Add and use python bootstrap toolchain (#18)

Summary:
The toolchains currently have a cycle which causes them to hang, since the python toolchain depends on itself to initialise itself. This breaks that.

Pull Request resolved: #18

Reviewed By: ndmitchell

Differential Revision: D40020345

Pulled By: ndmitchell

fbshipit-source-id: f6f50791c8cb36bce73013cb81c43bd108f5f5e1

examples/prelude/.buckconfig

@@ -5,3 +5,8 @@ name=BUILD
 prelude = ./prelude
 toolchains = ./toolchains
 root = .
+platform = no
+buck = no
+fbcode = no
+fbsource = no
+ovr_config = no

examples/prelude/toolchain/BUILD

@@ -1,5 +1,5 @@
 load(":cxx_hacks.bzl", "cxx_hacks")
-load(":toolchain.bzl", "cxx_toolchain", "python_toolchain")
+load(":toolchain.bzl", "cxx_toolchain", "python_toolchain", "python_bootstrap_toolchain")
 
 # Required to support the $(cxx-header-tree) macro
 cxx_hacks(
@@ -21,7 +21,7 @@ python_toolchain(
 # the bootstrap toolchain is used
 # to poentially initialize other
 # toolchains
-python_toolchain(
+python_bootstrap_toolchain(
     name="python_bootstrap",
     visibility=["PUBLIC"],
-)
+)

examples/prelude/toolchain/toolchain.bzl

@@ -20,6 +20,10 @@ load(
     "PythonPlatformInfo",
     "PythonToolchainInfo",
 )
+load(
+    "@prelude//python_bootstrap:python_bootstrap.bzl",
+    "PythonBootstrapToolchainInfo",
+)
 
 DEFAULT_MAKE_COMP_DB = "prelude//cxx/tools:make_comp_db"
 DEFAULT_MAKE_PEX_INPLACE = "prelude//python/tools:make_pex_inplace"
@@ -130,3 +134,15 @@ python_toolchain = rule(
     },
     is_toolchain_rule = True,
 )
+
+def _python_bootstrap_toolchain(_ctx):
+    return [
+        DefaultInfo(),
+        PythonBootstrapToolchainInfo(interpreter = "python"),
+    ]
+
+python_bootstrap_toolchain = rule(
+    impl = _python_bootstrap_toolchain,
+    attrs = {},
+    is_toolchain_rule = True,
+)