Add link-group-mappings subtarget for shared library file to generate…

… the summary for the specified link group.

Summary:
Currently we have a subtarget that prints dependencies of link groups:

```
christylee@devbig394:~/fbsource/fbcode(47c0c0319)$ buck2 build @//mode/opt-lg tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:main_bin[link-group-mappings] --show-output
Buck UI: https://www.internalfb.com/buck2/be198590-6945-4e63-8faa-bfe68322ec9a
Network: Up: 0B  Down: 0B
Jobs completed: 4. Time elapsed: 0.0s.
BUILD SUCCEEDED
fbcode//tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:main_bin[link-group-mappings] buck-out/v2/gen/fbcode/c6f87ea505222064/tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test/__main_bin__/main_bin.link_group_map.json

christylee@devbig394:~/fbsource(87742e131|remote/master)$ cat buck-out/v2/gen/fbcode/c6f87ea505222064/tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test/__main_bin__/main_bin.link_group_map.json | jq
{
  "NO_MATCH": [
    "fbcode//common/init:kill"
  ],
  "shared_lib_2": [
    "fbcode//tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:D"
  ],
  "shared_lib_1": [
    "fbcode//tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:A",
    "fbcode//third-party-buck/platform010/build/libgcc:stdc++-legacy",
    "fbcode//third-party-buck/platform010/build/libgcc:stdc++",
    "fbcode//third-party-buck/platform010/build/libgcc:gcc_s",
    "fbcode//third-party-buck/platform010/build/libgcc:atomic",
    "fbcode//third-party-buck/platform010/build/openmp:headers",
    "fbcode//third-party-buck/platform010/build/glibc:pthread",
    "fbcode//third-party-buck/platform010/build/glibc:m",
    "fbcode//third-party-buck/platform010/build/glibc:mvec",
    "fbcode//third-party-buck/platform010/build/glibc:c",
    "fbcode//tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:C"
  ]
}
```

This gets unwieldy when we have a sufficiently large number of link groups. This diff creates a subtarget for each link group shared library that prints the dependencies that compose of that shared library. We want something that looks like this:

```
buck2 build @//mode/opt-lg tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:main_bin[shared-libraries][libshared_lib_2.so][link-group-mappings]

{
  "shared_lib_2": [
    "fbcode//tools/build/test/cpp/link_groups/cpp_rule_link_group_basic_test:D"
  ],
}
```

Reviewed By: christycylee

Differential Revision: D61422475

fbshipit-source-id: 4bebf87b304b398177adfe5e473874c706c12315

cxx/cxx_executable.bzl

@@ -160,6 +160,7 @@ load(
  "PDB_SUB_TARGET",
  "get_dumpbin_providers",
  "get_pdb_providers",
+ "get_shared_library_name",
 )
 load(
  ":preprocessor.bzl",
@@ -580,34 +581,13 @@ def cxx_executable(ctx: AnalysisContext, impl_params: CxxRuleConstructorParams,
  for shlib in shared_libs
  if shlib.soname.is_str()
  }
- sub_targets["shared-libraries"] = [DefaultInfo(
- default_output = ctx.actions.write_json(
- binary.output.basename + ".shared-libraries.json",
- {
- "libraries": [
- "{}:{}[shared-libraries][{}]".format(ctx.label.path, ctx.label.name, soname)
- for soname in str_soname_shlibs
- ],
- "librariesdwp": [
- "{}:{}[shared-libraries][{}][dwp]".format(ctx.label.path, ctx.label.name, soname)
- for soname, shlib in str_soname_shlibs.items()
- if shlib.lib.dwp
- ],
- "rpathtree": ["{}:{}[rpath-tree]".format(ctx.label.path, ctx.label.name)] if shared_libs_symlink_tree else [],
- },
- ),
- sub_targets = {
- soname: [DefaultInfo(
- default_output = shlib.lib.output,
- sub_targets = {"dwp": [DefaultInfo(default_output = shlib.lib.dwp)]} if shlib.lib.dwp else {},
- )]
- for soname, shlib in str_soname_shlibs.items()
- },
- )]
 
+ readable_mappings = {}
+ soname_to_group_mappings = {}
  if link_group_mappings:
- readable_mappings = {}
  for node, group in link_group_mappings.items():
+ soname = get_shared_library_name(linker_info, group, True)
+ soname_to_group_mappings[soname] = group
  readable_mappings[group] = readable_mappings.get(group, []) + ["{}//{}:{}".format(node.cell, node.package, node.name)]
 
  sub_targets[LINK_GROUP_MAPPINGS_SUB_TARGET] = [DefaultInfo(
@@ -627,6 +607,43 @@ def cxx_executable(ctx: AnalysisContext, impl_params: CxxRuleConstructorParams,
  ),
  )]
 
+ shared_libraries_sub_targets = {}
+ for soname, shlib in str_soname_shlibs.items():
+ targets = {"dwp": [DefaultInfo(default_output = shlib.lib.dwp)]} if shlib.lib.dwp else {}
+
+ group = soname_to_group_mappings.get(soname)
+ if group in readable_mappings:
+ output_json_file = binary.output.basename + "." + group + LINK_GROUP_MAPPINGS_FILENAME_SUFFIX
+ targets[LINK_GROUP_MAPPINGS_SUB_TARGET] = [DefaultInfo(
+ default_output = ctx.actions.write_json(
+ output_json_file,
+ {group: readable_mappings[group]},
+ ),
+ )]
+ shared_libraries_sub_targets[soname] = [DefaultInfo(
+ default_output = shlib.lib.output,
+ sub_targets = targets,
+ )]
+
+ sub_targets["shared-libraries"] = [DefaultInfo(
+ default_output = ctx.actions.write_json(
+ binary.output.basename + ".shared-libraries.json",
+ {
+ "libraries": [
+ "{}:{}[shared-libraries][{}]".format(ctx.label.path, ctx.label.name, soname)
+ for soname in str_soname_shlibs
+ ],
+ "librariesdwp": [
+ "{}:{}[shared-libraries][{}][dwp]".format(ctx.label.path, ctx.label.name, soname)
+ for soname, shlib in str_soname_shlibs.items()
+ if shlib.lib.dwp
+ ],
+ "rpathtree": ["{}:{}[rpath-tree]".format(ctx.label.path, ctx.label.name)] if shared_libs_symlink_tree else [],
+ },
+ ),
+ sub_targets = shared_libraries_sub_targets,
+ )]
+
  # If we have some resources, write it to the resources JSON file and add
  # it and all resources to "runtime_files" so that we make to materialize
  # them with the final binary.