pw_presubmit: Add a 'host_clang_fuzz' step

This CL adds three things:

* A 'pw_strict_host_clang_fuzz' toolchain based on
  `pw_toolchain_host_clang.fuzz`.
* A top-level `//:host_clang_fuzz` GN targets which builds
  `//:pigweed_defaults` with the above toolchain.
* A `gn_fuzz_build` presubmit step which builds the above target. This
  step is added to the `SECURITY` presubmit program.

As of this CL, this new step does not do much; it merely builds and runs
existing targets such as unit tests with additional fuzzer and ASan
instrumentation. Subsequent CLs will add FuzzTest and its dependencies
to allow `pw_fuzz_test`s to run in "unit test" mode, enabling bounded
fuzzing in presubmit.

Change-Id: I4f0786b49ebf9d9370277551af46033d3b01c1cf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/143150
Commit-Queue: Aaron Green <[email protected]>
Reviewed-by: Rob Mohr <[email protected]>

BUILD.gn

@@ -207,6 +207,10 @@ _build_pigweed_default_at_all_optimization_levels("host_clang") {
   toolchain_prefix = "$_internal_toolchains:pw_strict_host_clang_"
 }
 
+group("host_clang_fuzz") {
+  deps = [ ":pigweed_default($_internal_toolchains:pw_strict_host_clang_fuzz)" ]
+}
+
 # GCC is only supported for Windows. Pigweed doesn't yet provide a Windows
 # clang toolchain, and Pigweed does not provide gcc toolchains for macOS and
 # Linux.

pw_presubmit/py/pw_presubmit/pigweed_presubmit.py

@@ -381,6 +381,12 @@ def gn_arm_build(ctx: PresubmitContext):
     ninja_targets=_at_all_optimization_levels(f'host_{_HOST_COMPILER}'),
 )
 
+gn_fuzz_build = build.GnGenNinja(
+    name='gn_fuzz_build',
+    path_filter=_BUILD_FILE_FILTER,
+    ninja_targets=('host_clang_fuzz',),
+)
+
 gn_docs_build = build.GnGenNinja(name='gn_docs_build', ninja_targets=('docs',))
 
 gn_host_tools = build.GnGenNinja(
@@ -989,6 +995,7 @@ def owners_lint_checks(ctx: PresubmitContext):
     # keep-sorted: start
     gn_crypto_mbedtls_build,
     gn_crypto_micro_ecc_build,
+    gn_fuzz_build,
     gn_software_update_build,
     # keep-sorted: end
 )

targets/host/target_toolchains.gni

@@ -373,6 +373,18 @@ pw_internal_host_toolchains = [
       default_configs += _internal_clang_default_configs
     }
   },
+  {
+    name = "pw_strict_host_clang_fuzz"
+    _toolchain_base = pw_toolchain_host_clang.fuzz
+    forward_variables_from(_toolchain_base, "*", _excluded_members)
+    defaults = {
+      forward_variables_from(_toolchain_base.defaults, "*")
+      forward_variables_from(_host_common, "*")
+      forward_variables_from(_pigweed_internal, "*")
+      forward_variables_from(_os_specific_config, "*")
+      default_configs += _internal_clang_default_configs
+    }
+  },
   {
     name = "pw_strict_host_gcc_debug"
     _toolchain_base = pw_toolchain_host_gcc.debug