* In `nix-store --export', abort if the contents of a path has

changed.  This prevents corrupt paths from spreading to other
  machines.  Note that checking the hash is cheap because we're
  hashing anyway (because of the --sign feature).

src/libstore/local-store.cc

@@ -930,16 +930,19 @@ struct HashAndWriteSink : Sink
 {
     Sink & writeSink;
     HashSink hashSink;
-    bool hashing;
     HashAndWriteSink(Sink & writeSink) : writeSink(writeSink), hashSink(htSHA256)
     {
-        hashing = true;
     }
     virtual void operator ()
         (const unsigned char * data, unsigned int len)
     {
         writeSink(data, len);
-        if (hashing) hashSink(data, len);
+        hashSink(data, len);
+    }
+    Hash currentHash()
+    {
+        HashSink hashSinkClone(hashSink);
+        return hashSinkClone.finish();
     }
 };
 
@@ -970,6 +973,15 @@ void LocalStore::exportPath(const Path & path, bool sign,
     
     dumpPath(path, hashAndWriteSink);
 
+    /* Refuse to export paths that have changed.  This prevents
+       filesystem corruption from spreading to other machines. */
+    Hash hash = hashAndWriteSink.currentHash();
+    Hash storedHash = queryPathHash(path);
+    if (hash != storedHash)
+        throw Error(format("hash of path `%1%' has changed from `%2%' to `%3%'!") % path
+            % printHash(storedHash) % printHash(hash));
+    printMsg(lvlError, printHash(hash));
+
     writeInt(EXPORT_MAGIC, hashAndWriteSink);
 
     writeString(path, hashAndWriteSink);
@@ -982,8 +994,7 @@ void LocalStore::exportPath(const Path & path, bool sign,
     writeString(deriver, hashAndWriteSink);
 
     if (sign) {
-        Hash hash = hashAndWriteSink.hashSink.finish();
-        hashAndWriteSink.hashing = false;
+        Hash hash = hashAndWriteSink.currentHash();
  
         writeInt(1, hashAndWriteSink);
         

src/libutil/hash.cc

@@ -289,6 +289,13 @@ HashSink::HashSink(HashType ht) : ht(ht)
     start(ht, *ctx);
 }
     
+HashSink::HashSink(const HashSink & h)
+{
+    ht = h.ht;
+    ctx = new Ctx;
+    *ctx = *h.ctx;
+}
+    
 HashSink::~HashSink()
 {
     delete ctx;

src/libutil/hash.hh

@@ -96,6 +96,7 @@ private:
 
 public:
     HashSink(HashType ht);
+    HashSink(const HashSink & h);
     ~HashSink();
     virtual void operator () (const unsigned char * data, unsigned int len);
     Hash finish();