Make the gn copy() tool honor dependencies.

tools/gn/ninja_copy_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_copy_target_writer.h"
 
 #include "base/strings/string_util.h"
 #include "tools/gn/ninja_utils.h"
 #include "tools/gn/output_file.h"
 #include "tools/gn/scheduler.h"
@@ skipping 53 matching lines @@
   const SubstitutionList& output_subst_list =
       target_->action_values().outputs();
   CHECK_EQ(1u, output_subst_list.list().size())
       << "Should have one entry exactly.";
   const SubstitutionPattern& output_subst = output_subst_list.list()[0];
 
   std::string tool_name =
       GetNinjaRulePrefixForToolchain(settings_) +
       Toolchain::ToolTypeToName(Toolchain::TYPE_COPY);
 
+  OutputFile input_dep =
+      WriteInputDepsStampAndGetDep(std::vector<const Target*>());
+
   // Note that we don't write implicit deps for copy steps. "copy" only
   // depends on the output files themselves, rather than having includes
   // (the possibility of generated #includes is the main reason for implicit
   // dependencies).
   //
   // It would seem that specifying implicit dependencies on the deps of the
   // copy command would still be harmeless. But Chrome implements copy tools
   // as hard links (much faster) which don't change the timestamp. If the
   // ninja rule looks like this:
   //   output: copy input | foo.stamp
   // The copy will not make a new timestamp on the output file, but the
   // foo.stamp file generated from a previous step will have a new timestamp.
   // The copy rule will therefore look out-of-date to Ninja and the rule will
   // get rebuilt.
   //
   // If this copy is copying a generated file, not listing the implicit
   // dependency will be fine as long as the input to the copy is properly
   // listed as the output from the step that generated it.
   //
   // Moreover, doing this assumes that the copy step is always a simple
   // locally run command, so there is no need for a toolchain dependency.
+  //
+  // Note that there is the need in some cases for order-only dependencies
+  // where a command might need to make sure something else runs before it runs
+  // to avoid conflicts. Such cases should be avoided where possible, but
+  // sometimes that's not possible.
   for (size_t i = 0; i < target_->sources().size(); i++) {
     const SourceFile& input_file = target_->sources()[i];
 
     OutputFile output_file =
         SubstitutionWriter::ApplyPatternToSourceAsOutputFile(
             target_->settings(), output_subst, input_file);
     output_files->push_back(output_file);
 
     out_ << "build ";
     path_output_.WriteFile(out_, output_file);
     out_ << ": " << tool_name << " ";
     path_output_.WriteFile(out_, input_file);
+    if (!input_dep.value().empty()) {
+      out_ << " || ";
+      path_output_.WriteFile(out_, input_dep);
+    }
     out_ << std::endl;
   }
 }