[ThinLTO] Also prune Thin-* files from the ThinLTO cache

[hans]

Summary

Such files (Thin-%%%%%%.tmp.o) are supposed to be deleted immediately
after they're used (either by renaming or deletion). However, we've seen
instances on Windows where this doesn't happen, probably due to the
filesystem being flaky. This is effectively a resource leak which has
prevented us from using the ThinLTO cache on Windows.

Since those temporary files are in the thinlto cache directory which we
prune periodically anyway, allowing them to be pruned too seems like a
tidy way to solve the problem.

[rnk]

Generally speaking, I think LLVM's temp file deletion isn't reliable on Windows. You can see this in your build directory, do find . -iname '*.obj.tmp', and that (at least for me) shows many files left by clang-cl.

lld/test/COFF/lto-cache.ll

 ; REQUIRES: x86
 ; NetBSD: noatime mounts currently inhibit 'touch' from updating atime
 ; UNSUPPORTED: system-netbsd
 
 ; RUN: opt -module-hash -module-summary %s -o %t.o
 ; RUN: opt -module-hash -module-summary %p/Inputs/lto-cache.ll -o %t2.o
 
 ; RUN: rm -Rf %t.cache && mkdir %t.cache
 ; Create two files that would be removed by cache pruning due to age.
-; We should only remove files matching the pattern "llvmcache-*".
-; RUN: touch -t 197001011200 %t.cache/llvmcache-foo %t.cache/foo
+; We should only remove files matching "llvmcache-*" or "Thin-*".
+; RUN: touch -t 197001011200 %t.cache/llvmcache-foo %t.cache/Thin-123.tmp.o %t.cache/foo
 ; RUN: lld-link /lldltocache:%t.cache /lldltocachepolicy:prune_after=1h /out:%t3 /entry:main %t2.o %t.o
 
 ; Two cached objects, plus a timestamp file and "foo", minus the file we removed.
 ; RUN: ls %t.cache | count 4
 
 target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-windows-msvc"
 
 define void @globalfunc() #0 {
 entry:
   ret void
 }

llvm/lib/Support/CachePruning.cpp

bool llvm::pruneCache(StringRef Path, CachePruningPolicy Policy) {
 
   // Walk the entire directory cache, looking for unused files.
   std::error_code EC;
   SmallString<128> CachePathNative;
   sys::path::native(Path, CachePathNative);
   // Walk all of the files within this directory.
   for (sys::fs::directory_iterator File(CachePathNative, EC), FileEnd;
        File != FileEnd && !EC; File.increment(EC)) {
-    // Ignore any files not beginning with the string "llvmcache-". This
+    // Ignore filenames not beginning with "llvmcache-" or "Thin-". This
     // includes the timestamp file as well as any files created by the user.
     // This acts as a safeguard against data loss if the user specifies the
     // wrong directory as their cache directory.
-    if (!sys::path::filename(File->path()).startswith("llvmcache-"))
+    StringRef filename = sys::path::filename(File->path());
+    if (!filename.startswith("llvmcache-") && !filename.startswith("Thin-"))
       continue;
 
     // Look at this file. If we can't stat it, there's nothing interesting
     // there.
     ErrorOr<sys::fs::basic_file_status> StatusOrErr = File->status();
     if (!StatusOrErr) {
       LLVM_DEBUG(dbgs() << "Ignore " << File->path() << " (can't stat)\n");
       continue;