[PCH] Recover gracefully if the ASTReader detects that a file is different

from the one stored in the PCH/AST, while trying to load a SLocEntry.

We verify that all files of the PCH did not change before loading it but this is not enough because:

- The AST may have been 1) kept around, 2) to do queries on it.
- We may have 1) verified the PCH and 2) started parsing.

Between 1) and 2) files may change and we are going to have crashes because the rest of clang
cannot deal with the ASTReader failing to read a SLocEntry.

Handle this by recovering gracefully in such a case, by initializing the SLocEntry
with the info from the PCH/AST as well as reporting failure by the ASTReader.

rdar://10888929

llvm-svn: 151004

clang/include/clang/Basic/SourceManager.h

@@ -519,7 +519,7 @@ class SourceManager : public RefCountedBase<SourceManager> {
   ///
   /// Negative FileIDs are indexes into this table. To get from ID to an index,
   /// use (-ID - 2).
-  std::vector<SrcMgr::SLocEntry> LoadedSLocEntryTable;
+  mutable std::vector<SrcMgr::SLocEntry> LoadedSLocEntryTable;
 
   /// \brief The starting offset of the next local SLocEntry.
   ///
@@ -576,6 +576,8 @@ class SourceManager : public RefCountedBase<SourceManager> {
   // Cache for the "fake" buffer used for error-recovery purposes.
   mutable llvm::MemoryBuffer *FakeBufferForRecovery;
 
+  mutable SrcMgr::ContentCache *FakeContentCacheForRecovery;
+
   /// \brief Lazily computed map of macro argument chunks to their expanded
   /// source location.
   typedef std::map<unsigned, SourceLocation> MacroArgsMap;
@@ -1260,9 +1262,9 @@ public:
   const SrcMgr::SLocEntry &getLoadedSLocEntry(unsigned Index,
                                               bool *Invalid = 0) const {
     assert(Index < LoadedSLocEntryTable.size() && "Invalid index");
-    if (!SLocEntryLoaded[Index])
-      ExternalSLocEntries->ReadSLocEntry(-(static_cast<int>(Index) + 2));
-    return LoadedSLocEntryTable[Index];
+    if (SLocEntryLoaded[Index])
+      return LoadedSLocEntryTable[Index];
+    return loadSLocEntry(Index, Invalid);
   }
 
   const SrcMgr::SLocEntry &getSLocEntry(FileID FID, bool *Invalid = 0) const {
@@ -1313,6 +1315,9 @@ public:
 
 private:
   const llvm::MemoryBuffer *getFakeBufferForRecovery() const;
+  const SrcMgr::ContentCache *getFakeContentCacheForRecovery() const;
+
+  const SrcMgr::SLocEntry &loadSLocEntry(unsigned Index, bool *Invalid) const;
 
   /// \brief Get the entry with the given unwrapped FileID.
   const SrcMgr::SLocEntry &getSLocEntryByID(int ID) const {
@@ -1322,8 +1327,9 @@ private:
     return getLocalSLocEntry(static_cast<unsigned>(ID));
   }
 
-  const SrcMgr::SLocEntry &getLoadedSLocEntryByID(int ID) const {
-    return getLoadedSLocEntry(static_cast<unsigned>(-ID - 2));
+  const SrcMgr::SLocEntry &getLoadedSLocEntryByID(int ID,
+                                                  bool *Invalid = 0) const {
+    return getLoadedSLocEntry(static_cast<unsigned>(-ID - 2), Invalid);
   }
 
