Expose cc_binary.link_extra_lib label

Recently Bazel introduced support for C++ binaries linking against
//tools/cpp:link_extra_lib which typically just depends on the label flag
//tools/cpp:link_extra_libs.

For normal C++ binaries this is the intended behaviour. However, certain
special C++ binaries really do not want to be linked against any other targets
for a variety of reasons: conflicting dependencies, pure C binaries that should
not pull in C++ runtime libraries, or other very low level binaries that want
to remain untouched.

Our experience building many thousands of C++ targets shows that there are few
cases where opting out linking of extra libraries always makes sense. To
provide this flexibility, expose cc_binary.link_extra_lib, which can be set to
an empty library such as @bazel_tools//tools/cpp:empty_lib.

PiperOrigin-RevId: 548117556
Change-Id: Iaaa35ec9115031bfaa124ca2441aafe823145899

Author: melver

src/main/java/com/google/devtools/build/lib/bazel/rules/cpp/BazelCppRuleClasses.java

@@ -535,6 +535,23 @@ public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env)
                   .allowedFileTypes()
                   .allowedRuleClasses("cc_library"))
           .add(attr(":default_malloc", LABEL).value(CppRuleClasses.DEFAULT_MALLOC))
+          /*<!-- #BLAZE_RULE($cc_binary_base).ATTRIBUTE(link_extra_lib) -->
+          Control linking of extra libraries.
+          <p>
+            By default, C++ binaries are linked against <code>//tools/cpp:link_extra_lib</code>,
+            which by default depends on the label flag <code>//tools/cpp:link_extra_libs</code>.
+            Without setting the flag, this library is empty by default. Setting the label flag
+            allows linking optional dependencies, such as overrides for weak symbols, interceptors
+            for shared library functions, or special runtime libraries (for malloc replacements,
+            prefer <code>malloc</code> or <code>--custom_malloc</code>). Setting this attribute to
+            <code>None</code> disables this behaviour.
+          </p>
+          <!-- #END_BLAZE_RULE.ATTRIBUTE -->*/
+          .add(
+              attr("link_extra_lib", LABEL)
+                  .value(env.getToolsLabel("//tools/cpp:link_extra_lib"))
+                  .allowedFileTypes()
+                  .allowedRuleClasses("cc_library"))
           /*<!-- #BLAZE_RULE($cc_binary_base).ATTRIBUTE(stamp) -->
           Whether to encode build information into the binary. Possible values:
           <ul>

src/main/starlark/builtins_bzl/common/cc/cc_binary_attrs.bzl

@@ -70,7 +70,7 @@ cc_binary_attrs_with_aspects = {
         default = configuration_field(fragment = "cpp", name = "custom_malloc"),
         aspects = [graph_structure_aspect],
     ),
-    "_link_extra_lib": attr.label(
+    "link_extra_lib": attr.label(
         default = Label("@" + semantics.get_repo() + "//tools/cpp:link_extra_lib"),
         providers = [CcInfo],
         aspects = [graph_structure_aspect],
@@ -118,6 +118,6 @@ cc_binary_attrs_without_aspects["malloc"] = attr.label(
 cc_binary_attrs_without_aspects["_default_malloc"] = attr.label(
     default = configuration_field(fragment = "cpp", name = "custom_malloc"),
 )
-cc_binary_attrs_without_aspects["_link_extra_lib"] = attr.label(
+cc_binary_attrs_without_aspects["link_extra_lib"] = attr.label(
     default = Label("@" + semantics.get_repo() + "//tools/cpp:link_extra_lib"),
 )

src/main/starlark/builtins_bzl/common/cc/semantics.bzl

@@ -128,7 +128,7 @@ def _get_cc_runtimes(ctx, is_library):
     if is_library:
         return []
 
-    runtimes = [ctx.attr._link_extra_lib]
+    runtimes = [ctx.attr.link_extra_lib]
 
     if ctx.fragments.cpp.custom_malloc != None:
         runtimes.append(ctx.attr._default_malloc)

src/test/java/com/google/devtools/build/lib/packages/util/Crosstool.java

@@ -583,8 +583,8 @@ public void write() throws IOException {
                 "    name = 'interface_library_builder',",
                 "    srcs = ['build_interface_so'],",
                 ")",
-                // We add an empty :link_extra_lib target in case we need it.
-                "cc_library(name = 'link_extra_lib')",
+                // We add a :link_extra_lib target in case we need it.
+                "cc_library(name = 'link_extra_lib', srcs = ['linkextra.cc'])",
                 // We add an empty :malloc target in case we need it.
                 "cc_library(name = 'malloc')",
                 // Fake targets to get us through loading/analysis.

src/test/java/com/google/devtools/build/lib/query2/testutil/AbstractQueryTest.java

@@ -1023,7 +1023,13 @@ public void testNoImplicitDeps() throws Exception {
 
     // Implicit dependencies:
     String hostDepsExpr = helper.getToolsRepository() + "//tools/cpp:malloc";
-    hostDepsExpr += " + " + helper.getToolsRepository() + "//tools/cpp:link_extra_lib";
+    hostDepsExpr +=
+        " + "
+            + helper.getToolsRepository()
+            + "//tools/cpp:link_extra_lib"
+            + " + "
+            + helper.getToolsRepository()
+            + "//tools/cpp:linkextra.cc";
     if (!analysisMock.isThisBazel()) {
       hostDepsExpr += " + //tools/cpp:malloc.cc";
     }

src/test/java/com/google/devtools/build/lib/rules/cpp/CcBinaryThinLtoObjDirTest.java

@@ -82,7 +82,9 @@ private CppLinkAction getIndexAction(LtoBackendAction backendAction) throws Exce
   @Before
   public void createBasePkg() throws IOException {
     scratch.overwriteFile(
-        "base/BUILD", "cc_library(name = 'system_malloc', visibility = ['//visibility:public'])");
+        "base/BUILD",
+        "cc_library(name = 'system_malloc', visibility = ['//visibility:public'])",
+        "cc_library(name = 'empty_lib', visibility = ['//visibility:public'])");
   }
 
   public void createBuildFiles(String... extraCcBinaryParameters) throws Exception {
@@ -104,6 +106,7 @@ public void createBuildFiles(String... extraCcBinaryParameters) throws Exception
         "          srcs = ['binfile.cc', ],",
         "          deps = [ ':lib', ':tree', ':tree_2', 'tree_empty'], ",
         String.join("", extraCcBinaryParameters),
+        "          link_extra_lib = '//base:empty_lib', ",
         "          malloc = '//base:system_malloc')",
         "cc_library(name = 'lib',",
         "        srcs = ['libfile.cc'],",
@@ -200,13 +203,15 @@ public void createTestFiles(String extraTestParameters, String extraLibraryParam
         "    srcs = ['bin_test.cc', ],",
         "    deps = [ ':lib', ':tree', ], ",
         extraTestParameters,
+        "    link_extra_lib = '//base:empty_lib', ",
         "    malloc = '//base:system_malloc'",
         ")",
         "cc_test(",
         "    name = 'bin_test2',",
         "    srcs = ['bin_test2.cc', ],",
         "    deps = [ ':lib', ':tree', ], ",
         extraTestParameters,
+        "    link_extra_lib = '//base:empty_lib', ",
         "    malloc = '//base:system_malloc'",
         ")",
         "cc_library(",

src/test/java/com/google/devtools/build/lib/rules/cpp/CcBinaryThinLtoTest.java

@@ -81,7 +81,9 @@ private CppLinkAction getIndexAction(LtoBackendAction backendAction) throws Exce
   @Before
   public void createBasePkg() throws IOException {
     scratch.overwriteFile(
-        "base/BUILD", "cc_library(name = 'system_malloc', visibility = ['//visibility:public'])");
+        "base/BUILD",
+        "cc_library(name = 'system_malloc', visibility = ['//visibility:public'])",
+        "cc_library(name = 'empty_lib', visibility = ['//visibility:public'])");
   }
 
   public void createBuildFiles(String... extraCcBinaryParameters) throws Exception {
@@ -93,6 +95,7 @@ public void createBuildFiles(String... extraCcBinaryParameters) throws Exception
         "          srcs = ['binfile.cc', ],",
         "          deps = [ ':lib' ], ",
         String.join("", extraCcBinaryParameters),
+        "          link_extra_lib = '//base:empty_lib', ",
         "          malloc = '//base:system_malloc')",
         "cc_library(name = 'lib',",
         "        srcs = ['libfile.cc'],",
@@ -116,13 +119,15 @@ public void createTestFiles(String extraTestParameters, String extraLibraryParam
         "    srcs = ['bin_test.cc', ],",
         "    deps = [ ':lib' ], ",
         extraTestParameters,
+        "    link_extra_lib = '//base:empty_lib', ",
         "    malloc = '//base:system_malloc'",
         ")",
         "cc_test(",
         "    name = 'bin_test2',",
         "    srcs = ['bin_test2.cc', ],",
         "    deps = [ ':lib' ], ",
         extraTestParameters,
+        "    link_extra_lib = '//base:empty_lib', ",
         "    malloc = '//base:system_malloc'",
         ")",
         "cc_library(",

