GN: Don't link source sets into static libraries

tools/gn/ninja_binary_target_writer.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/ninja_binary_target_writer.h"
 
 #include <set>
 
 #include "base/strings/string_util.h"
 #include "tools/gn/config_values_extractors.h"
@@ skipping 408 matching lines @@
     std::vector<const Target*>* linkable_deps,
     std::vector<const Target*>* non_linkable_deps) const {
   // Only these types of outputs have libraries linked into them. Child deps of
   // static libraries get pushed up the dependency tree until one of these is
   // reached, and source sets don't link at all.
   bool can_link_libs =
       (target_->output_type() == Target::EXECUTABLE ||
        target_->output_type() == Target::SHARED_LIBRARY);
 
   if (dep->output_type() == Target::SOURCE_SET) {
-    if (target_->output_type() == Target::SOURCE_SET) {
-      // When a source set depends on another source set, add it as a data
-      // dependency so if the user says "ninja second_source_set" it will
-      // also compile the first (what you would expect) even though we'll
-      // never do anything with the first one's files.
-      non_linkable_deps->push_back(dep);
-    } else {
-      // Linking in a source set, copy its object files.
+    // Source sets have their object files linked into final targets (shared
+    // libraries and executables). Intermediate static libraries and other
+    // source sets just forward the dependency, otherwise the files in the
+    // source set can easily get linked more than once which will cause
+    // multiple definition errors.
+    //
+    // In the future, we may need a way to specify a "complete" static library
+    // for cases where you want a static library that includes all source sets
+    // (like if you're shipping that to customers to link against).
+    if (target_->output_type() != Target::SOURCE_SET &&
+        target_->output_type() != Target::STATIC_LIBRARY) {
+      // Linking in a source set to an executable or shared library, copy its
+      // object files.
       for (size_t i = 0; i < dep->sources().size(); i++) {
         SourceFileType input_file_type = GetSourceFileType(dep->sources()[i]);
         if (input_file_type != SOURCE_UNKNOWN &&
             input_file_type != SOURCE_H) {
           // Note we need to specify the target as the source_set target
           // itself, since this is used to prefix the object file name.
           extra_object_files->insert(helper_.GetOutputFileForSource(
               dep, dep->sources()[i], input_file_type));
         }
       }
@@ skipping 19 matching lines @@
     }
 
     // Data files.
     const std::vector<SourceFile>& data = target_->data();
     for (size_t i = 0; i < data.size(); i++) {
       out_ << " ";
       path_output_.WriteFile(out_, data[i]);
     }
   }
 }