[build] Add a new --cross-compile-build-swift-tools flag to disable cross-compiling the compiler

[finagolfin]

This is useful when building cross-compilation toolchains where you want the stdlib and corelibs cross-compiled but don't want the Swift compiler cross-compiled too with --cross-compile-hosts. This pull demonstrates using the new flag for the Android build presets.

I'm submitting this after discussing cross-compiling the corelibs on the Android CI with @drodriguez, as the easiest way to do so after #33724 is by using --cross-compile-hosts but we don't want to cross-compile the Swift compiler too.

To build this, I first had to unpack three Termux packages for Android, libandroid-spawn, libcurl, and libxml2 (I also unpacked libicu but the Android CI does that differently) as detailed here, because those libraries are dependencies of swift-corelibs-foundation.

I then passed that swift-android-aarch64-24-sdk path into this build-script invocation with the July 9 trunk source snapshot to build an Android cross-compilation toolchain that includes the corelibs:

./swift/utils/build-script -RA --android --android-ndk /home/butta/src/android-ndk-r21e/
--android-arch aarch64 --android-api-level 24
--android-icu-uc /home/butta/swift-android-aarch64-24-sdk/usr/lib/libicuuc.so
--android-icu-uc-include /home/butta/swift-android-aarch64-24-sdk/usr/include/
--android-icu-i18n /home/butta/swift-android-aarch64-24-sdk/usr/lib/libicui18n.so
--android-icu-i18n-include /home/butta/swift-android-aarch64-24-sdk/usr/include/
--android-icu-data /home/butta/swift-android-aarch64-24-sdk/usr/lib/libicudata.so
--xctest --cross-compile-hosts=android-aarch64 --install-swift --install-libdispatch
--install-foundation --install-xctest
--cross-compile-deps-path=/home/butta/swift-android-aarch64-24-sdk
--cross-compile-build-swift-tools=0 --llvm-ninja-targets-for-cross-compile-hosts=help

swift-corelibs-foundation recently added a dependency on some posix_spawn functions on Android, which as discussed on the forum isn't officially supported before Android API 28, so I linked against Termux's backported library with these additional patches:

diff --git a/swift/stdlib/public/Platform/CMakeLists.txt b/swift/stdlib/public/Platform/CMakeLists.txt
index a7a60a063d9..8894800509a 100644
--- a/swift/stdlib/public/Platform/CMakeLists.txt
+++ b/swift/stdlib/public/Platform/CMakeLists.txt
@@ -99,6 +99,7 @@ foreach(sdk ${SWIFT_SDKS})
         OUTPUT "${glibc_modulemap_out}"
         FLAGS
             "-DCMAKE_SDK=${sdk}"
+            "-DSDK_INCLUDE_PATH=${SWIFT_${sdk}_${arch}_ICU_UC_INCLUDE}"
             "-DGLIBC_INCLUDE_PATH=${SWIFT_SDK_${sdk}_ARCH_${arch}_LIBC_INCLUDE_DIRECTORY}"
             "-DGLIBC_ARCH_INCLUDE_PATH=${SWIFT_SDK_${sdk}_ARCH_${arch}_LIBC_ARCHITECTURE_INCLUDE_DIRECTORY}")
 
@@ -142,6 +143,7 @@ foreach(sdk ${SWIFT_SDKS})
         SOURCE "${glibc_modulemap_source}"
         OUTPUT "${glibc_sysroot_relative_modulemap_out}"
         FLAGS "-DCMAKE_SDK=${sdk}"
+              "-DSDK_INCLUDE_PATH=${SWIFT_${sdk}_${arch}_ICU_UC_INCLUDE}"
               "-DGLIBC_INCLUDE_PATH=${absolute_libc_include_path}"
               "-DGLIBC_ARCH_INCLUDE_PATH=${absolute_libc_arch_include_path}")
 
