### I/O

- Add support for the Chirp filesystem. To configure and build, chirp 3.2.2 must be installed.
- When a `TFile` object is deleted, make sure that CINT also 'removes' any global variables that might point to it.
- Fix support for the automatic addition to the current directory (for `TTree` and `TH1` for example) in `TKey::Read(TObject*).`
- In `TKey`, properly handle error in the I/O routines.
- Explicitly check the validity of the zipped buffer before calling `R__unzip`, this allow for better error recovery.
- When double checking whether a checksum difference is sustantial, ignore the std namespace. Use `CompareContent` also in the case of where the class is versioned but the 'current' streamerInfo has not yet been built.
- Prevent the I/O engine from mistakenly applying schema evolution to the `TObject::fBits.`
- Make sure that when a streamer info of a base class is used to stream memberwise that is always not-optimized. If the StreamerInfo on file has the same version as the StreamerInfo in memory but the one on file need to be 'not optimized' while the one in memory is not yet built, make sure it will not be optimized.
- Fix the reading of empty collection of object when reading without the library.
- If the sequence of actions for streaming member-wise is not created correctly (i.e. where `fReadMemberWise` was null previously), we now explicitly issue a Fatal error:

      Fatal in <ReadSequence>: The sequence of actions to read AliESDVertex:7 member-wise was not initialized.
      aborting

- Add new optional parameter maxbuf to `TXMLEngine::ParseFile()` allowing the specification of the XML file size to be parsed. This fixes issue #78864.
- Add function `TBuffer::AutoExpand` to centralize the automatic buffer extension policy. This enable the ability to tweak it later (for example instead of always doubling the size, increasing by only at most 2Mb or take hints from the number of entries already in a TBasket).
- Migrate the class `TFileMerger` from the proofplayer library to ROOT I/O library and update `hadd` to rely on `TFileMerger` for everything but the command line argument parsing.
- Fix support for schema evolution from:

         TParameter<Int_t>* MyInt; //-> version 1

  to

         TParameter<Int_t> MyInt; // version 2

- The TZIPFile compressed archive reader now supports the Zip64 format for archives and file members greater than 2 and 4 GB, respectively.

#### MakeProject

- Add support for the case when the requested project/files in path that in not under the current directory
- Generate the code/dictonary only for pair type that do not already have a dictionary

#### Object Merging

We introduced a new explicit interface for providing merging capability. If a class has a method with the name and signature:

       Long64_t Merge(TCollection *input, TFileMergeInfo*);

it will be used by a `TFileMerger` (and thus by PROOF) to merge one or more other objects into the current object. `Merge` should return a negative value if the merging failed.

If this method does not exist, the `TFileMerger` will use a method with the name and signature:

       Long64_t Merge(TCollection *input);

TClass now provides a quick access to these merging function via `TClass::GetMerge`. The wrapper function is automatically created by rootcint and can be installed via `TClass::SetMerge`. The wrapper function should have the signature/type ROOT::MergeFunc_t:

       Long64_t (*)(void *thisobj, TCollection *input, TFileMergeInfo*);

We added the new `Merge` function to `TTree` and `THStack`. We also added the new `Merge` function to `TQCommand` as the existing `TQCommand::Merge` does not have the right semantic (in part because `TQCommand` is a collection).

In `TFileMerger`, we added a `PrintLevel` to allow hadd to request more output than regular `TFileMerger`.

We removed all hard dependencies of `TFileMerger` on `TH1` and `TTree`. (Soft dependencies still exist to be able to disable the merging of `TTrees` and to be able to disable the `AutoAdd` behavior of `TH1`).

The object `TFileMergeInfo` can be used inside the `Merge` function to pass information between runs of the `Merge` (see below). In particular it contains:

       TDirectory  *fOutputDirectory;  // Target directory where the merged object will be written.
       Bool_t       fIsFirst;          // True if this is the first call to Merge for this series of object.
       TString      fOptions;          // Additional text based option being passed down to customize the merge.
       TObject     *fUserData;         // Place holder to pass extra information.  This object will be deleted at the end of each series of objects.

The default in `TFileMerger` is to call `Merge` for every object in the series (i.e the collection has exactly one element) in order to save memory (by not having all the object in memory at the same time).

However for histograms, the default is to first load all the objects and then merge them in one go ; this is customizable when creating the `TFileMerger` object.

#### Asynchronous Prefetching

The prefetching mechanism uses two new classes (`TFilePrefetch` and `TFPBlock`) to prefetch in advance a block of tree entries. There is a thread which takes care of actually transferring the blocks and making them available to the main requesting thread. Therefore, the time spent by the main thread waiting for the data before processing considerably decreases. Besides the prefetching mechanisms there is also a local caching option which can be enabled by the user. Both capabilities are disabled by default and must be explicitly enabled by the user.

In order to enable the prefetching the user must set the rootrc environment variable `TFile.AsyncPrefetching` as follows: `gEnv->SetValue("TFile.AsyncPrefetching", 1)`. Only when the prefetching is enabled can the user set the local cache directory in which the file transferred will be saved. For subsequent reads of the same file the system will use the local copy of the file from cache. To set up a local cache directory, the client can use the following commands:

    TString cachedir="file:/tmp/xcache/";
    // or using xrootd on port 2000
    // TString cachedir="root://localhost:2000//tmp/xrdcache1/";
    gEnv->SetValue("Cache.Directory", cachedir.Data());

The `TFilePrefetch` class is responsible for actually reading and storing the requests received from the main thread. It also creates the working thread which will transfer all the information. Apart from managing the block requests, it also deals with caching the blocks on the local machine and retrieving them when necessary.

The `TFPBlock` class represents the encapsulation of a block request. It contains the chunks to be prefetched and also serves as a container for the information read.