Implement reference transactions

docs/index.rst

@@ -74,6 +74,7 @@ Table of Contents
    oid
    packing
    references
+   transactions
    remotes
    repository
    revparse

docs/references.rst

@@ -88,6 +88,21 @@ Example::
 .. autoclass:: pygit2.RefLogEntry
    :members:
 
+Reference Transactions
+=======================
+
+For atomic updates of multiple references, use transactions. See the
+:doc:`transactions` documentation for details.
+
+Example::
+
+    # Update multiple refs atomically
+    with repo.transaction() as txn:
+        txn.lock_ref('refs/heads/master')
+        txn.lock_ref('refs/heads/develop')
+        txn.set_target('refs/heads/master', new_oid, message='Release')
+        txn.set_target('refs/heads/develop', dev_oid, message='Continue dev')
+
 Notes
 ====================
 

docs/transactions.rst

@@ -0,0 +1,120 @@
+**********************************************************************
+Reference Transactions
+**********************************************************************
+
+Reference transactions allow you to update multiple references atomically.
+All reference updates within a transaction either succeed together or fail
+together, ensuring repository consistency.
+
+Basic Usage
+===========
+
+Use the :meth:`Repository.transaction` method as a context manager. The
+transaction commits automatically when the context exits successfully, or
+rolls back if an exception is raised::
+
+    with repo.transaction() as txn:
+        txn.lock_ref('refs/heads/master')
+        txn.set_target('refs/heads/master', new_oid, message='Update master')
+
+Atomic Multi-Reference Updates
+===============================
+
+Transactions are useful when you need to update multiple references
+atomically::
+
+    # Swap two branches atomically
+    with repo.transaction() as txn:
+        txn.lock_ref('refs/heads/branch-a')
+        txn.lock_ref('refs/heads/branch-b')
+
+        # Get current targets
+        ref_a = repo.lookup_reference('refs/heads/branch-a')
+        ref_b = repo.lookup_reference('refs/heads/branch-b')
+
+        # Swap them
+        txn.set_target('refs/heads/branch-a', ref_b.target, message='Swap')
+        txn.set_target('refs/heads/branch-b', ref_a.target, message='Swap')
+
+Automatic Rollback
+==================
+
+If an exception occurs during the transaction, changes are automatically
+rolled back::
+
+    try:
+        with repo.transaction() as txn:
+            txn.lock_ref('refs/heads/master')
+            txn.set_target('refs/heads/master', new_oid)
+
+            # If this raises an exception, the ref update is rolled back
+            validate_commit(new_oid)
+    except ValidationError:
+        # Master still points to its original target
+        pass
+
+Manual Commit
+=============
+
+While the context manager is recommended, you can manually manage
+transactions::
+
+    from pygit2 import ReferenceTransaction
+
+    txn = ReferenceTransaction(repo)
+    try:
+        txn.lock_ref('refs/heads/master')
+        txn.set_target('refs/heads/master', new_oid, message='Update')
+        txn.commit()
+    finally:
+        del txn  # Ensure transaction is freed
+
+API Reference
+=============
+
+Repository Methods
+------------------
+
+.. automethod:: pygit2.Repository.transaction
+
+The ReferenceTransaction Type
+------------------------------
+
+.. autoclass:: pygit2.ReferenceTransaction
+   :members:
+   :special-members: __enter__, __exit__
+
+Usage Notes
+===========
+
+- Always lock a reference with :meth:`~ReferenceTransaction.lock_ref` before
+  modifying it
+- Transactions operate on reference names, not Reference objects
+- Symbolic references can be updated with
+  :meth:`~ReferenceTransaction.set_symbolic_target`
+- References can be deleted with :meth:`~ReferenceTransaction.remove`
+- The signature parameter defaults to the repository's configured identity
+
+Thread Safety
+=============
+
+Transactions are thread-local and must be used from the thread that created
+them. Attempting to use a transaction from a different thread raises
+:exc:`RuntimeError`::
+
+    # This is safe - each thread has its own transaction
+    def thread1():
+        with repo.transaction() as txn:
+            txn.lock_ref('refs/heads/branch1')
+            txn.set_target('refs/heads/branch1', oid1)
+
+    def thread2():
+        with repo.transaction() as txn:
+            txn.lock_ref('refs/heads/branch2')
+            txn.set_target('refs/heads/branch2', oid2)
+
+    # Both threads can run concurrently without conflicts
+
+Different threads can hold transactions simultaneously as long as they don't
+attempt to lock the same references. If two threads try to acquire locks in
+different orders, libgit2 will detect potential deadlocks and raise an error.

