Add option to revert to dynamic stdlib

Author: vzsg

README.md

@@ -85,10 +85,37 @@ $ heroku config:set SWIFT_BUILD_CONFIGURATION=debug
 $ git commit -m "Change to debug configuration on Heroku" --allow-empty
 $ git push heroku master
 ...
-remote: -----> Building package (debug configuration)
+remote: -----> Building package (debug configuration, static stdlib)
 ...
 ```
 
+### Statically linked standard library
+
+Swift supports static linkage for its standard library and Foundation, which has two advantages:
+
+1. The resulting binary is more self-contained, and can run on other machines running the same version of the same distribution _without having to install the Swift toolchain._
+1. As only the relevant parts of the standard library/Foundation are copied into the binary, the resulting image will be smaller. A smaller image deploys faster too.
+
+The buildpack uses static linkage by default.
+
+However, if your project does not build, throwing errors about missing symbols for example, you can opt-in for the previous, dynamic linkage.
+
+Define a configuration variable called `SWIFT_DYNAMIC_STDLIB` with any non-empty value, then deploy again.
+
+```shell
+$ heroku config:set SWIFT_DYNAMIC_STDLIB=true
+$ git commit -m "Change to dynamic stdlib on Heroku" --allow-empty
+$ git push heroku master
+```
+
+To use static linkage again, _unset_ the configuration variable and deploy again.
+
+```shell
+$ heroku config:unset SWIFT_DYNAMIC_STDLIB
+$ git commit -m "Change to dynamic stdlib on Heroku" --allow-empty
+$ git push heroku master
+```
+
 ### Other build arguments
 
 If you want to pass extra flags to the `swift build` command, you can do so by setting the `SWIFT_BUILD_FLAGS` config variable.

bin/compile

@@ -36,6 +36,13 @@ if [ -f "$ENV_DIR/SWIFT_BUILD_FLAGS" ]; then
   SWIFT_BUILD_FLAGS=`cat "$ENV_DIR/SWIFT_BUILD_FLAGS"`
 fi
 
+if [ -f "$ENV_DIR/SWIFT_DYNAMIC_STDLIB" ]; then
+  SWIFT_DYNAMIC_STDLIB=`cat "$ENV_DIR/SWIFT_DYNAMIC_STDLIB" | tr -d '[[:space:]]'`
+  SWIFT_BACKTRACE_EXECUTABLE="swift-backtrace"
+else
+  SWIFT_BACKTRACE_EXECUTABLE="swift-backtrace-static"
+fi
+
 mkdir -p "$CACHE_DIR/$STACK"
 source "$BIN_DIR/steps/swiftenv"
 
@@ -55,4 +62,4 @@ mkdir -p $BUILD_DIR/.profile.d
 
 set-env PATH '$HOME/.swift-bin:$PATH'
 set-env LD_LIBRARY_PATH '$LD_LIBRARY_PATH:$HOME/.swift-lib'
-set-env SWIFT_BACKTRACE 'enable=yes,sanitize=yes,threads=all,images=all,interactive=no,output-to=stderr,symbolicate=fast,swift-backtrace=.swift-bin/swift-backtrace-static'
+set-env SWIFT_BACKTRACE "enable=yes,sanitize=yes,threads=all,images=all,interactive=no,output-to=stderr,symbolicate=fast,swift-backtrace=.swift-bin/$SWIFT_BACKTRACE_EXECUTABLE"

bin/steps/swift-build

@@ -1,2 +1,7 @@
-puts-step "Building package ($SWIFT_BUILD_CONFIGURATION configuration)"
-swift build $SWIFT_BUILD_FLAGS -c "$SWIFT_BUILD_CONFIGURATION" --static-swift-stdlib
+if [ ! -z "$SWIFT_DYNAMIC_STDLIB" ]; then
+  puts-step "Building package ($SWIFT_BUILD_CONFIGURATION configuration, dynamic stdlib)"
+  swift build $SWIFT_BUILD_FLAGS -c "$SWIFT_BUILD_CONFIGURATION"
+else
+  puts-step "Building package ($SWIFT_BUILD_CONFIGURATION configuration, static stdlib)"
+  swift build $SWIFT_BUILD_FLAGS -c "$SWIFT_BUILD_CONFIGURATION" --static-swift-stdlib
+fi

bin/steps/swift-install

@@ -5,10 +5,14 @@ puts-step "Installing dynamic libraries"
 mkdir -p $BUILD_DIR/.swift-lib
 find -L .build/$SWIFT_BUILD_CONFIGURATION -regex "$DYNAMIC_LIBRARY_REGEX" -type f -exec cp -a {} $BUILD_DIR/.swift-lib \; || true 2>/dev/null
 
+if [ ! -z "$SWIFT_DYNAMIC_STDLIB" ]; then
+  find -L $SWIFT_PREFIX/usr/lib/swift/linux -regex "$DYNAMIC_LIBRARY_REGEX" -type f -exec cp -a {} $BUILD_DIR/.swift-lib \; || true 2>/dev/null
+fi
+
 puts-step "Installing binaries"
 mkdir -p $BUILD_DIR/.swift-bin
 find -L .build/$SWIFT_BUILD_CONFIGURATION ! -regex "$DYNAMIC_LIBRARY_REGEX" -type f -perm /a+x -exec cp -a {} $BUILD_DIR/.swift-bin \; || true 2>/dev/null
-cp -a $SWIFT_PREFIX/usr/libexec/swift/linux/swift-backtrace-static $BUILD_DIR/.swift-bin
+cp -a $SWIFT_PREFIX/usr/libexec/swift/linux/$SWIFT_BACKTRACE_EXECUTABLE $BUILD_DIR/.swift-bin
 
 puts-step "Installing resources"
 find -L .build/$SWIFT_BUILD_CONFIGURATION/* -regex '.*\.resources$' -exec cp -a {} $BUILD_DIR/.swift-bin \; || true 2>/dev/null