Stop input and output of cc_library from clobbering each other.

Before this change:
Any given cc_library can only contribute one library with a given name to
targets which depend on it. If an input library has the same name as the
cc_library which it is an input to, the decision of which to use is based
on the link mode. e.g.,

cc_library(
    name = "foo",
    srcs = ["foo.c", "libfoo.so"],
)

will only contribute libfoo.a (a static library containing foo.o) in static mode,
while it will only contribute libfoo.so (the precompiled shared library) in dynamic
mode.

This change alters cc_library's behavior in this case:
* If libfoo.a would be empty (i.e., there are no linkable sources), then
this is allowed. The libfoo.so from srcs is simply passed through. (Previously,
the empty libfoo.a would be forwarded.)
* Otherwise, this is an error.

In the case where there are multiple libraries in the srcs with the same
library identifier (lib[name].[a|so|lo]), cc_library will still choose one
based on the link mode. This behavior has not changed.

Similarly, cc_library will still choose one of its own outputs based on the
link mode. That behavior has not changed either.

RELNOTES[INC]: It is now an error to include a precompiled library (.a, .lo, .so)
in a cc_library which would generate a library with the same name
(e.g., libfoo.so in cc_library foo) if that library also contains other linkable
sources.

--
MOS_MIGRATED_REVID=127569615

2 files changed, 62 insertions, 7 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLibraryHelper.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLibraryHelper.java
index d6b6a2b2f1..c43404e3b9 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLibraryHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLibraryHelper.java
@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.rules.cpp;
 
 import com.google.common.base.Function;
+import com.google.common.base.Joiner;
 import com.google.common.base.Predicates;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
@@ -913,14 +914,50 @@ public final class CcLibraryHelper {
     CcLinkingOutputs originalLinkingOutputs = ccLinkingOutputs;
     if (!(
         staticLibraries.isEmpty() && picStaticLibraries.isEmpty() && dynamicLibraries.isEmpty())) {
+
+      CcLinkingOutputs.Builder newOutputsBuilder = new CcLinkingOutputs.Builder();
+      if (!ccOutputs.isEmpty()) {
+        // Add the linked outputs of this rule iff we had anything to put in them, but then
+        // make sure we're not colliding with some library added from the inputs.
+        newOutputsBuilder.merge(originalLinkingOutputs);
+        Iterable<LibraryToLink> allLibraries =
+            Iterables.concat(staticLibraries, picStaticLibraries, dynamicLibraries);
+        for (LibraryToLink precompiledLibrary : allLibraries) {
+          List<LibraryToLink> matchingLibs =
+              originalLinkingOutputs.getLibrariesWithSameIdentifierAs(precompiledLibrary);
+          if (!matchingLibs.isEmpty()) {
+            Iterable<String> matchingLibArtifactNames =
+                Iterables.transform(
+                    matchingLibs,
+                    new Function<LibraryToLink, String>() {
+                      @Override
+                      public String apply(LibraryToLink input) {
+                        return input.getOriginalLibraryArtifact().getFilename();
+                      }
+                    });
+            ruleContext.ruleError(
+                "Can't put "
+                    + precompiledLibrary.getArtifact().getFilename()
+                    + " into the srcs of a "
+                    + ruleContext.getRuleClassNameForLogging()
+                    + " with the same name ("
+                    + ruleContext.getRule().getName()
+                    + ") which also contains other code or objects to link; it shares a name with "
+                    + Joiner.on(", ").join(matchingLibArtifactNames)
+                    + " (output compiled and linked from the non-library sources of this rule), "
+                    + "which could cause confusion");
+          }
+        }
+      }
+
       // Merge the pre-compiled libraries (static & dynamic) into the linker outputs.
-      ccLinkingOutputs = new CcLinkingOutputs.Builder()
-          .merge(ccLinkingOutputs)
-          .addStaticLibraries(staticLibraries)
-          .addPicStaticLibraries(picStaticLibraries)
-          .addDynamicLibraries(dynamicLibraries)
-          .addExecutionDynamicLibraries(dynamicLibraries)
-          .build();
+      ccLinkingOutputs =
+          newOutputsBuilder
+              .addStaticLibraries(staticLibraries)
+              .addPicStaticLibraries(picStaticLibraries)
+              .addDynamicLibraries(dynamicLibraries)
+              .addExecutionDynamicLibraries(dynamicLibraries)
+              .build();
     }
 
     DwoArtifactsCollector dwoArtifacts =
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLinkingOutputs.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLinkingOutputs.java
index 9d2ef270cf..aab459989c 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLinkingOutputs.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcLinkingOutputs.java
@@ -69,6 +69,24 @@ public class CcLinkingOutputs {
   }
 
   /**
+   * Returns all libraries in this CcLinkingOutputs with the same library identifier - i.e., those
+   * which would be considered different forms of the same library by getPreferredLibrary.
+   */
+  public List<LibraryToLink> getLibrariesWithSameIdentifierAs(LibraryToLink input) {
+    Iterable<LibraryToLink> allLibraries =
+        Iterables.concat(
+            staticLibraries, picStaticLibraries, dynamicLibraries, executionDynamicLibraries);
+    ImmutableList.Builder<LibraryToLink> result = new ImmutableList.Builder<>();
+    for (LibraryToLink library : allLibraries) {
+      if (libraryIdentifierOf(library.getOriginalLibraryArtifact())
+          .equals(libraryIdentifierOf(input.getOriginalLibraryArtifact()))) {
+        result.add(library);
+      }
+    }
+    return result.build();
+  }
+
+  /**
    * Add the ".a", ".pic.a" and/or ".so" files in appropriate order of preference depending on the
    * link preferences.
    *