[Frontend] Return an error on bad inputs to PrecompiledPreabmle

[ilya-biryukov]

Summary

Instead of failing with assertions.
Fixed a crash found by oss-fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12865

include/clang/Frontend/PrecompiledPreamble.h

public:
   /// The returned CommentHandler will be added to the preprocessor if not null.
   virtual CommentHandler *getCommentHandler();
 };
 
 enum class BuildPreambleError {
   CouldntCreateTempFile = 1,
   CouldntCreateTargetInfo,
   BeginSourceFileFailed,
-  CouldntEmitPCH
+  CouldntEmitPCH,
+  BadInputs
 };
 
 class BuildPreambleErrorCategory final : public std::error_category {
 public:
   const char *name() const noexcept override;
   std::string message(int condition) const override;
 };
 

lib/Frontend/ASTUnit.cpp

if (NewPreamble) {
       switch (static_cast<BuildPreambleError>(NewPreamble.getError().value())) {
       case BuildPreambleError::CouldntCreateTempFile:
         // Try again next time.
         PreambleRebuildCountdown = 1;
         return nullptr;
       case BuildPreambleError::CouldntCreateTargetInfo:
       case BuildPreambleError::BeginSourceFileFailed:
       case BuildPreambleError::CouldntEmitPCH:
+      case BuildPreambleError::BadInputs:
         // These erros are more likely to repeat, retry after some period.
         PreambleRebuildCountdown = DefaultPreambleRebuildInterval;
         return nullptr;
       }
       llvm_unreachable("unexpected BuildPreambleError");
     }
   }
 

lib/Frontend/PrecompiledPreamble.cpp

if (!Clang->hasTarget())
     return BuildPreambleError::CouldntCreateTargetInfo;
 
   // Inform the target of the language options.
   //
   // FIXME: We shouldn't need to do this, the target should be immutable once
   // created. This complexity should be lifted elsewhere.
   Clang->getTarget().adjust(Clang->getLangOpts());
 
-  assert(Clang->getFrontendOpts().Inputs.size() == 1 &&
-         "Invocation must have exactly one source file!");
-  assert(Clang->getFrontendOpts().Inputs[0].getKind().getFormat() ==
-             InputKind::Source &&
-         "FIXME: AST inputs not yet supported here!");
-  assert(Clang->getFrontendOpts().Inputs[0].getKind().getLanguage() !=
-             InputKind::LLVM_IR &&
-         "IR inputs not support here!");
+  if (Clang->getFrontendOpts().Inputs.size() != 1 ||
+      Clang->getFrontendOpts().Inputs[0].getKind().getFormat() !=
+          InputKind::Source ||
+      Clang->getFrontendOpts().Inputs[0].getKind().getLanguage() ==
+          InputKind::LLVM_IR) {
+    return BuildPreambleError::BadInputs;
+  }
 
   // Clear out old caches and data.
   Diagnostics.Reset();
   ProcessWarningOptions(Diagnostics, Clang->getDiagnosticOpts());
 
   VFS =
       createVFSFromCompilerInvocation(Clang->getInvocation(), Diagnostics, VFS);
 
std::string BuildPreambleErrorCategory::message(int condition) const {
   case BuildPreambleError::CouldntCreateTempFile:
     return "Could not create temporary file for PCH";
   case BuildPreambleError::CouldntCreateTargetInfo:
     return "CreateTargetInfo() return null";
   case BuildPreambleError::BeginSourceFileFailed:
     return "BeginSourceFile() return an error";
   case BuildPreambleError::CouldntEmitPCH:
     return "Could not emit PCH";
+  case BuildPreambleError::BadInputs:
+    return "Command line arguments must contain exactly one source file";
   }
   llvm_unreachable("unexpected BuildPreambleError");
 }