Add remotely debuggable Swift document (bazelbuild#576)

Author: keith

README.md

@@ -107,6 +107,11 @@ bazel run @build_bazel_rules_swift//tools/dump_toolchains
 **Linux hosts:** At this time, Bazel uses whichever `swift` executable is
 encountered first on your `PATH`.
 
+## Supporting remote builds
+
+To make remote builds work correctly with debugging and general
+reproducibility see [this doc](doc/debuggable_remote_swift.md)
+
 ## Future Work
 
 - Support for building and linking to shared libraries (`.dylib`/`.so`) written

doc/debuggable_remote_swift.md

@@ -0,0 +1,112 @@
+# Debuggable Remotely Built Swift
+
+This is a guide to using remotely built Swift modules in local debug builds.
+
+At the time of writing, `lldb` depends on debugging options embedded in `.swiftmodule` files. These options include paths that are only valid on the build host. For local builds, this all just works, but for remote builds, it doesn't.
+
+The solution is two parts:
+
+1. Use `-no-serialize-debugging-options` globally, to prevent embedded paths
+2. Use `-serialize-debugging-options` locally in one empty module
+
+Globally disabling debugging options makes those `.swiftmodule`s usable on any machine. Locally enabling debugging options for one module provides lldb with enough info to make debugging work.
+
+An lldb bug has been filed here: https://bugs.swift.org/browse/SR-11485
+
+### Disable Debugging Options Globally
+
+To globally disable debugging options, use the `swift.cacheable_swiftmodules` feature in rules_swift. For example, your `.bazelrc` could look like this:
+
+```
+build --features=swift.cacheable_swiftmodules
+```
+
+What this does is ensure all modules are built explicitly with `-no-serialize-debugging-options`. It has to be explicit because `swiftc` enables `-serialize-debugging-options` in some cases.
+
+### Add Debug Build Config
+
+In a `BUILD` file - in this example, the root `BUILD` file, define the following [`config_setting`](https://docs.bazel.build/versions/master/be/general.html#config_setting). This will allow targets to conditionally depend on the locally built debugging module.
+
+```python
+config_setting(
+    name = "debug",
+    values = {
+        "compilation_mode": "dbg",
+    },
+)
+```
+
+### Define Local Debug Target
+
+In the `BUILD` file of your choice, define a `swift_library` with:
+
+* A single empty source file
+* Enables `-serialize-debugging-options`
+* Built locally, not remote - using the `no-remote` tag
+
+Here is one way to define the `BUILD` file, using a [`genrule`](https://docs.bazel.build/versions/master/be/general.html#genrule) to create the empty swift file.
+
+```python
+genrule(
+    name = "empty",
+    outs = ["empty.swift"],
+    cmd = "touch $(OUTS)",
+)
+
+swift_library(
+    name = "_LocalDebugOptions",
+    srcs = [":empty"],
+    copts = [
+        "-Xfrontend",
+        "-serialize-debugging-options",
+    ],
+    module_name = "_LocalDebugOptions",
+    tags = ["no-remote"],
+    visibility = ["//visibility:public"],
+)
+```
+
+### Update Top Level Test Targets
+
+Finally, for each top-level test target (`ios_unit_test`, `ios_ui_test`*, etc), conditionally add the local debugging module to the deps. This is done via the [debug config](#add-debug-build-config). In the past this also had to be done for `ios_application` targets but that has since been fixed in lldb.
+
+```python
+debug_deps = select({
+    "//:debug": ["//some/path:_LocalDebugOptions"],
+    "//conditions:default": [],
+})
+
+ios_unit_test(
+    name = "...",
+    deps = debug_deps + [
+        # ...
+    ],
+    # ...
+)
+```
+
+##### Note about `ios_unit_test`
+
+When using a test host, the debugging module must be added to the test host target only, not the unit test target. _However_, for tests without a test host, the debugging module must be added to the unit test target.
+
+### LLDB Settings
+
+Additional settings may be required, depending on your build setup. For example, an Xcode Run Script may look like:
+
+```
+echo "settings set target.sdk-path $SDKROOT"
+echo "settings set target.swift-framework-search-paths $FRAMEWORK_SEARCH_PATHS"
+```
+
+Other settings you can try customizing are:
+
+* `target.clang-module-search-paths`
+* `target.debug-file-search-paths`
+* `target.sdk-path`
+* `target.swift-extra-clang-flags`
+* `target.swift-framework-search-paths`
+* `target.swift-module-search-paths`
+* `target.use-all-compiler-flags`
+* `symbols.clang-modules-cache-path`
+
+These settings would be written to some project specific lldbinit file which you can include directly in Xcode's scheme.