src/test/java/com/google/devtools/build/lib/rules/cpp/CcCommonTest.java

@@ -1275,4 +1275,30 @@ public void testNoCoptsDisabled() throws Exception {
         "This attribute was removed. See https://github.com/bazelbuild/bazel/issues/8706 for"
             + " details.");
   }
+
+  @Test
+  public void testLinkExtra() throws Exception {
+    ConfiguredTarget target =
+        scratchConfiguredTarget(
+            "mypackage",
+            "mybinary",
+            "cc_binary(name = 'mybinary',",
+            "          srcs = ['mybinary.cc'])");
+    List<String> artifactNames = baseArtifactNames(getLinkerInputs(target));
+    assertThat(artifactNames).contains("liblink_extra_lib.a");
+  }
+
+  @Test
+  public void testNoLinkExtra() throws Exception {
+    ConfiguredTarget target =
+        scratchConfiguredTarget(
+            "mypackage",
+            "mybinary",
+            "cc_library(name = 'empty_lib')",
+            "cc_binary(name = 'mybinary',",
+            "          srcs = ['mybinary.cc'],",
+            "          link_extra_lib = ':empty_lib')");
+    List<String> artifactNames = baseArtifactNames(getLinkerInputs(target));
+    assertThat(artifactNames).doesNotContain("liblink_extra_lib.a");
+  }
 }

src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java

@@ -2142,10 +2142,12 @@ public void testRpathIsNotAddedWhenThereAreNoSoDeps() throws Exception {
         "    srcs = ['malloc.cc'],",
         "    linkstatic = 1,",
         ")",
+        "cc_library(name = 'empty_lib')",
         "cc_binary(",
         "    name = 'main',",
         "    srcs = ['main.cc'],",
         "    malloc = ':malloc',",
+        "    link_extra_lib = ':empty_lib',",
         "    linkstatic = 0,",
         ")");
 
@@ -2225,11 +2227,13 @@ public void testRpathRootIsAddedEvenWithTransitionedDepsOnly() throws Exception
         "    srcs = ['malloc.cc'],",
         "    linkstatic = 1,",
         ")",
+        "cc_library(name = 'empty_lib')",
         "cc_binary(",
         "    name = 'main',",
         "    srcs = ['main.cc'],",
         "    linkstatic = 0,",
         "    malloc = ':malloc',",
+        "    link_extra_lib = ':empty_lib',",
         "    deps = ['dep1'],",
         ")",
         "",

src/test/java/com/google/devtools/build/lib/skyframe/rewinding/RewindingTestsHelper.java

@@ -2106,6 +2106,8 @@ final void runGeneratedHeaderRewound_lostInInputDiscovery(SpawnShim shim) throws
             "Extracting include lines from genheader/consumes.cc",
             "Extracting include lines from tools/cpp/malloc.cc",
             "Compiling tools/cpp/malloc.cc",
+            "Extracting include lines from tools/cpp/linkextra.cc",
+            "Compiling tools/cpp/linkextra.cc",
             String.format(
                 "Extracting include lines from %s-out/k8-fastbuild/bin/genheader/gen.h",
                 TestConstants.PRODUCT_NAME),
@@ -2170,6 +2172,8 @@ final void runGeneratedHeaderRewound_lostInActionExecution(SpawnShim shim) throw
             "Extracting include lines from genheader/consumes.cc",
             "Extracting include lines from tools/cpp/malloc.cc",
             "Compiling tools/cpp/malloc.cc",
+            "Extracting include lines from tools/cpp/linkextra.cc",
+            "Compiling tools/cpp/linkextra.cc",
             String.format(
                 "Extracting include lines from %s-out/k8-fastbuild/bin/genheader/gen.h",
                 TestConstants.PRODUCT_NAME),
@@ -2263,6 +2267,8 @@ final void runGeneratedTransitiveHeaderRewound_lostInInputDiscovery(SpawnShim sh
             "Extracting include lines from genheader/consumes.cc",
             "Extracting include lines from tools/cpp/malloc.cc",
             "Compiling tools/cpp/malloc.cc",
+            "Extracting include lines from tools/cpp/linkextra.cc",
+            "Compiling tools/cpp/linkextra.cc",
             "Compiling genheader/intermediate.cc",
             String.format(
                 "Extracting include lines from %s-out/k8-fastbuild/bin/genheader/gen.h",
@@ -2332,6 +2338,8 @@ final void runGeneratedTransitiveHeaderRewound_lostInActionExecution(SpawnShim s
             "Extracting include lines from genheader/intermediate.cc",
             "Extracting include lines from tools/cpp/malloc.cc",
             "Compiling tools/cpp/malloc.cc",
+            "Extracting include lines from tools/cpp/linkextra.cc",
+            "Compiling tools/cpp/linkextra.cc",
             "Extracting include lines from genheader/consumes.cc",
             "Compiling genheader/intermediate.cc",
             String.format(