nodemap: deal with data mmap error

If the file is too small, the mmapread call would raise a ValueError. We catch
that and ignore nodemap content (as we do without mmap). This make the repository
slightly slower (until the next write) but usable. Unlike the current crash.

Differential Revision: https://phab.mercurial-scm.org/D10458

mercurial/revlogutils/nodemap.py

@@ -53,7 +53,11 b' def persisted_data(revlog):'
     try:
         with revlog.opener(filename) as fd:
             if use_mmap:
-                data = util.buffer(util.mmapread(fd, data_length))
+                try:
+                    data = util.buffer(util.mmapread(fd, data_length))
+                except ValueError:
+                    # raised when the read file is too small
+                    data = b''
             else:
                 data = fd.read(data_length)
     except (IOError, OSError) as e:

tests/test-persistent-nodemap.t

@@ -799,9 +799,13 b' truncate the file'
 
 Check that Mercurial reaction to this event
 
-  $ hg -R corruption-test-repo log -r .
-  abort: index 00changelog.i is corrupted
-  [50]
+  $ hg -R corruption-test-repo log -r . --traceback
+  changeset:   5005:90d5d3ba2fc4
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     a2
+  
 
 
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