fix(filesystem_writer): segfault on bad_compression_ratio_error

When recompressing a filesystem where some blocks cannot be compressed
using the selected algorithm because of a `bad_compression_ratio_error`,
the resulting `block` was left empty after the refactoring done in
06f8728cc. It's a really easy fix, pretty much just doing what the code
did before the refactor: initialize `block` with the decompressed data
before attempting to compress it. Since `clang-format` reformatted the
whole lambda after the change, I've pulled the lamdba body out into a
separate function, which makes this change much bigger than it is.

src/writer/filesystem_writer.cpp

@@ -387,46 +387,10 @@ class rewritten_fsblock : public fsblock::impl {
     std::promise<void> prom;
     future_ = prom.get_future();
 
-    wg.add_job([this, prom = std::move(prom),
-                meta = std::move(meta)]() mutable {
-      try {
-        shared_byte_buffer block;
-
-        {
-          // TODO: we don't have to do this for uncompressed blocks
-          block_decompressor bd(data_comp_type_, data_);
-          auto buffer = bd.start_decompression(malloc_byte_buffer::create());
-          bd.decompress_frame(bd.uncompressed_size());
-
-          if (!meta) {
-            meta = bd.metadata();
-          }
-
-          pctx_->bytes_in += buffer.size(); // TODO: data_.size()?
-
-          try {
-            if (meta) {
-              block = bc_.compress(buffer, *meta);
-            } else {
-              block = bc_.compress(buffer);
-            }
-          } catch (bad_compression_ratio_error const&) {
-            comp_type_ = compression_type::NONE;
-          }
-        }
-
-        pctx_->bytes_out += block.size();
-
-        {
-          std::lock_guard lock(mx_);
-          block_data_.emplace(std::move(block));
-        }
-
-        prom.set_value();
-      } catch (...) {
-        prom.set_exception(std::current_exception());
-      }
-    });
+    wg.add_job(
+        [this, prom = std::move(prom), meta = std::move(meta)]() mutable {
+          compress_job(std::move(prom), std::move(meta));
+        });
   }
 
   void wait_until_compressed() override { future_.get(); }
@@ -480,6 +444,46 @@ class rewritten_fsblock : public fsblock::impl {
   }
 
  private:
+  void compress_job(std::promise<void> prom, std::optional<std::string> meta) {
+    try {
+      shared_byte_buffer block;
+
+      {
+        // TODO: we don't have to do this for uncompressed blocks
+        block_decompressor bd(data_comp_type_, data_);
+        block = bd.start_decompression(malloc_byte_buffer::create());
+        bd.decompress_frame(bd.uncompressed_size());
+
+        if (!meta) {
+          meta = bd.metadata();
+        }
+
+        pctx_->bytes_in += block.size(); // TODO: data_.size()?
+
+        try {
+          if (meta) {
+            block = bc_.compress(block, *meta);
+          } else {
+            block = bc_.compress(block);
+          }
+        } catch (bad_compression_ratio_error const&) {
+          comp_type_ = compression_type::NONE;
+        }
+      }
+
+      pctx_->bytes_out += block.size();
+
+      {
+        std::lock_guard lock(mx_);
+        block_data_.emplace(std::move(block));
+      }
+
+      prom.set_value();
+    } catch (...) {
+      prom.set_exception(std::current_exception());
+    }
+  }
+
   section_type const type_;
   block_compressor const& bc_;
   mutable std::recursive_mutex mx_;

test/tool_main_test.cpp

@@ -1623,6 +1623,16 @@ TEST_P(mkdwarfs_recompress_test, recompress) {
     EXPECT_EQ(2, history.size());
   }
 
+  {
+    auto t = tester(image);
+    ASSERT_EQ(0, t.run({"-i", image_file, "-o", "-", "--recompress", "-l1"}))
+        << t.err();
+    auto fs = t.fs_from_stdout();
+    EXPECT_TRUE(fs.find("/random"));
+    auto history = fs.info_as_json(history_opts)["history"];
+    EXPECT_EQ(2, history.size());
+  }
+
   {
     auto t = tester(image);
     EXPECT_NE(0, t.run({"-i", image_file, "-o", "-", "--recompress=foo"}));