cmd/link: -buildmode=c-shared exports many functions, not just //export functions [1.15 backport]

[gopherbot]

@networkimprov requested issue #30674 (fixed by 6f7b553 in Go 1.16) to be considered for backport to the next 1.15 minor release.

@gopherbot please backport to 1.15

buildmode=pie became the default in 1.15, so folks have to manually set buildmode=exe to work around this bug.

[networkimprov]

Filed in case fix for #40795 depends on this.

[dmitshur]

We've discussed this in a release meeting and it didn't seem like this is needed for #43592. CC @ianlancetaylor, @jeremyfaller.

@networkimprov If you still think that CL 262797 (fix for #30674) should be backported, please provide a rationale as described in the third paragraph of https://golang.org/wiki/MinorReleases. It seems to be about buildmode=c-shared, which doesn't match the current backport rationale.

[networkimprov]

You need to ask the authors of the fixes for this and #40795; somewhere I read this might be needed for its backport.

cc @qmuntal

[qmuntal]

e463c28 adds a hardcoded dummy variable tagged with __declspec(dllexport) and removes the -Wl,--export-all-symbols flag from the linker arguments. This tells mingw-w64 to only add the symbols tagged with __declspec(dllexport) into the DLL export table. The dummy variable is necessary because the default behavior of mingw-w64 is to export everything, even if -Wl,--export-all-symbols is not present, unless there is a symbol tagged with __declspec(dllexport).

What 6f7b553 does is add the __declspec(dllexport) tag to all methods that should be exported.

Backporting e463c28 without either backporting 6f7b553 or setting again -Wl,--export-all-symbols for c-shared means that the generated binary won't honor the //export comment and that will only export the dummy variable (_cgo_dummy_export).

[dmitshur]

Thank you for the rationale, it's very helpful.

We've discussed this candidate and agreed it is also okay to backport, so both e463c28 (#43592) and 6f7b553 (this issue) will be backported. Only Go 1.15 needs this backport.