   /// createExpansionLoc - Implements the common elements of storing an

clang/lib/Basic/SourceManager.cpp

@@ -370,7 +370,8 @@ LineTableInfo &SourceManager::getLineTable() {
 SourceManager::SourceManager(DiagnosticsEngine &Diag, FileManager &FileMgr)
   : Diag(Diag), FileMgr(FileMgr), OverridenFilesKeepOriginalName(true),
     ExternalSLocEntries(0), LineTable(0), NumLinearScans(0),
-    NumBinaryProbes(0), FakeBufferForRecovery(0) {
+    NumBinaryProbes(0), FakeBufferForRecovery(0),
+    FakeContentCacheForRecovery(0) {
   clearIDTables();
   Diag.setSourceManager(this);
 }
@@ -396,6 +397,7 @@ SourceManager::~SourceManager() {
   }
   
   delete FakeBufferForRecovery;
+  delete FakeContentCacheForRecovery;
 
   for (llvm::DenseMap<FileID, MacroArgsMap *>::iterator
          I = MacroArgsCacheMap.begin(),E = MacroArgsCacheMap.end(); I!=E; ++I) {
@@ -469,6 +471,25 @@ SourceManager::createMemBufferContentCache(const MemoryBuffer *Buffer) {
   return Entry;
 }
 
+const SrcMgr::SLocEntry &SourceManager::loadSLocEntry(unsigned Index,
+                                                      bool *Invalid) const {
+  assert(!SLocEntryLoaded[Index]);
+  if (ExternalSLocEntries->ReadSLocEntry(-(static_cast<int>(Index) + 2))) {
+    if (Invalid)
+      *Invalid = true;
+    // If the file of the SLocEntry changed we could still have loaded it.
+    if (!SLocEntryLoaded[Index]) {
+      // Try to recover; create a SLocEntry so the rest of clang can handle it.
+      LoadedSLocEntryTable[Index] = SLocEntry::get(0,
+                                 FileInfo::get(SourceLocation(),
+                                               getFakeContentCacheForRecovery(),
+                                               SrcMgr::C_User));
+    }
+  }
+
+  return LoadedSLocEntryTable[Index];
+}
+
 std::pair<int, unsigned>
 SourceManager::AllocateLoadedSLocEntries(unsigned NumSLocEntries,
                                          unsigned TotalSize) {
@@ -491,6 +512,18 @@ const llvm::MemoryBuffer *SourceManager::getFakeBufferForRecovery() const {
   return FakeBufferForRecovery;
 }
 
+/// \brief As part of recovering from missing or changed content, produce a
+/// fake content cache.
+const SrcMgr::ContentCache *
+SourceManager::getFakeContentCacheForRecovery() const {
+  if (!FakeContentCacheForRecovery) {
+    FakeContentCacheForRecovery = new ContentCache();
+    FakeContentCacheForRecovery->replaceBuffer(getFakeBufferForRecovery(),
+                                               /*DoNotFree=*/true);
+  }
+  return FakeContentCacheForRecovery;
+}
+
 //===----------------------------------------------------------------------===//
 // Methods to create new FileID's and macro expansions.
 //===----------------------------------------------------------------------===//

clang/lib/Serialization/ASTReader.cpp

@@ -1111,6 +1111,10 @@ ASTReader::ASTReadResult ASTReader::ReadSLocEntryRecord(int ID) {
       return Failure;
     }
 
+    // We will detect whether a file changed and return 'Failure' for it, but
+    // we will also try to fail gracefully by setting up the SLocEntry.
+    ASTReader::ASTReadResult Result = Success;
+
     bool OverriddenBuffer = Record[6];
     
     std::string OrigFilename(BlobStart, BlobStart + BlobLen);
@@ -1149,7 +1153,7 @@ ASTReader::ASTReadResult ASTReader::ReadSLocEntryRecord(int ID) {
 #endif
         )) {
       Error(diag::err_fe_pch_file_modified, Filename);
-      return Failure;
+      Result = Failure;
     }
 
     SourceLocation IncludeLoc = ReadSourceLocation(*F, Record[1]);
@@ -1193,6 +1197,9 @@ ASTReader::ASTReadResult ASTReader::ReadSLocEntryRecord(int ID) {
                                            Filename);
       SourceMgr.overrideFileContents(File, Buffer);
     }
+
+    if (Result == Failure)
+      return Failure;
     break;
   }