Building Swift fails on linux with module redefinition errors

[keith]

When building the Swift compiler on Linux with:

LDFLAGS="-Xlinker -z -Xlinker nostart-stop-gc" CC=/usr/bin/clang CXX=/usr/bin/clang++ ./swift/utils/build-script --release-debuginfo --sccache --llvm-max-parallel-lto-link-jobs 1 --llvm-cmake-options='-DLLVM_ENABLE_LLD=' --extra-cmake-options="-DSWIFT_USE_LINKER= -DLLVM_USE_LINKER=/home/ksmiley/.bin/ld.lld" --reconfigure --bootstrapping off

While also having swiftc 5.7.1 in my $PATH, I end up getting these build failures:

/opt/swift/swift-5.7.1/usr/lib/swift/dispatch/module.modulemap:8:8: error: redefinition of module 'DispatchIntrospection'
module DispatchIntrospection [system] [extern_c] {
       ^
/home/ksmiley/dev/oss-swift/swift-corelibs-libdispatch/dispatch/module.modulemap:8:8: note: previously defined here
module DispatchIntrospection [system] [extern_c] {
       ^
/opt/swift/swift-5.7.1/usr/lib/swift/dispatch/module.modulemap:13:8: error: redefinition of module 'CDispatch'
module CDispatch [system] [extern_c] {
       ^
/home/ksmiley/dev/oss-swift/swift-corelibs-libdispatch/dispatch/module.modulemap:13:8: note: previously defined here
module CDispatch [system] [extern_c] {
       ^
ninja: build stopped: subcommand failed.

Dispatch seems to be the first module that fails, but there are many errors with the same issue. Eventually I also see:

/home/ksmiley/dev/oss-swift/build/Ninja-RelWithDebInfoAssert/swift-linux-x86_64/lib/swift/host/SwiftDiagnostics.swiftmodule/x86_64-unknown-linux-gnu.private.swiftinterface:5:8: error: failed to build module 'SwiftSyntax' for importation due to the errors above; the textual interface may be broken by project issues or a compiler bug
import SwiftSyntax
       ^
/home/ksmiley/dev/oss-swift/build/Ninja-RelWithDebInfoAssert/swift-linux-x86_64/lib/swift/host/SwiftParser.swiftmodule/x86_64-unknown-linux-gnu.private.swiftinterface:5:8: error: failed to build module 'SwiftDiagnostics' for importation due to the errors above; the textual interface may be broken by project issues or a compiler bug
import SwiftDiagnostics
       ^
/opt/swift/swift-5.7.1/usr/lib/swift/dispatch/module.modulemap:1:8: error: redefinition of module 'Dispatch'
module Dispatch {
       ^
/home/ksmiley/dev/oss-swift/swift-corelibs-libdispatch/dispatch/module.modulemap:1:8: note: previously defined here
module Dispatch {
       ^

But I believe the message is correct that the issue is the other duplicate modules.

My impression was that building with an existing Swift compiler installation was expected (or maybe even required), but I am attempting to build without these in my $PATH assuming that if it still works that way it won't be able to find these modulemaps, so maybe I'll side step this.

Steps to reproduce

• Checkout 5b9aef91f8e96dcc82ac3828c18fa26473d17fe6 and friends
• Build with the command above

Environment

• Swift 5.7.1 release
• Ubuntu 20.04

[keith]

might have been fixed by passing -skip-early-swift-driver --skip-early-swiftsyntax from the docs (must have changed since my last build ~a month ago since I just re-ran my previous command)

[finagolfin]

I've seen this on Android too, with the prebuilt Swift toolchain package colliding with a trunk toolchain I'm building. The Linux CI doesn't have a prebuilt Swift toolchain, so it doesn't build those early Swift tools, but we'll need to fix this for when building the Swift compiler with a prebuilt Swift compiler becomes mandatory, so keep this open.

[redsun82]

I am just encountering this since the update to swift 5.8. Unfortunately I do require bootstrapping. Any workaround for this?

[finagolfin]

This has nothing to do with bootstrapping: it should work if you don't have a prebuilt toolchain in your path, as the linux CI doesn't, or if you pass the two build flags Keith mentioned.

[kateinoigakukun]

Just for record, dispatch/module.modulemap is no longer placed under source directory of swift-corelibs-libdispatch after https://github.com/apple/swift-corelibs-libdispatch/pull/785, so I just needed to clean the source directory by git clean -f to repair the build failure.

[finagolfin]

@keith, still seeing this or can we close? I haven't seen this in a while, and the linux CI builds with a prebuilt Swift 5.8 toolchain now.

[keith]

not sure