manifest-cache: ignore IOError while writing stable

manifest-cache: ignore IOError while writing

If the wcache directory is non writable for some reason (eg: belong to root).
Trying to write to it currently crash Mercurial. Instead we ignore the error and
skip writing that cache.

We should probably improve the user experience of multiple users interacting
with the same repository. However this is not an adventure for stable.

Differential Revision: https://phab.mercurial-scm.org/D8512

--- a/mercurial/manifest.py	Wed May 06 11:40:17 2020 -0700
+++ b/mercurial/manifest.py	Sat May 09 20:25:07 2020 +0200
@@ -1455,18 +1455,29 @@
         if not self._dirty or self._opener is None:
             return
         # rotate backwards to the first used node
-        with self._opener(
-            self._file, b'w', atomictemp=True, checkambig=True
-        ) as fp:
-            node = self._head.prev
-            while True:
-                if node.key in self._cache:
-                    fp.write(node.key)
-                    fp.write(struct.pack(b'>L', len(node.value)))
-                    fp.write(node.value)
-                if node is self._head:
-                    break
-                node = node.prev
+        try:
+            with self._opener(
+                self._file, b'w', atomictemp=True, checkambig=True
+            ) as fp:
+                node = self._head.prev
+                while True:
+                    if node.key in self._cache:
+                        fp.write(node.key)
+                        fp.write(struct.pack(b'>L', len(node.value)))
+                        fp.write(node.value)
+                    if node is self._head:
+                        break
+                    node = node.prev
+        except IOError:
+            # We could not write the cache (eg: permission error)
+            # the content can be missing.
+            #
+            # We could try harder and see if we could recreate a wcache
+            # directory were we coudl write too.
+            #
+            # XXX the error pass silently, having some way to issue an error
+            # log `ui.log` would be nice.
+            pass
 
     def __len__(self):
         if not self._read: