bug#25351: 26.0.50; Testcover doesn't work on code that creates simple lists and modifies them

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bug#25351: 26.0.50; Testcover doesn't work on code that creates simple lists and modifies them

Gemini Lasswell
Testcover marks forms which have only returned a single value while under
code coverage with a tan splotch to indicate insufficient test coverage,
It also identifies forms which will only ever return a single value and
suppresses the splotch on those. During the coverage run, it saves a
reference to the return value of those single value forms, and the next
time the form is evaluated, it uses `equal' to determine if the value
has changed, and if it has, Testcover reports an error.

The problem with this is that it is a common idiom to create a data
structure using an expression that always returns the same thing, and
then modify it later. Since Testcover keeps a reference not a copy, it
ends up with a reference to a modified object, which fails the
comparison with the next return value of the same expression.

For example:

(defun my-blank-record ()
  (list (list 'name) (list 'age)))

(defun my-make-record (name age)
  (let ((rec (my-blank-record)))
    (setf (cdr (assq 'name rec)) name)
    (setf (cdr (assq 'age rec)) age)
    rec))

(my-make-record "Jack" 10)
(my-make-record "Jill" 12)

To reproduce this problem, save the above code in a file called bug.el,
and then:

M-x testcover-start RET bug.el RET

The result will be testcover-1value's "Value of form marked with
‘1value’ does vary" error.



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bug#25351: Patch for bug#25351: 26.0.50; Testcover doesn't work on code that creates simple lists and modifies them

Gemini Lasswell
Here is a patch which fixes this bug by making a copy of the result of
the first evaluation of a form to compare with the results of
subsequent evaluations.

This patch is based on the two patches I sent yesterday to bug#25316.

Because Testcover may encounter circular structures in the code it
instruments, I wrote a modified version of copy-tree which can handle
circular structures. It's possibly generally useful enough to belong
in subr.el alongside copy-tree instead of being an internal Testcover
function, but I will submit that idea as a separate wishlist item.

There isn't currently a version of `equal' which works for circular
structures, so Testcover's handling of them is not fully implemented,
but it now avoids stopping the code under coverage with an error
message.


0001-Stop-Testcover-from-producing-spurious-1value-errors.patch (7K) Download Attachment
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bug#25351: Patch for bug#25351: 26.0.50; Testcover doesn't work on code that creates simple lists and modifies them

Gemini Lasswell
I've pushed this patch to master.