Code Samples

Example 8.1, “Using the Repository Layer” contains a code segment (written in C) that illustrates some of the concepts we've been discussing. It uses both the repository and filesystem interfaces (as can be determined by the prefixes svn_repos_ and svn_fs_ of the function names, respectively) to create a new revision in which a directory is added. You can see the use of an APR pool, which is passed around for memory allocation purposes. Also, the code reveals a somewhat obscure fact about Subversion error handling—all Subversion errors must be explicitly handled to avoid memory leakage (and in some cases, application failure).

Example 8.1. Using the Repository Layer

/* Convert a Subversion error into a simple boolean error code.
 *
 * NOTE:  Subversion errors must be cleared (using svn_error_clear())
 *        because they are allocated from the global pool, else memory
 *        leaking occurs.
 */
#define INT_ERR(expr)                           \
  do {                                          \
    svn_error_t *__temperr = (expr);            \
    if (__temperr)                              \
      {                                         \
        svn_error_clear(__temperr);             \
        return 1;                               \
      }                                         \
    return 0;                                   \
  } while (0)

/* Create a new directory at the path NEW_DIRECTORY in the Subversion
 * repository located at REPOS_PATH.  Perform all memory allocation in
 * POOL.  This function will create a new revision for the addition of
 * NEW_DIRECTORY.  Return zero if the operation completes
 * successfully, non-zero otherwise.
 */
static int
make_new_directory(const char *repos_path,
                   const char *new_directory,
                   apr_pool_t *pool)
{
  svn_error_t *err;
  svn_repos_t *repos;
  svn_fs_t *fs;
  svn_revnum_t youngest_rev;
  svn_fs_txn_t *txn;
  svn_fs_root_t *txn_root;
  const char *conflict_str;

  /* Open the repository located at REPOS_PATH. 
   */
  INT_ERR(svn_repos_open(&repos, repos_path, pool));

  /* Get a pointer to the filesystem object that is stored in REPOS. 
   */
  fs = svn_repos_fs(repos);

  /* Ask the filesystem to tell us the youngest revision that
   * currently exists. 
   */
  INT_ERR(svn_fs_youngest_rev(&youngest_rev, fs, pool));

  /* Begin a new transaction that is based on YOUNGEST_REV.  We are
   * less likely to have our later commit rejected as conflicting if we
   * always try to make our changes against a copy of the latest snapshot
   * of the filesystem tree. 
   */
  INT_ERR(svn_repos_fs_begin_txn_for_commit2(&txn, repos, youngest_rev,
                                             apr_hash_make(pool), pool));

  /* Now that we have started a new Subversion transaction, get a root
   * object that represents that transaction. 
   */
  INT_ERR(svn_fs_txn_root(&txn_root, txn, pool));
  
  /* Create our new directory under the transaction root, at the path
   * NEW_DIRECTORY. 
   */
  INT_ERR(svn_fs_make_dir(txn_root, new_directory, pool));

  /* Commit the transaction, creating a new revision of the filesystem
   * which includes our added directory path.
   */
  err = svn_repos_fs_commit_txn(&conflict_str, repos, 
                                &youngest_rev, txn, pool);
  if (! err)
    {
      /* No error?  Excellent!  Print a brief report of our success.
       */
      printf("Directory '%s' was successfully added as new revision "
             "'%ld'.\n", new_directory, youngest_rev);
    }
  else if (err->apr_err == SVN_ERR_FS_CONFLICT)
    {
      /* Uh-oh.  Our commit failed as the result of a conflict
       * (someone else seems to have made changes to the same area 
       * of the filesystem that we tried to modify).  Print an error
       * message.
       */
      printf("A conflict occurred at path '%s' while attempting "
             "to add directory '%s' to the repository at '%s'.\n", 
             conflict_str, new_directory, repos_path);
    }
  else
    {
      /* Some other error has occurred.  Print an error message.
       */
      printf("An error occurred while attempting to add directory '%s' "
             "to the repository at '%s'.\n", 
             new_directory, repos_path);
    }

