[ThinLTO/gold] Avoid race with cache pruner by copying to temp files

[tejohnson]

Summary

This will avoid the race condition described in the review for D37993.

I believe that the Path parameter to AddBufferFn is no longer utilized.
I would prefer to remove that as a follow up clean up patch to reduce
the diffs in this patch.

[pcc]

LGTM

test/tools/gold/X86/cache.ll

 ; RUN:     --plugin-opt=cache-policy=cache_size_bytes=128k:prune_interval=0s \
 ; RUN:     -o %t3.o %t2.o %t.o
 ; RUN: ls %t.cache | count 5
 
 
 ; This should remove it.
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=save-temps \
 ; RUN:     --plugin-opt=cache-dir=%t.cache \
 ; RUN:     --plugin-opt=cache-policy=cache_size_bytes=32k:prune_interval=0s \
-; RUN:     -o %t3.o %t2.o %t.o
+; RUN:     -o %t4.o %t2.o %t.o
 ; RUN: ls %t.cache | count 4
+; With save-temps we can confirm that the cached files were copied into temp
+; files to avoid a race condition with the cached files being pruned, since the
+; gold plugin-api only accepts native objects passed back as files.
+; RUN: ls %t4.o.o1
+; RUN: ls %t4.o.o2
 
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define void @globalfunc() #0 {
 entry:
   ret void
 }

tools/gold/gold-plugin.cpp

auto AddStream =
     int FD = getOutputFileName(Filename, /*TempOutFile=*/!SaveTemps,
                                Filenames[Task], Task);
     return llvm::make_unique<lto::NativeObjectStream>(
         llvm::make_unique<llvm::raw_fd_ostream>(FD, true));
   };
 
   auto AddBuffer = [&](size_t Task, std::unique_ptr<MemoryBuffer> MB,
                        StringRef Path) {
-    // Note that this requires that the memory buffers provided to AddBuffer are
-    // backed by a file.
-    Filenames[Task] = Path;
+    *AddStream(Task)->OS << MB->getBuffer();
   };
 
   NativeObjectCache Cache;
   if (!options::cache_dir.empty())
     Cache = check(localCache(options::cache_dir, AddBuffer));
 
   check(Lto->run(AddStream, Cache));