Pass absolute path to gold when using Clang

Prior to this change Bazel's C++ toolchain autoconfiguration generated
toolchain that passed `-fuse-ld=gold` when gold linker was detected.
After this change this will be `-fuse-ld=/usr/bin/ld.gold` or similar
depending on what clang reported.

Gcc will still use -fuse-ld=gold (as it doesn't accept absolute
path argument to `-fuse-ld`).

This is to make the Clang autogenerated toolchain not depend on PATH if
we can help it. Gcc is still leaking the PATH.

The leak:

1) CC is installed in a non-standard location, or is checked-in into the
   workspace.
2) CC is passed to Bazel's cc_configure, Bazel autoconfigures the
   toolchain using CC.
3) Remote worker has CC installed in the same non-standard location, or
   gets CC in action inputs.
4) If Host system has gold on its PATH (potentially to the surprise
   of the user, user might assume that since they provide CC explicitly
   only that installation is used), Bazel will autoconfigure the
   toolchain to use gold linker.
5) Remote system doesn't have gold installed, build fails.

Alternative solutions:
* Pass -B$(dirname $(which gcc)) to CC and pass empty PATH to all
  Bazel's C++ autoconfiguration commands
* Do not detect gold automatically at all, require environment variable
  to be explicitly set to enable gold

Both are backwards incompatible changes. Solution in this PR is
backwards compatible. It's also not fixing the issue for Gcc.

Closes #8580.

PiperOrigin-RevId: 256623542

tools/cpp/unix_cc_configure.bzl

@@ -167,21 +167,51 @@ def _is_linker_option_supported(repository_ctx, cc, option, pattern):
     ])
     return result.stderr.find(pattern) == -1
 
-def _is_gold_supported(repository_ctx, cc):
-    """Checks that `gold` is supported by the C compiler."""
+def _find_gold_linker_path(repository_ctx, cc):
+    """Checks if `gold` is supported by the C compiler.
+
+    Args:
+      repository_ctx: repository_ctx.
+      cc: path to the C compiler.
+
+    Returns:
+      String to put as value to -fuse-ld= flag, or None if gold couldn't be found.
+    """
     result = repository_ctx.execute([
         cc,
-        "-fuse-ld=gold",
+        str(repository_ctx.path("tools/cpp/empty.cc")),
         "-o",
         "/dev/null",
         # Some macos clang versions don't fail when setting -fuse-ld=gold, adding
         # these lines to force it to. This also means that we will not detect
         # gold when only a very old (year 2010 and older) is present.
         "-Wl,--start-lib",
         "-Wl,--end-lib",
-        str(repository_ctx.path("tools/cpp/empty.cc")),
+        "-fuse-ld=gold",
+        "-v",
     ])
-    return result.return_code == 0
+    if result.return_code != 0:
+        return None
+
+    for line in result.stderr.splitlines():
+        if line.find("gold") == -1:
+            continue
+        for flag in line.split(" "):
+            if flag.find("gold") == -1:
+                continue
+
+            # flag is '-fuse-ld=gold' for GCC or "/usr/lib/ld.gold" for Clang
+            # strip space, single quote, and double quotes
+            flag = flag.strip(" \"'")
+
+            # remove -fuse-ld= from GCC output so we have only the flag value part
+            flag = flag.replace("-fuse-ld=", "")
+            return flag
+    auto_configure_warning(
+        "CC with -fuse-ld=gold returned 0, but its -v output " +
+        "didn't contain 'gold', falling back to the default linker.",
+    )
+    return None
 
 def _add_compiler_option_if_supported(repository_ctx, cc, option):
     """Returns `[option]` if supported, `[]` otherwise. Doesn't %-escape the option."""
@@ -348,7 +378,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
         "",
         False,
     ), ":")
-    supports_gold_linker = _is_gold_supported(repository_ctx, cc)
+    gold_linker_path = _find_gold_linker_path(repository_ctx, cc)
     cc_path = repository_ctx.path(cc)
     if not str(cc_path).startswith(str(repository_ctx.path(".")) + "/"):
         # cc is outside the repository, set -B
@@ -457,7 +487,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
             ),
             "%{cxx_flags}": get_starlark_list(cxx_opts + _escaped_cplus_include_paths(repository_ctx)),
             "%{link_flags}": get_starlark_list((
-                ["-fuse-ld=gold"] if supports_gold_linker else []
+                ["-fuse-ld=" + gold_linker_path] if gold_linker_path else []
             ) + _add_linker_option_if_supported(
                 repository_ctx,
                 cc,
@@ -532,6 +562,6 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
             "%{dbg_compile_flags}": get_starlark_list(["-g"]),
             "%{coverage_compile_flags}": coverage_compile_flags,
             "%{coverage_link_flags}": coverage_link_flags,
-            "%{supports_start_end_lib}": "True" if supports_gold_linker else "False",
+            "%{supports_start_end_lib}": "True" if gold_linker_path else "False",
         },
     )