[build-script] Use the build-script-impl from the swift repo that invoked build-script.

In the past if one wanted to be able to develop using multiple swift repos, one
was often stymied by build-script always using build-script-impl from the swift
directory. This includes the case where one invokes the build-script from a
hypothetical directory called swift-2.

Author: gottesmm

utils/SwiftBuildSupport.py

@@ -68,6 +68,26 @@ def _get_default_source_root():
     "SWIFT_BUILD_ROOT", os.path.join(SWIFT_SOURCE_ROOT, "build"))
 
 
+def _get_default_swift_repo_name():
+    result = ""
+
+    # Are we in a Swift checkout? Start from this file and check its parent
+    # directories.
+    #
+    # $SWIFT_SOURCE_ROOT/$SWIFT_REPO_NAME/utils/SwiftBuildSupport.py
+    (swift_path, parent_dirname) = os.path.split(os.path.dirname(__file__))
+    if parent_dirname != "utils":
+        return result
+    if not os.path.exists(os.path.join(swift_path, 'CMakeLists.txt')):
+        return result
+    return swift_path
+
+# Set SWIFT_REPO_NAME in your environment to control the name of the swift
+# directory name that is used.
+SWIFT_REPO_NAME = os.environ.get(
+    "SWIFT_REPO_NAME", _get_default_swift_repo_name())
+
+
 def _load_preset_files_impl(preset_file_names, substitutions={}):
     config = ConfigParser.SafeConfigParser(substitutions, allow_no_value=True)
     if config.read(preset_file_names) == []:

utils/build-script

@@ -30,6 +30,7 @@ sys.path.append(os.path.dirname(__file__))
 from SwiftBuildSupport import (
     HOME,
     SWIFT_BUILD_ROOT,
+    SWIFT_REPO_NAME,
     SWIFT_SOURCE_ROOT,
     get_all_preset_names,
     get_preset_options,
@@ -54,7 +55,7 @@ import swift_build_support.workspace    # noqa (E402)
 
 
 build_script_impl = os.path.join(
-    SWIFT_SOURCE_ROOT, "swift", "utils", "build-script-impl")
+    SWIFT_SOURCE_ROOT, SWIFT_REPO_NAME, "utils", "build-script-impl")
 
 
 def exit_rejecting_arguments(message, parser=None):
@@ -1096,7 +1097,8 @@ def main_preset():
         args.preset_file_names = [
             os.path.join(HOME, ".swift-build-presets"),
             os.path.join(
-                SWIFT_SOURCE_ROOT, "swift", "utils", "build-presets.ini")
+                SWIFT_SOURCE_ROOT, SWIFT_REPO_NAME, "utils",
+                "build-presets.ini")
         ]
 
     if args.show_presets: