This Engineering Notebook post discusses subtle problems with Leo's
paste-outline commands: paste-node, paste-retaining-clones, and
paste-as-template. A new unit test (in PR #3473
<https://github.com/leo-editor/leo-editor/pull/3473>) revealed these
problems. They may be ancient.
This post will be of interest only to hard-core Leo devs. Feel free to
ignore it. *Note:* Remember that I'll be discussing code in the *new *(6.7.5)
codebase.
*A subtle bug*
Dumps in a new unit test reveal that *c.recompute_all_parents* fails:
def recompute_all_parents(self) -> None:
"""
Recompute all v.parents arrays using neither positions
nor v.parents ivars.
"""
c = self
root_v = c.hiddenRootNode
# Clear all v.parents arrays.
root_v.parents = []
for v in c.alt_all_unique_nodes():
v.parents = []
# Loop invariant: Visit each *parent* vnode *once*.
# Child vnodes may be visited more than once.
# Visit the hidden root.
for child in root_v.children:
child.parents.append(root_v)
# Visit all other parents.
for parent in c.alt_all_unique_nodes():
for child in parent.children:
child.parents.append(parent)
How can this "obviously correct" code fail? What's going on??
Aha! c.alt_all_unique_nodes() can yield nodes with different ids than those
in root_v.children. In other words, *two distinct VNodes may share the same
gnx*. Big oops.
*The cause of the bug*
Leo calls *c.reassignNonClonedIndices* after calling *c.unarchive_to_vnode*.
But unarchive_to_vnode *itself* creates (or patches) parent/child links.
In short: unarchive_to_vnode must reallocate gnxs.
*fc.gnxDict*
Lurking behind all this code is the infamous *fc.gnxDict* dictionary. Keys
are gnxs; values are the corresponding VNodes. *c.recreateGnxDict*
*supposedly* recreates this dict. Various read/write/paste helpers save and
restore this dict for impossible-to-remember reasons.
I would love to remove this dict, but doing so probably won't work. Because
of undo, *Leo never deallocates VNodes!* Instead, I am leaning towards
replacing fc.gnxDict with, say, *c.all_gnxs_dict*. Leo will:
- only *add* keys to this dict.
- never *delete keys.*
*- *never save/restore this dict.
Only a detailed examination of Leo's entire codebase will show whether this
plan is feasible.
*Summary*
Let this post be a warning for anyone foolish enough to assert that some
code is "obviously correct."
It's impossible to say whether the bugs discussed here exist in Leo's
existing codebase without adding new unit tests to devel. I'm not going to
do that. It's too late to change Leo 6.7.4.
All calls to *c.recreateGnxDict* seem dubious. PR #3473
<https://github.com/leo-editor/leo-editor/pull/3473> is an opportunity to
re-engineer how Leo remembers existing VNodes.
All three paste-outline commands *and* their undo/redo helpers must likely
confront these issues explicitly. "Clever" solutions will likely fail.
Edward
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