revlog: automatically read from opened file handles

The revlog reading code commonly opens a new file handle for
reading on demand. There is support for passing a file handle
to revlog.revision(). But it is marked as an internal argument.

When revlogs are written, we write() data as it is available. But
we don't flush() data until all revisions are written.

Putting these two traits together, it is possible for an in-process
revlog reader during active writes to trigger the opening of a new
file handle on a file with unflushed writes. The reader won't have
access to all "available" revlog data (as it hasn't been flushed).
And with the introduction of the previous patch, this can lead to
the revlog raising an error due to a partial read.

I witnessed this behavior when applying changegroup data (via
`hg pull`) before issue6006 was fixed via different means. Having
this and the previous patch in play would have helped cause errors
earlier rather than manifesting as hash verification failures.

While this has been a long-standing issue, I believe the relatively
new delta computation code has tickled it into being more common.
This is because the new delta computation code will compute deltas
in more scenarios. This can lead to revlog reading. While the delta
computation code is probably supposed to reuse file handles, it
appears it isn't doing so in all circumstances.

But the issue runs deeper than that. Theoretically, any code can
access revision data during revlog writes. It appears we were just
getting lucky that it wasn't. (The "add revision callback" passed to
addgroup() provides an avenue to do this.)

If I changed the revlog's behavior to not cache the full revision
text or to clear caches after revision insertion during addgroup(),
I was able to produce crashes 100% of the time when writing changelog
revisions. This is because changelog's add revision callback attempts
to resolve the revision data to access the changed files list. And
without the revision's fulltext being cached, we performed a revlog
read, which required opening a new file handle. This attempted to read
unflushed data, leading to a partial read and a crash.

This commit teaches the revlog to store the file handles used for
writing multiple revisions during addgroup(). It also teaches the
code for resolving a file handle when reading to use these handles,
if available. This ensures that *any* reads (regardless of their
source) use the active writing file handles, if available. These
file handles have access to the unflushed data because they wrote it.
This allows reads to complete without issue.

Differential Revision: https://phab.mercurial-scm.org/D5267

mercurial/revlog.py

@@ -375,6 +375,9 b' class revlog(object):'
         # custom flags.
         self._flagprocessors = dict(_flagprocessors)
 
+        # 2-tuple of file handles being used for active writing.
+        self._writinghandles = None
+
         mmapindexthreshold = None
         v = REVLOG_DEFAULT_VERSION
         opts = getattr(opener, 'options', None)
@@ -505,8 +508,21 b' class revlog(object):'
     @contextlib.contextmanager
     def _datareadfp(self, existingfp=None):
         """file object suitable to read data"""
+        # Use explicit file handle, if given.
         if existingfp is not None:
             yield existingfp
+
+        # Use a file handle being actively used for writes, if available.
+        # There is some danger to doing this because reads will seek the
+        # file. However, _writeentry() performs a SEEK_END before all writes,
+        # so we should be safe.
+        elif self._writinghandles:
+            if self._inline:
+                yield self._writinghandles[0]
+            else:
+                yield self._writinghandles[1]
+
+        # Otherwise open a new file handle.
         else:
             if self._inline:
                 func = self._indexfp
@@ -1750,6 +1766,9 b' class revlog(object):'
         if fp:
             fp.flush()
             fp.close()
+            # We can't use the cached file handle after close(). So prevent
+            # its usage.
+            self._writinghandles = None
 
         with self._indexfp('r') as ifh, self._datafp('w') as dfh:
             for r in self:
@@ -1996,7 +2015,9 b' class revlog(object):'
         # if the file was seeked to before the end. See issue4943 for more.
         #
         # We work around this issue by inserting a seek() before writing.
-        # Note: This is likely not necessary on Python 3.
+        # Note: This is likely not necessary on Python 3. However, because
+        # the file handle is reused for reads and may be seeked there, we need
+        # to be careful before changing this.
         ifh.seek(0, os.SEEK_END)
         if dfh:
             dfh.seek(0, os.SEEK_END)
@@ -2029,6 +2050,9 b' class revlog(object):'
         this revlog and the node that was added.
         """
 
+        if self._writinghandles:
+            raise error.ProgrammingError('cannot nest addgroup() calls')
+
         nodes = []
 
         r = len(self)
@@ -2048,6 +2072,9 b' class revlog(object):'
             if dfh:
                 dfh.flush()
             ifh.flush()
+
+        self._writinghandles = (ifh, dfh)
+
         try:
             deltacomputer = deltautil.deltacomputer(self)
             # loop through our set of deltas
@@ -2109,7 +2136,10 b' class revlog(object):'
                     ifh.close()
                     dfh = self._datafp("a+")
                     ifh = self._indexfp("a+")
+                    self._writinghandles = (ifh, dfh)
         finally:
+            self._writinghandles = None
+
             if dfh:
                 dfh.close()
             ifh.close()