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc:733:40: error: 'struct utp_callback_arguments' has no member named 'u1'
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc: In lambda function:
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc:746:63: error: 'struct utp_callback_arguments' has no member named 'u1'
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc: In lambda function:
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/peer-io.cc:758:47: error: 'struct utp_callback_arguments' has no member named 'u1'
make[2]: *** [libtransmission/CMakeFiles/transmission.dir/build.make:429: libtransmission/CMakeFiles/transmission.dir/peer-io.cc.o] Error 1
make[2]: *** Waiting for unfinished jobs....
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/tr-utp.cc: In function 'uint64 {anonymous}::utp_callback(utp_callback_arguments*)':
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/tr-utp.cc:137:58: error: 'struct utp_callback_arguments' has no member named 'u1'
/home/users/builder/rpm/BUILD/transmission-4.0.0/libtransmission/tr-utp.cc:137:76: error: 'struct utp_callback_arguments' has no member named 'u2'

So it seems currently using system libutp is not possible (USE_SYSTEM_UTP), libutp is not optional (ENABLE_UTP) and build files for embedded libutp need tweaking so LIBUTP_ENABLE_WERROR does not get set to ON while keeping LIBUTP_STANDALONE_BUILD to ON since it's required for install target.

Which application of Transmission?

None

Which version of Transmission?

4.0.0

[ckerr]

tl;dr in terms of unblocking you I'd say go into third-party/libutp/CMakeLists.txt and just turn off the Werror option

-  option(LIBUTP_ENABLE_WERROR "${PROJECT_NAME}: Treat warnings as errors" ${LIBUTP_STANDALONE_BUILD})
+  option(LIBUTP_ENABLE_WERROR "${PROJECT_NAME}: Treat warnings as errors" OFF)

I see three possible workitems for Transmission here:

1. Your guess that the system libutp is incompatible is almost certainly correct. libutp has some semver-major changes but the upstream project doesn't do releases AFAIK and Transmission can't just run pkg-config to see what version is preinstalled. It's likely not detecting the version correctly; Transmission should probably sniff utp.h better and give a cleaner error message.

2. I don't understand why your build is enabling -Werror; it looks like somehow LIBUTP_STANDALONE_BUILD is set to ON which it shouldn't be unless you're building libutp in its own cmake directory. But anyway that CMakeLists.txt file comes from The Transmission project and I'm not sure why we ever enable -Werror when building for Transmission. (@mikedld might know?)

3. Maybe the WITH_UTP option to compile without UTP isn't needed anymore. It was added IIRC a decade ago when one distro had reservations about libutp but I think the library has proven itself safe since then. Also, the overwhelming majority of BitTorrent clients use UTP and so users who don't have it would be at a disadvantage.

For bookkeeping's sake I'm going to open separate tickets for those three issues, and I want to focus here in this ticket on getting it to build for you. @jpalus does disabling LIBUTP_STANDALONE_BUILD ubnlock you?

[jpalus]

it looks like somehow LIBUTP_STANDALONE_BUILD is set to ON which it shouldn't be unless you're building libutp in its own cmake directory.
does disabling LIBUTP_STANDALONE_BUILD ubnlock you?

Actually that was my very first attempt to drop the logic for setting LIBUTP_STANDALONE_BUILD to ON which results in yet another failure:

make[3]: *** No rule to make target 'install'.  Stop.
make[2]: *** [CMakeFiles/utp.dir/build.make:107: third-party/utp/src/utp-stamp/utp-install] Error 2

I guess that's because LIBUTP_STANDALONE_BUILD is used for LIBUTP_ENABLE_INSTALL as well:
https://github.com/transmission/libutp/blob/bf695bdfb047cdca9710ea9cffc4018669cf9548/CMakeLists.txt#L9

and the latter guards:
https://github.com/transmission/libutp/blob/bf695bdfb047cdca9710ea9cffc4018669cf9548/CMakeLists.txt#L92-L103

[ckerr]

OK. @mikedld wrote those and I'd rather give him a chance to weigh in instead of me second-guessing why that code is there.

In terms of just making this build for you, maybe remove the actual -Werror from add_compile_options() at https://github.com/transmission/libutp/blob/bf695bdfb047cdca9710ea9cffc4018669cf9548/CMakeLists.txt#L36 ?

[jpalus]

Just to be clear I was able to compile transmission 4.0.0 successfully after patching embedded libutp cmake file and forcing LIBUTP_ENABLE_WERROR to OFF.

[ckerr]

Awesome, I'm glad you got unblocked!

Since I've filed separate tickets for those workitems above (4750, 4751, 4752) I'm going to close this issue as completed. In particular 4752 would be the one for anyone else who hits this issue.