largefiles: fix a traceback in lfconvert if a largefile is missing (issue3519)

The largefile may be missing for various reasons, including that a remote
repository was cloned without the --all-largefiles option.  Therefore, it seems
reasonable to attempt to download the missing files and failing that, abort and
indicate the affected file and revision so the user can manually fix the
problem.

hgext/largefiles/lfcommands.py

@@ -141,7 +141,17 def _addchangeset(ui, rsrc, rdst, ctx, r
 
             hash = fctx.data().strip()
             path = lfutil.findfile(rsrc, hash)
-            ### TODO: What if the file is not cached?
+
+            # If one file is missing, likely all files from this rev are
+            if path is None:
+                cachelfiles(ui, rsrc, ctx.node())
+                path = lfutil.findfile(rsrc, hash)
+
+                if path is None:
+                    raise util.Abort(
+                        _("missing largefile \'%s\' from revision %s")
+                         % (f, node.hex(ctx.node())))
+
             data = ''
             fd = None
             try:

tests/test-lfconvert.t

@@ -1,3 +1,5
+  $ USERCACHE="$TESTTMP/cache"; export USERCACHE
+  $ mkdir "${USERCACHE}"
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > largefiles =
@@ -8,6 +10,7
   > minsize = 0.5
   > patterns = **.other
   >     **.dat
+  > usercache=${USERCACHE}
   > EOF
 
 "lfconvert" works
@@ -270,3 +273,14 round-trip: converting back to a normal
   pass
 
   $ cd ..
+
+Avoid a traceback if a largefile isn't available (issue3519)
+
+  $ hg clone -U largefiles-repo issue3519
+  $ rm "${USERCACHE}"/*
+  $ hg lfconvert --to-normal issue3519 normalized3519
+  initializing destination normalized3519
+  abort: missing largefile 'large' from revision d4892ec57ce212905215fad1d9018f56b99202ad
+  [255]
+
+