bash: Add a toolchain for local Bash.

Bazel automatically detects the local Bash and
creates a custom toolchain rule for it.

Later, rules that use Bash will require this
toolchain and retrieve Bash's path from it instead
of relying on hardcoded paths or the
--shell_executable flag.

See bazelbuild#4319

Change-Id: Idd8242a20d202b1f5a56cddac95b625c6c08ede9

src/main/java/com/google/devtools/build/lib/bazel/rules/tools.WORKSPACE

@@ -1,3 +1,6 @@
 local_repository(name = "bazel_tools", path = __embedded_dir__ + "/embedded_tools")
 bind(name = "cc_toolchain", actual = "@bazel_tools//tools/cpp:default-toolchain")
 bind(name = "java_toolchain", actual = "%java_toolchain%")
+
+load("@bazel_tools//tools/bash:bash_def.bzl", "bash_repositories")
+bash_repositories()

tools/bash/BUILD

@@ -16,6 +16,10 @@ filegroup(
 
 filegroup(
     name = "embedded_tools",
-    srcs = ["//tools/bash/runfiles:embedded_tools"],
+    srcs = [
+        "BUILD.tools",
+        "bash_def.bzl",
+        "//tools/bash/runfiles:embedded_tools",
+    ],
     visibility = ["//tools:__pkg__"],
 )

tools/bash/BUILD.tools

@@ -0,0 +1,6 @@
+exports_files(["bash_def.bzl"])
+
+toolchain_type(
+    name = "bash_toolchain_type",
+    visibility = ["//visibility:public"],
+)

tools/bash/bash_def.bzl

@@ -0,0 +1,75 @@
+def _bash_toolchain_impl(ctx):
+  return [platform_common.ToolchainInfo(path = ctx.attr.path)]
+
+def _is_windows(repository_ctx):
+  return repository_ctx.os.name.startswith("windows")
+
+def _bash_config_impl(repository_ctx):
+  bash_path = repository_ctx.os.environ.get("BAZEL_SH")
+  if not bash_path:
+    if _is_windows(repository_ctx):
+      bash_path = repository_ctx.which("bash.exe")
+      if bash_path:
+        # When the Windows Subsystem for Linux is installed there's a bash.exe
+        # under %WINDIR%\system32\bash.exe that launches Ubuntu Bash which
+        # cannot run native Windows programs so it's not what we want.
+        windir = repository_ctx.os.environ.get("WINDIR")
+        if windir and bash_path.startswith(windir):
+          bash_path = None
+    else:
+      bash_path = repository_ctx.which("bash")
+
+  if not bash_path:
+    bash_path = ""
+
+  if bash_path and _is_windows(repository_ctx):
+    bash_path = bash_path.replace("\\", "/")
+
+  os_label = None
+  if _is_windows(repository_ctx):
+    os_label = "@bazel_tools//platforms:windows"
+  elif repository_ctx.os.name.startswith("linux"):
+    os_label = "@bazel_tools//platforms:linux"
+  elif repository_ctx.os.name.startswith("mac"):
+    os_label = "@bazel_tools//platforms:osx"
+  else:
+    fail("Unknown OS")
+
+  repository_ctx.file("BUILD", """
+load("@bazel_tools//tools/bash:bash_def.bzl", "bash_toolchain")
+
+bash_toolchain(
+    name = "local_bash",
+    path = "{bash_path}",
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "local_bash_toolchain",
+    exec_compatible_with = [
+        "@bazel_tools//platforms:x86_64",
+        "{os_label}",
+    ],
+    toolchain = ":local_bash",
+    toolchain_type = "@bazel_tools//tools/bash:bash_toolchain_type",
+)
+""".format(bash_path = bash_path, os_label = os_label))
+
+bash_toolchain = rule(
+    attrs = {"path": attr.string()},
+    implementation = _bash_toolchain_impl,
+)
+
+bash_config = repository_rule(
+    environ = [
+        "WINDIR",
+        "PATH",
+    ],
+    local = True,
+    implementation = _bash_config_impl,
+)
+
+def bash_repositories():
+  """Detect the local Bash and register it as a toolchain."""
+  bash_config(name = "local_config_bash")
+  native.register_toolchains("@local_config_bash//:local_bash_toolchain")