pygit2/__init__.py

@@ -366,6 +366,7 @@
 from .repository import Repository
 from .settings import Settings
 from .submodules import Submodule
+from .transaction import ReferenceTransaction
 from .utils import to_bytes, to_str
 
 # Features
@@ -971,6 +972,8 @@ def clone_repository(
     'Settings',
     'submodules',
     'Submodule',
+    'transaction',
+    'ReferenceTransaction',
     'utils',
     'to_bytes',
     'to_str',

pygit2/_libgit2/ffi.pyi

@@ -239,12 +239,17 @@ class GitRemoteC:
 class GitReferenceC:
     pass
 
+class GitTransactionC:
+    pass
+
 def string(a: char_pointer) -> bytes: ...
 @overload
 def new(a: Literal['git_repository **']) -> _Pointer[GitRepositoryC]: ...
 @overload
 def new(a: Literal['git_remote **']) -> _Pointer[GitRemoteC]: ...
 @overload
+def new(a: Literal['git_transaction **']) -> _Pointer[GitTransactionC]: ...
+@overload
 def new(a: Literal['git_repository_init_options *']) -> GitRepositoryInitOptionsC: ...
 @overload
 def new(a: Literal['git_submodule_update_options *']) -> GitSubmoduleUpdateOptionsC: ...

pygit2/_run.py

@@ -81,6 +81,7 @@
     'revert.h',
     'stash.h',
     'submodule.h',
+    'transaction.h',
     'options.h',
     'callbacks.h',  # Bridge from libgit2 to Python
 ]

pygit2/decl/transaction.h

@@ -0,0 +1,8 @@
+int git_transaction_new(git_transaction **out, git_repository *repo);
+int git_transaction_lock_ref(git_transaction *tx, const char *refname);
+int git_transaction_set_target(git_transaction *tx, const char *refname, const git_oid *target, const git_signature *sig, const char *msg);
+int git_transaction_set_symbolic_target(git_transaction *tx, const char *refname, const char *target, const git_signature *sig, const char *msg);
+int git_transaction_set_reflog(git_transaction *tx, const char *refname, const git_reflog *reflog);
+int git_transaction_remove(git_transaction *tx, const char *refname);
+int git_transaction_commit(git_transaction *tx);
+void git_transaction_free(git_transaction *tx);

pygit2/decl/types.h

@@ -12,6 +12,8 @@ typedef struct git_submodule git_submodule;
 typedef struct git_transport git_transport;
 typedef struct git_tree git_tree;
 typedef struct git_packbuilder git_packbuilder;
+typedef struct git_transaction git_transaction;
+typedef struct git_reflog git_reflog;
 
 typedef int64_t git_off_t;
 typedef int64_t git_time_t;

pygit2/repository.py

@@ -78,6 +78,7 @@
 from .references import References
 from .remotes import RemoteCollection
 from .submodules import SubmoduleCollection
+from .transaction import ReferenceTransaction
 from .utils import StrArray, to_bytes
 
 if TYPE_CHECKING:
@@ -120,6 +121,7 @@ def _common_init(self) -> None:
         self.references = References(self)
         self.remotes = RemoteCollection(self)
         self.submodules = SubmoduleCollection(self)
+        self._active_transaction = None
 
         # Get the pointer as the contents of a buffer and store it for
         # later access
@@ -359,6 +361,22 @@ def resolve_refish(self, refish: str) -> tuple[Commit, Reference]:
 
         return (commit, reference)  # type: ignore
 
+    def transaction(self) -> ReferenceTransaction:
+        """Create a new reference transaction.
+
+        Returns a context manager that commits all reference updates atomically
+        when the context exits successfully, or performs no updates if an exception
+        is raised.
+
+        Example::
+
+            with repo.transaction() as txn:
+                txn.lock_ref('refs/heads/master')
+                txn.set_target('refs/heads/master', new_oid, message='Update')
+        """
+        txn = ReferenceTransaction(self)
+        return txn
+
     #
     # Checkout
     #