Make builds more hermetic by using prebuilt gcc toolchain

Previously the Rust builds were using the system gcc toolchain for host
builds.  This CL adds the -B flag to the compiler wrappers to pass in
the location of the prebuilt gcc toolchain.

Test: ./build.py
Change-Id: Ie53f16f2513d3335f00a25d205b3e93ce538070b

Author: chriswailes

config.py

@@ -66,7 +66,7 @@ def instantiate_template_file(template_path: Path, output_path: Path, make_exec:
         output_path.chmod(output_path.stat().st_mode | stat.S_IEXEC)
 
 
-def host_config(target: str, sysroot_flags: str) -> str:
+def host_config(target: str, toolchain_flags: str) -> str:
     cc_wrapper_name  = OUT_PATH_WRAPPERS / ('clang-%s' % target)
     cxx_wrapper_name = OUT_PATH_WRAPPERS / ('clang++-%s' % target)
 
@@ -76,15 +76,15 @@ def host_config(target: str, sysroot_flags: str) -> str:
         real_cc=CC_PATH,
         ld_option=LD_OPTIONS,
         target=target,
-        sysroot_flags=sysroot_flags)
+        toolchain_flags=toolchain_flags)
 
     instantiate_template_exec(
         HOST_CXX_WRAPPER_TEMPLATE,
         cxx_wrapper_name,
         real_cxx=CXX_PATH,
         ld_option=LD_OPTIONS,
         target=target,
-        sysroot_flags=sysroot_flags,
+        toolchain_flags=toolchain_flags,
         cxxstd=CXXSTD_PATH,
         cxx_linker_flags=CXX_LINKER_FLAGS)
 
@@ -116,18 +116,21 @@ def device_config(target: str) -> str:
 
 def configure():
     """Generates config.toml for the rustc build."""
-    sysroot = None
-    # Apple removed the normal sysroot at / on Mojave+, so we need
-    # to go hunt for it on OSX
-    # On pre-Mojave, this command will output the empty string.
+    host_toolchain_flags = None
+
     if build_platform.system() == 'darwin':
+        # Apple removed the normal sysroot at / on Mojave+, so we need
+        # to go hunt for it on OSX
+        # On pre-Mojave, this command will output the empty string.
         output = subprocess.check_output(
             ['xcrun', '--sdk', 'macosx', '--show-sdk-path'])
-        sysroot = output.rstrip().decode('utf-8')
-    host_sysroot_flags = ("--sysroot " + sysroot) if sysroot else ""
+        host_toolchain_flags = "--sysroot " + output.rstrip().decode('utf-8')
+    else:
+        # On Linux build hosts we need to set the path to the gcc toolchain.
+        host_toolchain_flags = "-B " + GCC_TOOLCHAIN_PATH.as_posix()
 
     host_configs = '\n'.join(
-        [host_config(target, host_sysroot_flags) for target in HOST_TARGETS])
+        [host_config(target, host_toolchain_flags) for target in HOST_TARGETS])
     device_configs = '\n'.join(
         [device_config(target) for target in DEVICE_TARGETS])
 

paths.py

@@ -16,9 +16,10 @@
 from pathlib import Path
 import build_platform
 
-STAGE0_RUST_VERSION = '1.55.0'
-CLANG_REVISION = 'r433403'
-CLANG_NAME: str = 'clang-{0}'.format(CLANG_REVISION)
+STAGE0_RUST_VERSION: str = '1.55.0'
+CLANG_REVISION:      str = 'r433403'
+CLANG_NAME:          str = 'clang-{0}'.format(CLANG_REVISION)
+GLIBC_VERSION:       str = '2.17-4.8'
 
 TOOLCHAIN_PATH:   Path = Path(__file__).parent.resolve()
 WORKSPACE_PATH:   Path = (TOOLCHAIN_PATH / '..' / '..').resolve()
@@ -33,12 +34,13 @@
 OUT_PATH_STDLIB_SRCS: Path = OUT_PATH_PACKAGE / 'src' / 'stdlibs'
 OUT_PATH_WRAPPERS:    Path = OUT_PATH / 'wrappers'
 
-LLVM_BUILD_PATH: Path = OUT_PATH_RUST_SOURCE / 'build' / 'x86_64-unknown-linux-gnu' / 'llvm' / 'build'
+LLVM_BUILD_PATH: Path = OUT_PATH_RUST_SOURCE / 'build' / build_platform.triple() / 'llvm' / 'build'
 
 PREBUILT_PATH:         Path = WORKSPACE_PATH / 'prebuilts'
 RUST_PREBUILT_PATH:    Path = PREBUILT_PATH / 'rust' / build_platform.prebuilt() / STAGE0_RUST_VERSION
 LLVM_PREBUILT_PATH:    Path = PREBUILT_PATH / 'clang' / 'host' / build_platform.prebuilt() / CLANG_NAME
 LLVM_CXX_RUNTIME_PATH: Path = LLVM_PREBUILT_PATH / 'lib64'
+GCC_TOOLCHAIN_PATH:    Path = PREBUILT_PATH / 'gcc' / build_platform.prebuilt() / 'host' / ('x86_64-linux-glibc' + GLIBC_VERSION)
 
 # We live at      prebuilts/rust/${BUILD_PLATFORM}/${VERSION}/bin
 # libc++ lives at prebuilts/clang/host/${BUILD_PLATFORM}

templates/host_cc_wrapper.template

@@ -1,2 +1,2 @@
 #!/bin/bash
-$real_cc $$* $ld_option --target=$target $sysroot_flags
+$real_cc $$* $ld_option --target=$target $toolchain_flags

templates/host_cxx_wrapper.template

@@ -1,2 +1,2 @@
 #!/bin/bash
-$real_cxx -I$cxxstd $$* $ld_option --target=$target $sysroot_flags $cxx_linker_flags -stdlib=libc++
+$real_cxx -I$cxxstd $$* $ld_option --target=$target $toolchain_flags $cxx_linker_flags -stdlib=libc++