  INT_ERR(err);
} 

Note that in Example 8.1, “Using the Repository Layer”, the code could just as easily have committed the transaction using svn_fs_commit_txn(). But the filesystem API knows nothing about the repository library's hook mechanism. If you want your Subversion repository to automatically perform some set of non-Subversion tasks every time you commit a transaction (like, for example, sending an email that describes all the changes made in that transaction to your developer mailing list), you need to use the libsvn_repos-wrapped version of that function, which adds the hook triggering functionality—in this case, svn_repos_fs_commit_txn(). (For more information regarding Subversion's repository hooks, see the section called “Implementing Repository Hooks”.)

Now let's switch languages. Example 8.2, “Using the Repository Layer with Python” is a sample program that uses Subversion's SWIG Python bindings to recursively crawl the youngest repository revision, and print the various paths reached during the crawl.

Example 8.2. Using the Repository Layer with Python

#!/usr/bin/python

"""Crawl a repository, printing versioned object path names."""

import sys
import os.path
import svn.fs, svn.core, svn.repos

def crawl_filesystem_dir(root, directory):
    """Recursively crawl DIRECTORY under ROOT in the filesystem, and return
    a list of all the paths at or below DIRECTORY."""

    # Print the name of this path.
    print directory + "/"
    
    # Get the directory entries for DIRECTORY.
    entries = svn.fs.svn_fs_dir_entries(root, directory)

    # Loop over the entries.
    names = entries.keys()
    for name in names:
        # Calculate the entry's full path.
        full_path = directory + '/' + name

        # If the entry is a directory, recurse.  The recursion will return
        # a list with the entry and all its children, which we will add to
        # our running list of paths.
        if svn.fs.svn_fs_is_dir(root, full_path):
            crawl_filesystem_dir(root, full_path)
        else:
            # Else it's a file, so print its path here.
            print full_path

def crawl_youngest(repos_path):
    """Open the repository at REPOS_PATH, and recursively crawl its
    youngest revision."""
    
    # Open the repository at REPOS_PATH, and get a reference to its
    # versioning filesystem.
    repos_obj = svn.repos.svn_repos_open(repos_path)
    fs_obj = svn.repos.svn_repos_fs(repos_obj)

    # Query the current youngest revision.
    youngest_rev = svn.fs.svn_fs_youngest_rev(fs_obj)
    
    # Open a root object representing the youngest (HEAD) revision.
    root_obj = svn.fs.svn_fs_revision_root(fs_obj, youngest_rev)

    # Do the recursive crawl.
    crawl_filesystem_dir(root_obj, "")
    
if __name__ == "__main__":
    # Check for sane usage.
    if len(sys.argv) != 2:
        sys.stderr.write("Usage: %s REPOS_PATH\n"
                         % (os.path.basename(sys.argv[0])))
        sys.exit(1)

    # Canonicalize the repository path.
    repos_path = svn.core.svn_path_canonicalize(sys.argv[1])

    # Do the real work.
    crawl_youngest(repos_path)

This same program in C would need to deal with APR's memory pool system. But Python handles memory usage automatically, and Subversion's Python bindings adhere to that convention. In C, you'd be working with custom datatypes (such as those provided by the APR library) for representing the hash of entries and the list of paths, but Python has hashes (called “dictionaries”) and lists as built-in datatypes, and provides a rich collection of functions for operating on those types. So SWIG (with the help of some customizations in Subversion's language bindings layer) takes care of mapping those custom datatypes into the native datatypes of the target language. This provides a more intuitive interface for users of that language.

The Subversion Python bindings can be used for working copy operations, too. In the previous section of this chapter, we mentioned the libsvn_client interface, and how it exists for the sole purpose of simplifying the process of writing a Subversion client. Example 8.3, “A Python Status Crawler” is a brief example of how that library can be accessed via the SWIG Python bindings to recreate a scaled-down version of the svn status command.

