cross-fingers.patch

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 06d0517a2..a30c75744 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2222,14 +2222,39 @@ void LocalDerivationGoal::registerOutputs()
             /* Apply hash rewriting if necessary. */
             if (!outputRewrites.empty()) {
                 debug("rewriting hashes in '%1%'; cross fingers", actualPath);
-
-                /* FIXME: this is in-memory. */
-                StringSink sink;
-                dumpPath(actualPath, sink);
-                deletePath(actualPath);
-                sink.s = make_ref<std::string>(rewriteStrings(*sink.s, outputRewrites));
-                StringSource source(*sink.s);
-                restorePath(actualPath, source);
+                
+                AutoDelete tmpDir(createTempDir(), true);
+                Path tmpFile = (Path) tmpDir + "/rewrite";
+                {
+                  AutoCloseFD fd = open(tmpFile.c_str(), O_WRONLY | O_CREAT | O_EXCL, 0600);
+                  if(!fd) throw SysError("creating temporary file '%s'", tmpFile);
+                  FdSink sink(fd.get());
+                  dumpPath(actualPath, sink);
+                  deletePath(actualPath);
+                }
+                
+                {
+                  AutoCloseFD fd = open(tmpFile.c_str(), O_RDWR);
+                  if(!fd) throw SysError("Opening temporary file '%s'", tmpFile);
+                  struct stat stat_buf;
+                  if(fstat(fd.get(), &stat_buf) == -1) throw SysError("fstat: '%s'", tmpFile);
+                  void *ptr = mmap(NULL, stat_buf.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd.get(), 0);
+                  if(!ptr) throw SysError("mmap: '%s'", tmpFile);
+                  for(auto & i : outputRewrites) {
+                    if(i.first == i.second) continue;
+                    void *j;
+                    while((j = memmem(ptr, stat_buf.st_size, i.first.c_str(), i.first.size())))
+                      memcpy(j, i.second.c_str(), i.first.size());
+                  }
+                  munmap(ptr, stat_buf.st_size);
+                }
+                
+                {
+                  AutoCloseFD fd = open(tmpFile.c_str(), O_RDONLY);
+                  if(!fd) throw SysError("Opening temporary file '%s'", tmpFile);
+                  FdSource source(fd.get());
+                  restorePath(actualPath, source);
+                }
             }
         };