[PATCH] coccinelle: avoid wrong transformation suggestions from commit.cocci

[PATCH] coccinelle: avoid wrong transformation suggestions from commit.cocci

[SZEDER Gábor]

The semantic patch 'contrib/coccinelle/commit.cocci' added in
2e27bd7731 (treewide: replace maybe_tree with accessor methods,
2018-04-06) is supposed to "ensure that all references to the
'maybe_tree' member of struct commit are either mutations or accesses
through get_commit_tree()".  So get_commit_tree() clearly must be able
to directly access the 'maybe_tree' member, and 'commit.cocci' has a
bit of a roundabout workaround to ensure that get_commit_tree()'s
direct access in its return statement is not transformed: after all
references to 'maybe_tree' have been transformed to a call to
get_commit_tree(), including the reference in get_commit_tree()
itself, the last rule transforms back a 'return get_commit_tree()'
statement, back then found only in get_commit_tree() itself, to a
direct access.

Unfortunately, already the very next commit shows that this workaround
is insufficient: 7b8a21dba1 (commit-graph: lazy-load trees for
commits, 2018-04-06) extends get_commit_tree() with a condition
directly accessing the 'maybe_tree' member, and Coccinelle with
'commit.cocci' promptly detects it and suggests a transformation to
avoid it.  This transformation is clearly wrong, because calling
get_commit_tree() to access 'maybe_tree' _in_ get_commit_tree() would
obviously lead to recursion.  Furthermore, the same commit added
another, more specialized getter function get_commit_tree_in_graph(),
whose legitimate direct access to 'maybe_tree' triggers a similar
wrong transformation suggestion.

Exclude both of these getter functions from the general rule in
'commit.cocci' that matches their direct accesses to 'maybe_tree'.
Also exclude load_tree_for_commit(), which, as static helper funcion
of get_commit_tree_in_graph(), has legitimate direct access to
'maybe_tree' as well.

The last rule transforming back 'return get_commit_tree()' statements
to direct accesses thus became unnecessary, remove it.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 contrib/coccinelle/commit.cocci | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/contrib/coccinelle/commit.cocci b/contrib/coccinelle/commit.cocci
index ac38525941..a7e9215ffc 100644
--- a/contrib/coccinelle/commit.cocci
+++ b/contrib/coccinelle/commit.cocci
@@ -10,11 +10,15 @@ expression c;
 - c->maybe_tree->object.oid.hash
 + get_commit_tree_oid(c)->hash
 
+// These excluded functions must access c->maybe_tree direcly.
 @@
+identifier f !~ "^(get_commit_tree|get_commit_tree_in_graph|load_tree_for_commit)$";
 expression c;
 @@
+  f(...) {...
 - c->maybe_tree
 + get_commit_tree(c)
+  ...}
 
 @@
 expression c;
@@ -22,9 +26,3 @@ expression s;
 @@
 - get_commit_tree(c) = s
 + c->maybe_tree = s
-
-@@
-expression c;
-@@
-- return get_commit_tree(c);
-+ return c->maybe_tree;
-- 
2.17.0.551.g86756ed296

Re: [PATCH] coccinelle: avoid wrong transformation suggestions from commit.cocci

[Derrick Stolee]

On 4/30/2018 5:31 AM, SZEDER Gábor wrote:
> The semantic patch 'contrib/coccinelle/commit.cocci' added in
> 2e27bd7731 (treewide: replace maybe_tree with accessor methods,
> 2018-04-06) is supposed to "ensure that all references to the
> 'maybe_tree' member of struct commit are either mutations or accesses
> through get_commit_tree()".  So get_commit_tree() clearly must be able
> to directly access the 'maybe_tree' member, and 'commit.cocci' has a
> bit of a roundabout workaround to ensure that get_commit_tree()'s
> direct access in its return statement is not transformed: after all
> references to 'maybe_tree' have been transformed to a call to
> get_commit_tree(), including the reference in get_commit_tree()
> itself, the last rule transforms back a 'return get_commit_tree()'
> statement, back then found only in get_commit_tree() itself, to a
> direct access.
>
> Unfortunately, already the very next commit shows that this workaround
> is insufficient: 7b8a21dba1 (commit-graph: lazy-load trees for
> commits, 2018-04-06) extends get_commit_tree() with a condition
> directly accessing the 'maybe_tree' member, and Coccinelle with
> 'commit.cocci' promptly detects it and suggests a transformation to
> avoid it.  This transformation is clearly wrong, because calling
> get_commit_tree() to access 'maybe_tree' _in_ get_commit_tree() would
> obviously lead to recursion.  Furthermore, the same commit added
> another, more specialized getter function get_commit_tree_in_graph(),
> whose legitimate direct access to 'maybe_tree' triggers a similar
> wrong transformation suggestion.

Thanks for catching this, Szeder. Sorry for the noise.

> Exclude both of these getter functions from the general rule in
> 'commit.cocci' that matches their direct accesses to 'maybe_tree'.
> Also exclude load_tree_for_commit(), which, as static helper funcion
> of get_commit_tree_in_graph(), has legitimate direct access to
> 'maybe_tree' as well.

This is an interesting feature of Coccinelle. Happy to learn it.

> The last rule transforming back 'return get_commit_tree()' statements
> to direct accesses thus became unnecessary, remove it.
>
> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>

I applied this locally on 'next' and ran the check. I succeeded with no 
changes.

Thanks!

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>


> ---
>   contrib/coccinelle/commit.cocci | 10 ++++------
>   1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/contrib/coccinelle/commit.cocci b/contrib/coccinelle/commit.cocci
> index ac38525941..a7e9215ffc 100644
> --- a/contrib/coccinelle/commit.cocci
> +++ b/contrib/coccinelle/commit.cocci
> @@ -10,11 +10,15 @@ expression c;
>   - c->maybe_tree->object.oid.hash
>   + get_commit_tree_oid(c)->hash
>   
> +// These excluded functions must access c->maybe_tree direcly.
>   @@
> +identifier f !~ "^(get_commit_tree|get_commit_tree_in_graph|load_tree_for_commit)$";
>   expression c;
>   @@
> +  f(...) {...
>   - c->maybe_tree
>   + get_commit_tree(c)
> +  ...}
>   
>   @@
>   expression c;
> @@ -22,9 +26,3 @@ expression s;
>   @@
>   - get_commit_tree(c) = s
>   + c->maybe_tree = s
> -
> -@@
> -expression c;
> -@@
> -- return get_commit_tree(c);
> -+ return c->maybe_tree;