diff --git a/swift/stdlib/public/Platform/bionic.modulemap.gyb b/swift/stdlib/public/Platform/bionic.modulemap.gyb
index e44f9082653..09a92df50bb 100644
--- a/swift/stdlib/public/Platform/bionic.modulemap.gyb
+++ b/swift/stdlib/public/Platform/bionic.modulemap.gyb
@@ -185,7 +185,7 @@ module SwiftGlibc [system] {
       export *
     }
     module spawn {
-      header "${GLIBC_INCLUDE_PATH}/spawn.h"
+      header "${SDK_INCLUDE_PATH}/spawn.h"
       export *
     }
     module syslog {
diff --git a/swift-corelibs-foundation/Sources/Foundation/CMakeLists.txt b/swift-corelibs-foundation/Sources/Foundation/CMakeLists.txt
index 016bf294..1473f4dc 100644
--- a/swift-corelibs-foundation/Sources/Foundation/CMakeLists.txt
+++ b/swift-corelibs-foundation/Sources/Foundation/CMakeLists.txt
@@ -167,6 +167,10 @@ if(CMAKE_SYSTEM_NAME STREQUAL Windows)
     $<TARGET_OBJECTS:CoreFoundationResources>)
 elseif(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
   target_link_options(Foundation PRIVATE "SHELL:-no-toolchain-stdlib-rpath")
+  if(CMAKE_SYSTEM_NAME STREQUAL Android)
+    target_link_libraries(Foundation PRIVATE android-spawn)
+    target_link_directories(Foundation PUBLIC ${CMAKE_FIND_ROOT_PATH}/usr/lib)
+  endif()
 endif()
 
 

Alternatively, since the build only fails when linking plutil against libFoundation.a and we can't run the tests for the cross-compiled corelibs, we could hack out building a static plutil or revert the small pull adding that dependency, as done in vgorloff/swift-everywhere-toolchain#116.

@drodriguez, let me know what you think.

[finagolfin]

This changes the way --build-swift-tools works, where before it would control building both native and cross-compiled host tools, but now it only affects the native ones. Since very few people were likely using this flag, I think it's better to split the functionality up like this for cross-compiling the host tools, rather than keeping --build-swift-tools controlling both even after --cross-compile-build-swift-tools is added.

[drodriguez]

I will have to re-read and play around with the posix_spawn problems to understand them. In general, I would say that things should work with vanilla Android, without special libraries or headers.

For the time being, let's see if this works in macOS and Linux and we are not going to break another platform by mistake.

[drodriguez]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 90f4c15bf87f039691be43ee86057637245847b2

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 90f4c15bf87f039691be43ee86057637245847b2

[finagolfin]

I will have to re-read and play around with the posix_spawn problems to understand them. In general, I would say that things should work with vanilla Android, without special libraries or headers.

The problem is that swiftlang/swift-corelibs-foundation#2928 added a dependency on some posix_spawnattr_* functions that were not added to Android till API 28, whereas this preset builds against API 21. There are various ways we could remedy that, such as asking CoreFoundation if it found those APIs and only using them if available?

let's see if this works in macOS and Linux and we are not going to break another platform by mistake

A single failing test because I added a new path variable to the Android preset, didn't know I had to do something else for that, will fix it.

[finagolfin]

I added a line to fix the single failing test on the CI and fixed the preset arch issue we discussed above.

@drodriguez, I think this is ready for you to try on your local version of the Android CI. I think the only remaining issue will be adding library dependencies that the corelibs depend on to the Android CI, which will require the Termux packages I mentioned above for the Android corelibs and perhaps the native Ubuntu packages to build the native linux corelibs, if those Ubuntu packages like libcurl aren't already installed on the Android CI.

[drodriguez]

The problem is that apple/swift-corelibs-foundation#2928 added a dependency on some posix_spawnattr_* functions that were not added to Android till API 28, whereas this preset builds against API 21. There are various ways we could remedy that, such as asking CoreFoundation if it found those APIs and only using them if available?

I remember that code 😀. I don't think anyone will object if we switch (again) all the direct usages of posix_spawnattr_* into some _CFPosixSpawn* equivalents. The treatment of the attributes was not done, but as long as it is limited to just POSIX_SPAWN_SETPGROUP, maybe it is doable with setpgid(0, /* the pgroup value */) in _CFPosixSpawnImplPre28 after the comment // This is the child (probably before the actions are processed).

[drodriguez]

@swift-ci please test

[finagolfin]

Alright, I will look into implementing that CF polyfill on top of that posix_spawn wrapper you added a couple years ago and submit a pull for that.

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 26f0200b8d9f5723533cb763ec2392c39e3de6f9

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 26f0200b8d9f5723533cb763ec2392c39e3de6f9

[finagolfin]

Gah, playing whack-a-mole with the non-Android CI: I had missed that preset variables need that s at the end, actually ran the single failing test this time to make sure it works now.

[drodriguez]

@swift-ci please test

[finagolfin]

@swift-ci smoke test

[finagolfin]

I've stripped this down to just adding the single build flag, with no changes to any of the Android presets, so the Foundation considerations no longer apply.

@drodriguez, if you have any feedback on this final version, let me know.

[finagolfin]

@swift-ci smoke test macos

[finagolfin]

Merging since this won't break anything, just wondered if Daniel had any input, still listening if he comes up with anything later.