Merge #2328: Always flush block and undo when switching to new file

ab4f184 Fixed integer comparison warning. (Eric Lombrozo)
189d8b7 Always flush block and undo when switching to new file (Pieter Wuille)

Pull request description:

  Fixing a possible blocks db corruption cause.

  > Previously, the undo weren't being flushed during a reindex because
  fKnown was set to true in FindBlockPos. That is the correct behaviour
  for block files as they aren't being touched, but undo files are
  touched.

  > This changes the behavior to always flush when switching to a new file
  (even for block files, though that isn't really necessary).

  Coming from bitcoin#6948.

ACKs for top commit:
  random-zebra:
    utACK ab4f184
  Fuzzbawls:
    ACK ab4f184

Tree-SHA512: c544861e74015afab3cb7162fd49f478e455f7a7b0bfa19c9e3c74ccccd343130dab770fe5a2ad88e017889fe075da999caa89bb06dc669d41d7ba573295cbea

src/validation.cpp

@@ -2629,8 +2629,6 @@ bool FindBlockPos(CValidationState& state, CDiskBlockPos& pos, unsigned int nAdd
 
     if (!fKnown) {
         while (vinfoBlockFile[nFile].nSize + nAddSize >= MAX_BLOCKFILE_SIZE) {
-            LogPrintf("Leaving block file %i: %s\n", nFile, vinfoBlockFile[nFile].ToString());
-            FlushBlockFile(true);
             nFile++;
             if (vinfoBlockFile.size() <= nFile) {
                 vinfoBlockFile.resize(nFile + 1);
@@ -2640,7 +2638,14 @@ bool FindBlockPos(CValidationState& state, CDiskBlockPos& pos, unsigned int nAdd
         pos.nPos = vinfoBlockFile[nFile].nSize;
     }
 
-    nLastBlockFile = nFile;
+    if ((int)nFile != nLastBlockFile) {
+        if (!fKnown) {
+            LogPrintf("Leaving block file %i: %s\n", nFile, vinfoBlockFile[nFile].ToString());
+        }
+        FlushBlockFile(!fKnown);
+        nLastBlockFile = nFile;
+    }
+
     vinfoBlockFile[nFile].AddBlock(nHeight, nTime);
     if (fKnown)
         vinfoBlockFile[nFile].nSize = std::max(pos.nPos + nAddSize, vinfoBlockFile[nFile].nSize);