--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/appendfile.py	Fri Mar 24 09:23:11 2006 -0800
@@ -0,0 +1,171 @@
+# appendfile.py - special classes to make repo updates atomic
+#
+# Copyright 2006 Vadim Gelfer <vadim.gelfer@gmail.com>
+#
+# This software may be used and distributed according to the terms
+# of the GNU General Public License, incorporated herein by reference.
+
+from demandload import *
+demandload(globals(), "cStringIO changelog manifest os tempfile")
+
+# writes to metadata files are ordered.  reads: changelog, manifest,
+# normal files.  writes: normal files, manifest, changelog.
+
+# manifest contains pointers to offsets in normal files.  changelog
+# contains pointers to offsets in manifest.  if reader reads old
+# changelog while manifest or normal files are written, it has no
+# pointers into new parts of those files that are maybe not consistent
+# yet, so will not read them.
+
+# localrepo.addchangegroup thinks it writes changelog first, then
+# manifest, then normal files (this is order they are available, and
+# needed for computing linkrev fields), but uses appendfile to hide
+# updates from readers.  data not written to manifest or changelog
+# until all normal files updated.  write manifest first, then
+# changelog.
+
+# with this write ordering, readers cannot see inconsistent view of
+# repo during update.
+
+class appendfile(object):
+    '''implement enough of file protocol to append to revlog file.
+    appended data is written to temp file.  reads and seeks span real
+    file and temp file.  readers cannot see appended data until
+    writedata called.'''
+
+    def __init__(self, fp):
+        fd, self.tmpname = tempfile.mkstemp()
+        self.tmpfp = os.fdopen(fd, 'ab+')
+        self.realfp = fp
+        self.offset = 0
+        # real file is not written by anyone else. cache its size so
+        # seek and read can be fast.
+        self.fpsize = os.fstat(fp.fileno()).st_size
+
+    def seek(self, offset):
+        '''virtual file offset spans real file and temp file.'''
+        self.offset = offset
+        if self.offset < self.fpsize:
+            self.realfp.seek(self.offset)
+        else:
+            self.tmpfp.seek(self.offset - self.fpsize)
+
+    def read(self, count=-1):
+        '''only trick here is reads that span real file and temp file.'''
+        fp = cStringIO.StringIO()
+        old_offset = self.offset
+        if self.offset < self.fpsize:
+            s = self.realfp.read(count)
+            fp.write(s)
+            self.offset += len(s)
+            if count > 0:
+                count -= len(s)
+        if count != 0:
+            if old_offset != self.offset:
+                self.tmpfp.seek(self.offset - self.fpsize)
+            s = self.tmpfp.read(count)
+            fp.write(s)
+            self.offset += len(s)
+        return fp.getvalue()
+
+    def write(self, s):
+        '''append to temp file.'''
+        self.tmpfp.write(s)
+        # all writes are appends, so offset must go to end of file.
+        self.offset = self.fpsize + self.tmpfp.tell()
+
+    def writedata(self):
+        '''copy data from temp file to real file.'''
+        self.tmpfp.seek(0)
+        s = self.tmpfp.read()
+        self.tmpfp.close()
+        self.realfp.seek(0, 2)
+        # small race here.  we write all new data in one call, but
+        # reader can see partial update due to python or os. file
+        # locking no help: slow, not portable, not reliable over nfs.
+        # only safe thing is write to temp file every time and rename,
+        # but performance bad when manifest or changelog gets big.
+        self.realfp.write(s)
+        self.realfp.close()
+
+    def __del__(self):
+        '''delete temp file even if exception raised.'''
+        try: os.unlink(self.tmpname)
+        except: pass
+
+class sharedfile(object):
+    '''let file objects share a single appendfile safely.  each
+    sharedfile has own offset, syncs up with appendfile offset before
+    read and after read and write.'''
+
+    def __init__(self, fp):
+        self.fp = fp
+        self.offset = 0
+
+    def seek(self, offset):
+        self.offset = offset
+
+    def read(self, count=-1):
+        try:
+            if self.offset != self.fp.offset:
+                self.fp.seek(self.offset)
+            return self.fp.read(count)
+        finally:
+            self.offset = self.fp.offset
+
+    def write(self, s):
+        try:
+            return self.fp.write(s)
+        finally:
+            self.offset = self.fp.offset
+
+    def close(self):
+        # revlog wants this.
+        pass
+
+    def flush(self):
+        # revlog wants this.
+        pass
+
+    def writedata(self):
+        self.fp.writedata()
+
+class appendopener(object):
+    '''special opener for files that only read or append.'''
+
+    def __init__(self, opener):
+        self.realopener = opener
+        # key: file name, value: appendfile object
+        self.fps = {}
+
+    def __call__(self, name, mode='r'):
+        '''open file.  return same cached appendfile object for every
+        later call.'''
+
+        assert mode in 'ra'
+        fp = self.fps.get(name)
+        if fp is None:
+            fp = appendfile(self.realopener(name, 'a+'))
+            self.fps[name] = fp
+        return sharedfile(fp)
+
+    def writedata(self):
+        '''copy data from temp files to real files.'''
+        # write .d file before .i file.
+        fps = self.fps.items()
+        fps.sort()
+        for name, fp in fps:
+            fp.writedata()
+
+# files for changelog and manifest are in different appendopeners, so
+# not mixed up together.
+
+class appendchangelog(changelog.changelog, appendopener):
+    def __init__(self, opener):
+        appendopener.__init__(self, opener)
+        changelog.changelog.__init__(self, self)
+
+class appendmanifest(manifest.manifest, appendopener):
+    def __init__(self, opener):
+        appendopener.__init__(self, opener)
+        manifest.manifest.__init__(self, self)