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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@364464 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 3 deletions

diff --git a/lib/Frontend/TestModuleFileExtension.cpp b/lib/Frontend/TestModuleFileExtension.cpp
index 561fe1012a..f8ed341934 100644
--- a/lib/Frontend/TestModuleFileExtension.cpp
+++ b/lib/Frontend/TestModuleFileExtension.cpp
@@ -48,7 +48,12 @@ TestModuleFileExtension::Reader::Reader(ModuleFileExtension *Ext,
   // Read the extension block.
   SmallVector<uint64_t, 4> Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    llvm::Expected<llvm::BitstreamEntry> MaybeEntry =
+        Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      (void)MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock:
     case llvm::BitstreamEntry::EndBlock:
@@ -61,8 +66,12 @@ TestModuleFileExtension::Reader::Reader(ModuleFileExtension *Ext,
 
     Record.clear();
     StringRef Blob;
-    unsigned RecCode = Stream.readRecord(Entry.ID, Record, &Blob);
-    switch (RecCode) {
+    Expected<unsigned> MaybeRecCode =
+        Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecCode)
+      fprintf(stderr, "Failed reading rec code: %s\n",
+              toString(MaybeRecCode.takeError()).c_str());
+    switch (MaybeRecCode.get()) {
     case FIRST_EXTENSION_RECORD_ID: {
       StringRef Message = Blob.substr(0, Record[0]);
       fprintf(stderr, "Read extension block message: %s\n",