Example 8.3. A Python Status Crawler

#!/usr/bin/env python

"""Crawl a working copy directory, printing status information."""

import sys
import os.path
import getopt
import svn.core, svn.client, svn.wc

def generate_status_code(status):
    """Translate a status value into a single-character status code,
    using the same logic as the Subversion command-line client."""
    code_map = { svn.wc.svn_wc_status_none        : ' ',
                 svn.wc.svn_wc_status_normal      : ' ',
                 svn.wc.svn_wc_status_added       : 'A',
                 svn.wc.svn_wc_status_missing     : '!',
                 svn.wc.svn_wc_status_incomplete  : '!',
                 svn.wc.svn_wc_status_deleted     : 'D',
                 svn.wc.svn_wc_status_replaced    : 'R',
                 svn.wc.svn_wc_status_modified    : 'M',
                 svn.wc.svn_wc_status_merged      : 'G',
                 svn.wc.svn_wc_status_conflicted  : 'C',
                 svn.wc.svn_wc_status_obstructed  : '~',
                 svn.wc.svn_wc_status_ignored     : 'I',
                 svn.wc.svn_wc_status_external    : 'X',
                 svn.wc.svn_wc_status_unversioned : '?',
               }
    return code_map.get(status, '?')

def do_status(wc_path, verbose):
    # Build a client context baton.
    ctx = svn.client.svn_client_ctx_t()

    def _status_callback(path, status):
        """A callback function for svn_client_status."""

        # Print the path, minus the bit that overlaps with the root of
        # the status crawl
        text_status = generate_status_code(status.text_status)
        prop_status = generate_status_code(status.prop_status)
        print '%s%s  %s' % (text_status, prop_status, path)
        
    # Do the status crawl, using _status_callback() as our callback function.
    revision = svn.core.svn_opt_revision_t()
    revision.type = svn.core.svn_opt_revision_head
    svn.client.svn_client_status2(wc_path, revision, _status_callback,
                                  svn.core.svn_depth_infinity, verbose,
                                  0, 0, 1, ctx)

def usage_and_exit(errorcode):
    """Print usage message, and exit with ERRORCODE."""
    stream = errorcode and sys.stderr or sys.stdout
    stream.write("""Usage: %s OPTIONS WC-PATH
Options:
  --help, -h    : Show this usage message
  --verbose, -v : Show all statuses, even uninteresting ones
""" % (os.path.basename(sys.argv[0])))
    sys.exit(errorcode)
    
if __name__ == '__main__':
    # Parse command-line options.
    try:
        opts, args = getopt.getopt(sys.argv[1:], "hv", ["help", "verbose"])
    except getopt.GetoptError:
        usage_and_exit(1)
    verbose = 0
    for opt, arg in opts:
        if opt in ("-h", "--help"):
            usage_and_exit(0)
        if opt in ("-v", "--verbose"):
            verbose = 1
    if len(args) != 1:
        usage_and_exit(2)
            
    # Canonicalize the repository path.
    wc_path = svn.core.svn_path_canonicalize(args[0])

    # Do the real work.
    try:
        do_status(wc_path, verbose)
    except svn.core.SubversionException, e:
        sys.stderr.write("Error (%d): %s\n" % (e.apr_err, e.message))
        sys.exit(1)

As was the case in Example 8.2, “Using the Repository Layer with Python”, this program is pool-free and uses, for the most part, normal Python data types. The call to svn_client_ctx_t() is deceiving because the public Subversion API has no such function—this just happens to be a case where SWIG's automatic language generation bleeds through a little bit (the function is a sort of factory function for Python's version of the corresponding complex C structure). Also note that the path passed to this program (like the last one) gets run through svn_path_canonicalize(), because to not do so runs the risk of triggering the underlying Subversion C library's assertions about such things, which translate into rather immediate and unceremonious program abortion.