[Frontend] Verify that the bitstream is not empty before reading

the serialised diagnostics

Clang should avoid calling report_fatal_error when the file with the serialised
diagnostics is empty. This commit changes Clang's serialised diagnostic reader,
now it reports an appropriate error instead of crashing.

rdar://31939877

Differential Revision: https://reviews.llvm.org/D35069

llvm-svn: 307384

clang/lib/Frontend/SerializedDiagnosticReader.cpp

@@ -27,6 +27,9 @@ std::error_code SerializedDiagnosticReader::readDiagnostics(StringRef File) {
   llvm::BitstreamCursor Stream(**Buffer);
   Optional<llvm::BitstreamBlockInfo> BlockInfo;
 
+  if (Stream.AtEndOfStream())
+    return SDError::InvalidSignature;
+
   // Sniff for the signature.
   if (Stream.Read(8) != 'D' ||
       Stream.Read(8) != 'I' ||

clang/test/Index/read-empty-diags.test

@@ -0,0 +1,2 @@
+// RUN: not c-index-test -read-diagnostics %S/Inputs/empty.dia 2>&1 | FileCheck %s
+// CHECK: Trouble deserializing file (Invalid File): Invalid diagnostics signature