clang: Diag running out of file handles while looking for files

clang would only print "file not found" when it's unable to find a
header file.  If the reason for that is a file handle leak, that's not a
very useful error message.  For errors that aren't in a small whitelist
("file not found", "file is directory"), print an error with the
strerror() output.

This changes behavior in corner cases: If clang was out of file handles
while looking in one -I dir but then suddenly wasn't when looking in the
next -I dir, and both directories contained a file with the desired
name, previously we'd silently return the file from the second
directory. For this reason, it's important to ignore "is a directory"
for this new diag: if a file foo/foo exists and -I -Ifoo are passed, an
include of "foo" should successfully open file "foo" in directory "foo/"
instead of complaining that "./foo" is a directory.

No test since we mostly hit this when there's a handle leak somewhere,
and currently there isn't one. I manually tested this with the repro
steps in comment 2 on the bug below.

Fixes PR42524.

Differential Revision: https://reviews.llvm.org/D65956

llvm-svn: 368322

clang/lib/Lex/HeaderSearch.cpp

@@ -309,9 +309,18 @@ const FileEntry *HeaderSearch::getFileAndSuggestModule(
     ModuleMap::KnownHeader *SuggestedModule) {
   // If we have a module map that might map this header, load it and
   // check whether we'll have a suggestion for a module.
-  auto File = getFileMgr().getFile(FileName, /*OpenFile=*/true);
-  if (!File)
+  llvm::ErrorOr<const FileEntry *> File =
+      getFileMgr().getFile(FileName, /*OpenFile=*/true);
+  if (!File) {
+    // For rare, surprising errors (e.g. "out of file handles"), diag the EC
+    // message.
+    std::error_code EC = File.getError();
+    if (EC != std::errc::no_such_file_or_directory &&
+        EC != std::errc::is_a_directory) {
+      Diags.Report(IncludeLoc, diag::err_cannot_open_file) << EC.message();
+    }
     return nullptr;
+  }
 
   // If there is a module that corresponds to this header, suggest it.
   if (!findUsableModuleForHeader(*File, Dir ? Dir : (*File)->getDir(),