Test the just-built libraries when building unittests on Darwin

cmake/modules/AddSwiftUnittests.cmake

@@ -40,8 +40,13 @@ function(add_swift_unittest test_dirname)
   endif()
 
   if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+    # Add an @rpath to the swift library directory.
     set_target_properties(${test_dirname} PROPERTIES
       BUILD_RPATH ${SWIFT_LIBRARY_OUTPUT_INTDIR}/swift/macosx)
+    # Force all the swift libraries to be found via rpath.
+    add_custom_command(TARGET "${test_dirname}" POST_BUILD
+      COMMAND "${SWIFT_SOURCE_DIR}/utils/swift-rpathize.py"
+              "$<TARGET_FILE:${test_dirname}>")
   elseif("${SWIFT_HOST_VARIANT}" STREQUAL "android")
     swift_android_lib_for_arch(${SWIFT_HOST_VARIANT_ARCH} android_system_libs)
     set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_DIRECTORIES

utils/swift-rpathize.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python
+
+# On Darwin, dynamic libraries have an install name.  At link time, the
+# linker can work with a dylib anywhere in the filesystem, but it will
+# write the dylib's install name into the resulting image, and at load
+# time that dylib will normally be expected to be found at exactly that
+# path.  However, if the install name in an image begins with `@rpath`,
+# it will instead be searched for in the image's runtime search path
+# list.  That list may contain absolute paths, but it may also contain
+# paths beginning with `@executable_path` or `@loader_path`, meaning the
+# path containing the running executable or the image being loaded,
+# respectively.
+#
+# Many of Swift's dylibs are meant to be installed on the system, which
+# means they have install names like this:
+#   /usr/lib/swift/libswiftFoo.dylib
+# To support back-deployment, they also provide magic override symbols
+# ($ld$install_name) for all the OS versions preceding the addition of
+# of the library.  When the linker finds a dylib with a matching override
+# for the OS deployment target, it ignores the normal install name and
+# uses the override path in the linked image's load command.  Swift's
+# libraries use override paths that begin with `@rpath`, and Swift
+# builds images with a runtime search path list that starts with
+# /usr/lib/swift but then falls back on a path relative to the image;
+# thus, apps will use the system libraries if available but will
+# otherwise use fallback libraries.
+#
+# When we're working on Swift, we usually want to test the libraries
+# we just built rather than the system libraries.  There are two ways
+# to achieve that.  The first is to override dyld's runtime search path
+# with DYLD_LIBRARY_PATH; this will take precedence over even an
+# absolute install name.  The second is to make sure the dylibs are
+# loaded via an @rpath install name and then link the program with an
+# rpath that will use the just-built libraries.  Unfortunately, the
+# toolchain will ordinarily use an absolute install name instead of
+# an @rpath if the deployment target is old enough, subverting testing.
+#
+# This script looks for dependent dylibs with an absolute path in
+# /usr/lib/swift and changes them to use @rpath.
+
+import argparse
+import re
+import subprocess
+import sys
+
+
+def main(arguments):
+    parser = argparse.ArgumentParser(
+        description='Change absolute install names to use @rpath')
+    parser.add_argument('bin', help='the binary')
+
+    args = parser.parse_args(arguments)
+    rpathize(args.bin)
+
+
+def rpathize(filename):
+    dylibsOutput = subprocess.check_output(
+        ['xcrun', 'dyldinfo', '-dylibs', filename])
+
+    # The output from dyldinfo -dylibs is a line of header followed by one
+    # install name per line, indented with spaces.
+    dylib_regex = re.compile(
+        r"^\s*(?P<path>/usr/lib/swift/(?P<filename>.*\.dylib))\s*$")
+
+    # Build a command to invoke install_name_tool.
+    command = ['install_name_tool']
+    for line in dylibsOutput.splitlines():
+        match = dylib_regex.match(line)
+        if match:
+            command.append('-change')
+            command.append(match.group('path'))
+            command.append('@rpath/' + match.group('filename'))
+            continue
+
+    # Don't run the command if we didn't find any dylibs to change:
+    # it's invalid to invoke install_name_tool without any operations.
+    if len(command) == 1:
+        return
+
+    # The last argument is the filename to operate on.
+    command.append(filename)
+
+    subprocess.check_call(command)
+
+
+sys.exit(main(sys.argv[1:]))