[clangd] Avoid range-loop init-list lifetime subtleties.

[sammccall]

Summary

The original code appears to be OK per the spec, but we've had 3 reports of
crashes with certain unofficial builds of clangd that look a lot like old
compilers (GCC 5.4?) getting lifetime rules wrong.

Fixes https://github.com/clangd/clangd/issues/800

[kadircet]

thanks! it is unfortunate that history won't remember what an adventure it was to figure out the issue :D

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

292

nit: I'd make it static const just to make sure they are not going away this time + it will save up some stack space.

nit: Maybe also get rid of the struct's name ? (static const struct { ... } Files[] = { ... };)

[kadircet]

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

292

as discussed, CachedFile entries are actually pointers to members, so NVM the static.

[sammccall]

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

292

There are pointers to members in here! The fact that static const even compiles is a massive footgun :-(
We should write a clang-tidy check for that.

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

bool DirectoryBasedGlobalCompilationDatabase::DirectoryCache::load(
     llvm::vfs::FileSystem &FS) {
   dlog("Probing directory {0}", Path);
   std::string Error;
 
   // Load from the specially-supported compilation databases (JSON + Fixed).
   // For these, we know the files they read and cache their metadata so we can
   // cheaply validate whether they've changed, and hot-reload if they have.
   // (As a bonus, these are also VFS-clean)!
-  struct CDBFile {
+  struct {
     CachedFile *File;
     // Wrapper for {Fixed,JSON}CompilationDatabase::loadFromBuffer.
     llvm::function_ref<std::unique_ptr<tooling::CompilationDatabase>(
         PathRef,
         /*Data*/ llvm::StringRef,
         /*ErrorMsg*/ std::string &)>
         Parser;
+  } Files[] = {
+      {&CompileCommandsJson, parseJSON},
+      {&BuildCompileCommandsJson, parseJSON},
+      {&CompileFlagsTxt, parseFixed},
   };
-  for (const auto &Entry : {CDBFile{&CompileCommandsJson, parseJSON},
-                            CDBFile{&BuildCompileCommandsJson, parseJSON},
-                            CDBFile{&CompileFlagsTxt, parseFixed}}) {
+  for (const auto &Entry : Files) {
     bool Active = ActiveCachedFile == Entry.File;
     auto Loaded = Entry.File->load(FS, Active);
     switch (Loaded.Result) {
     case CachedFile::LoadResult::FileNotFound:
       if (Active) {
         log("Unloaded compilation database from {0}", Entry.File->Path);
         ActiveCachedFile = nullptr;
         CDB = nullptr;