BitStream reader: propagate errors

The bitstream reader handles errors poorly. This has two effects:

 * Bugs in file handling (especially modules) manifest as an "unexpected end of
   file" crash
 * Users of clang as a library end up aborting because the code unconditionally
   calls `report_fatal_error`

The bitstream reader should be more resilient and return Expected / Error as
soon as an error is encountered, not way late like it does now. This patch
starts doing so and adopting the error handling where I think it makes sense.
There's plenty more to do: this patch propagates errors to be minimally useful,
and follow-ups will propagate them further and improve diagnostics.

https://bugs.llvm.org/show_bug.cgi?id=42311
<rdar://problem/33159405>

Differential Revision: https://reviews.llvm.org/D63518

llvm-svn: 364464

1 files changed, 11 insertions, 5 deletions

diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index 2f4f5ef64c2..d724bbce374 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -924,7 +924,7 @@ failure:
   return false;
 }
 
-bool FrontendAction::Execute() {
+llvm::Error FrontendAction::Execute() {
   CompilerInstance &CI = getCompilerInstance();
 
   if (CI.hasFrontendTimer()) {
@@ -939,12 +939,18 @@ bool FrontendAction::Execute() {
       CI.hasPreprocessor()) {
     StringRef Cache =
         CI.getPreprocessor().getHeaderSearchInfo().getModuleCachePath();
-    if (!Cache.empty())
-      GlobalModuleIndex::writeIndex(CI.getFileManager(),
-                                    CI.getPCHContainerReader(), Cache);
+    if (!Cache.empty()) {
+      if (llvm::Error Err = GlobalModuleIndex::writeIndex(
+              CI.getFileManager(), CI.getPCHContainerReader(), Cache)) {
+        // FIXME this drops the error on the floor, but
+        // Index/pch-from-libclang.c seems to rely on dropping at least some of
+        // the error conditions!
+        consumeError(std::move(Err));
+      }
+    }
   }
 
-  return true;
+  return llvm::Error::success();
 }
 
 void FrontendAction::EndSourceFile() {