Lazy/on-demand decompression of clusters

src/cluster.cpp

@@ -230,8 +230,6 @@ CompressedCluster::CompressedCluster(std::shared_ptr<const Reader> reader, Compr
     readHeader<uint64_t>(*ds_);
   else
     readHeader<uint32_t>(*ds_);
-
-  readBlobs(*ds_);
 }
 
 bool
@@ -284,16 +282,17 @@ CompressedCluster::readHeader(IDataStream& ds)
 }
 
 void
-CompressedCluster::readBlobs(IDataStream& ds)
+CompressedCluster::ensureBlobIsDecompressed(blob_index_t n) const
 {
-  const size_t n = count().v;
-  for ( size_t i = 0; i < n; ++i )
-    blobs_.push_back(ds.readBlob(getBlobSize(blob_index_t(i)).v));
+  for ( size_t i = blobs_.size(); i <= n.v; ++i )
+    blobs_.push_back(ds_->readBlob(getBlobSize(blob_index_t(i)).v));
 }
 
 Blob
 CompressedCluster::getBlob(blob_index_t n) const
 {
+  std::lock_guard<std::mutex> lock(blobAccessMutex_);
+  ensureBlobIsDecompressed(n);
   ASSERT(n.v, <, blobs_.size());
   const IDataStream::Blob& blob = blobs_[n.v];
   return idsBlob2zimBlob(blob, 0, blob.size());
@@ -302,6 +301,8 @@ CompressedCluster::getBlob(blob_index_t n) const
 Blob
 CompressedCluster::getBlob(blob_index_t n, offset_t offset, zsize_t size) const
 {
+  std::lock_guard<std::mutex> lock(blobAccessMutex_);
+  ensureBlobIsDecompressed(n);
   ASSERT(n.v, <, blobs_.size());
   return idsBlob2zimBlob(blobs_[n.v], offset.v, size.v);
 }

src/cluster.h

@@ -28,6 +28,7 @@
 #include <iosfwd>
 #include <vector>
 #include <memory>
+#include <mutex>
 
 #include "zim_types.h"
 #include "zim/error.h"
@@ -110,13 +111,15 @@ namespace zim
       template<typename OFFSET_TYPE>
       void readHeader(IDataStream& ds);
 
-      void readBlobs(IDataStream& ds);
+      void ensureBlobIsDecompressed(blob_index_t i) const;
 
     private: // data
       const std::shared_ptr<IDataStream> ds_;
       const CompressionType compression_;
       std::vector<size_t> blobSizes_;
-      std::vector<IDataStream::Blob> blobs_;
+
+      mutable std::mutex blobAccessMutex_;
+      mutable std::vector<IDataStream::Blob> blobs_;
   };
 
 }