[RFC] New macro SVN_WC__DB_WITH_TXN(my_func_call(...)) doesn't need a baton

The "call a function within a transaction" paradigm is used heavily in wc_db.c.  The current implementation uses a callback function with a standard signature, passing all other parameters to it via a "baton":   struct base_remove_baton {     param1;     param2;     ...   };   /* ...      Implements svn_wc__db_txn_callback_t. */   static svn_error_t *db_base_remove(void *baton,                                      wcroot,                                      local_relpath,                                      scratch_pool)   {     struct base_remove_baton *rb = baton;     ... rb->param1 ...   }   In the caller:   {     struct base_remove_baton rb;     rb.param1 = param1;     rb.param2 = param2;     ...     SVN_ERR(svn_wc__db_with_txn(wcroot, local_relpath, db_base_remove,                                 &rb, scratch_pool));   } But this is unnecessarily verbose.  To simplify it, we can write a statement macro that takes a complete, arbitrary function call as an argument.  Pseudo-code (omitting error handling): #define WITH_TXN(db, wrapped_function_call)   {     start_txn(db)     if (wrapped_function_call) succeeds:       commit_txn(db)     else:       roll_back_txn(db)   } (The argument need not be a function call, it can be any expression of type svn_error_t*.) Then the usage looks like this:   /* ... */   static svn_error_t *db_base_remove(param1,                                      param2,                                      ...,                                      scratch_pool)   {     ... param1 ...   }   In the caller:   {     SVN_WC__DB_WITH_TXN(wcroot,       db_base_remove(param1, param2, ..., scratch_pool));   } This style can be used not only for DB transactions but anywhere we need to wrap a call inside a resource acquisition and release.  We already use this style for working with mutex locks:   SVN_MUTEX__WITH_LOCK(mutex, expr) And this style could also be used in place of:   with_txnlist_lock() in fs_fs.c and probably others.   The main advantage is:   * Simpler usage. Potential down-sides:   * Macros can be harder to debug in some environments?  But we already accept that svn_wc__db_with_txn is wrapped in SVN_ERR so I don't see a problem. The attached patch contains an implementation and some examples of usage.  I would tweak the macro definition and support functions a bit, and there is a bug in that patch somewhere, but you get the idea. I would prefer to be working with the simpler style of code, so I would like to introduce and use this macro. What does anybody think of introducing this form, in principle and in practice? - Julian -- Certified & Supported Apache Subversion Downloads: http://www.wandisco.com/subversion/download