memset less in addToStoreFromDump

resizing a std::string clears the newly added bytes, which is not
necessary here and comes with a ~1.4% slowdown on our test nixos config.

〉 nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'

before:

  Time (mean ± σ):      4.486 s ±  0.003 s    [User: 3.978 s, System: 0.507 s]
  Range (min … max):    4.482 s …  4.492 s    10 runs

after:

  Time (mean ± σ):      4.429 s ±  0.002 s    [User: 3.929 s, System: 0.500 s]
  Range (min … max):    4.427 s …  4.433 s    10 runs

src/libstore/local-store.cc

@@ -18,6 +18,8 @@
 #include <algorithm>
 #include <cstring>
 
+#include <memory>
+#include <new>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/select.h>
@@ -1130,7 +1132,11 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
        path. */
     bool inMemory = false;
 
-    std::string dump;
+    struct Free {
+        void operator()(void* v) { free(v); }
+    };
+    std::unique_ptr<char, Free> dumpBuffer(nullptr);
+    std::string_view dump;
 
     /* Fill out buffer, and decide whether we are working strictly in
        memory based on whether we break out because the buffer is full
@@ -1139,13 +1145,18 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
         auto oldSize = dump.size();
         constexpr size_t chunkSize = 65536;
         auto want = std::min(chunkSize, settings.narBufferSize - oldSize);
-        dump.resize(oldSize + want);
+        if (auto tmp = realloc(dumpBuffer.get(), oldSize + want)) {
+            dumpBuffer.release();
+            dumpBuffer.reset((char*) tmp);
+        } else {
+            throw std::bad_alloc();
+        }
         auto got = 0;
         Finally cleanup([&]() {
-            dump.resize(oldSize + got);
+            dump = {dumpBuffer.get(), dump.size() + got};
         });
         try {
-            got = source.read(dump.data() + oldSize, want);
+            got = source.read(dumpBuffer.get() + oldSize, want);
         } catch (EndOfFile &) {
             inMemory = true;
             break;
@@ -1171,7 +1182,8 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
         else
             writeFile(tempPath, bothSource);
 
-        dump.clear();
+        dumpBuffer.reset();
+        dump = {};
     }
 
     auto [hash, size] = hashSink->finish();