cmd/go: remove -buildmode=shared (not c-shared)

[rsc]

-buildmode=shared has been broken since modules, and it is apparently unused.
See #47257 (comment).
Let's remove it.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[electricface]

I really want to use this buildmode=shared mode.

[scott-cotton]

@rsc how has it been broken since modules?

[ainar-g]

Just to clarify, this is not related to --buildmode=plugin, right?

[ianlancetaylor]

@electricface Why do you want to use it? Are you using it today?

@scott-cotton You can't use -buildmode=shared with a module and then use -linkshared to link against it from other modules. The whole idea of -buildmode=shared doesn't fit very well with modules. Each separate program can be linked against different versions of dependent modules, so if it did work people would in practice wind up with many different versions of the shared libraries of the same modules.

@ainar-g Correct, this is not about -buildmode=plugin, although plugins have their own, different, problems.

[electricface]

@ianlancetaylor
I am a debian-baseed Linux Desktop Environment developer. Our team have developed lot of daemon programs with golang. But these programs have significantly larger binary file size and use more ram than C or C++ programs. We currently working on these two problems. An idea is making some modules pluggable, which means only load modules that we need.

As other C or C++ programs are using shared-library, we are researching on some shared-library-like mechanism of golang. Inspired by buildmode=plugin and goloader, we found a way that bases on buildmode=shared to make modules pluggable recently, you can found a demo here. This solution has not been using in production environment, because of some problems hasn't been solved yet. I found out that programs built with -linkeshared can not be debugged using gdb and dlv.

After this commit, our "unoffical plugin" solution can work fine with go 1.14.9~1.16.7. Programs built in this way can run stably. But this won't work on go 1.17. Program will exit with a "fatal error: unreachable method called. linker bug?"

Plan of remove buildmode=shared make me feel bad.

[ianlancetaylor]

@electricface It sounds like you want plugins, which are supported (though not very well) by -buildmode=plugin. We aren't going to support -buildmode=shared just to support plugins, since we already have -buildmode=plugin.

[electricface]

@ianlancetaylor
I know that using static linking can eliminate the problems caused by the mismatched version of the dependent dynamic library.
But in the linux desktop system ecosystem, C or C++ languages are mostly used. These languages generally use dynamic libraries to save memory. The main reason for not using -buildmode=plugin is that static linking cannot save memory. For example, if the modules are divided into smaller ones and a lot of modules are added, each module occupies a relatively large amount of memory, and the overall occupancy will eventually become unacceptably large. Compared with C or C++ languages, they only use less memory to implement these functions, while the golang implementation uses several times the memory.
There is even an idea in our team that we must change the development language if we can't reduce the memory. I personally like to use golang for development, so I plan to study this part of the technology.

[rsc]

"I want to use it" is different from "I am using it today".
We believe that it is not possible to use it today, and furthermore it is difficult to provide.
We shouldn't advertise something that doesn't actually work.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[mewmew]

Just to confirm, this proposal suggests to remove --buildmode=shared while keeping --buildmode=c-shared? I am using --buildmode=c-shared for a number of projects and would be very sad to see it go.

[ianlancetaylor]

This proposal is only about -buildmode=shared.