[PATCH v3 0/4] add and apply a rule to find "unused" init+free

This series adds a coccinelle rule to find and remove code where the only reference to a variable in a given function is to malloc() & free() it, where "malloc" and "free" also match "strbuf_init/strbuf_release", and then later in the series anything that looks like a init/free pattern.

Changes since v2:

 * Make the wider rule revert-able, as requested by Junio in
   https://lore.kernel.org/git/xmqqsfnw65zu.fsf@gitster.g/

 * We now find and remove "malloc" followed by an optional "init" and
   "release".

 * We now match { 0 } initializers, in addition to things that look
   like "INIT" macros.

Ævar Arnfjörð Bjarmason (4):
  cocci: add and apply a rule to find "unused" strbufs
  cocci: catch unused "strbuf" using an xmalloc() pattern
  cocci: remove "when strict" from unused.cocci
  cocci: generalize "unused" rule to cover more than "strbuf"

 builtin/fetch.c                 |  3 +-
 builtin/merge.c                 |  4 --
 builtin/repack.c                |  2 -
 contrib/coccinelle/unused.cocci | 88 +++++++++++++++++++++++++++++++++
 contrib/scalar/scalar.c         |  3 +-
 diff.c                          |  2 -
 6 files changed, 90 insertions(+), 12 deletions(-)
 create mode 100644 contrib/coccinelle/unused.cocci

Range-diff against v2:
1:  d14036521ab ! 1:  49e9ccb5819 cocci: add and apply a rule to find "unused" variables
    @@ Metadata
     Author: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
      ## Commit message ##
    -    cocci: add and apply a rule to find "unused" variables
    +    cocci: add and apply a rule to find "unused" strbufs
     
    -    Add a coccinelle rule to remove variable initialization followed by
    -    calling a "release" function. See extensive commentary in the new
    -    "unused.cocci" for how it works, and what it's intended to find and
    -    replace.
    +    Add a coccinelle rule to remove "struct strbuf" initialization
    +    followed by calling "strbuf_release()" function.
    +
    +    See extensive commentary in the new "unused.cocci" for how it works,
    +    and what it's intended to find and replace.
     
         The inclusion of "contrib/scalar/scalar.c" is because "spatch" was
         manually run on it (we don't usually run spatch on contrib).
    @@ Commit message
         The use of "with strict" here will be explained and amended in the
         following commit.
     
    -    1. https://lore.kernel.org/git/042d624b8159364229e95d35e9309f12b67f8173.1652977582.git.gitgitgadget@gmail.com/
    -
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
      ## builtin/fetch.c ##
    @@ builtin/merge.c: static void merge_name(const char *remote, struct strbuf *msg)
     
      ## contrib/coccinelle/unused.cocci (new) ##
     @@
    -+// This rule finds sequences of "unused" declerations, init and
    -+// release(). E.g.:
    ++// This rule finds sequences of "unused" declerations and uses of
    ++// "struct strbuf".
    ++//
    ++// I.e. this finds cases where we only declare the variable, and then
    ++// release it, e.g.:
     +//
     +//	struct strbuf buf = STRBUF_INIT;
     +//      [.. no other use of "buf" in the function ..]
     +//	strbuf_release(&buf)
     +//
    ++// Or:
    ++//
    ++//	struct strbuf buf;
    ++//	[.. no other use of "buf" in the function ..]
    ++//	strbuf_init(&buf, 0);
    ++//	[.. no other use of "buf" in the function ..]
    ++//	strbuf_release(&buf)
    ++//
     +// To do do this we find (continued below)...
     +@@
     +type T;
     +identifier I;
    -+// STRBUF_INIT, but also e.g. STRING_LIST_INIT_DUP (so no anchoring)
    -+constant INIT =~ "_INIT";
    -+// I = get_worktrees() etc.
    -+identifier INIT_ASSIGN1 =~ "^get_worktrees$";
    ++// STRBUF_INIT
    ++constant INIT_MACRO =~ "^STRBUF_INIT$";
     +// strbuf_init(&I, ...) etc.
    -+identifier INIT_CALL1 =~ "^[a-z_]*_init$";
    -+// stbuf_release(), string_list_clear() etc.
    -+identifier REL1 =~ "^[a-z_]*_(release|clear|free)$";
    -+// release_patch(), clear_pathspec() etc.
    -+identifier REL2 =~ "^(release|clear|free)_[a-z_]*$";
    ++identifier INIT_CALL1 =~ "^strbuf_init$";
    ++// strbuf_release()
    ++identifier REL1 =~ "^strbuf_release$";
     +@@
     +
     +// .. A declaration like "struct strbuf buf;"...
     +(
     +- T I;
    ++// ... or "struct strbuf buf = { 0 };" ...
    ++|
    ++- T I = { 0 };
     +// ... or "struct STRBUF buf = STRBUF_INIT;" ...
     +|
    -+- T I = INIT;
    ++- T I = INIT_MACRO;
     +)
     +
     +// ... Optionally followed by lines that make no use of "buf", "&buf"
    @@ contrib/coccinelle/unused.cocci (new)
     +     when strict
     +// .. (only) make use of "buf" or "&buf" to call something like
     +// "strbuf_init(&buf, ...)" ...
    -+(
     +- \( INIT_CALL1 \)( \( I \| &I \), ...);
    -+|
    -+// .. or e.g. "worktrees = get_worktrees();", i.e. a known "assignment
    -+// init" ...
    -+- I = \( INIT_ASSIGN1 \)(...);
    -+)
     +...>
     +
     +// ... and then no mention of "buf" or "&buf" until we get to a
     +// strbuf_release(&buf) at the end ...
    -+(
    -+- \( REL1 \| REL2 \)( \( I \| &I \), ...);
    -+|
    -+- \( REL1 \| REL2 \)( \( &I \| I \) );
    -+)
    ++- \( REL1 \)( \( &I \| I \) );
     +// ... and no use *after* either, e.g. we don't want to delete
     +// init/strbuf_release() patterns, where "&buf" could be used
     +// afterwards.
     +  ... when != \( I \| &I \)
     +      when strict
    -+// Note that we're intentionally loose in accepting e.g. a
    -+// "strbuf_init(&buf)" followed by a "string_list_clear(&buf,
    -+// 0)". It's assumed that the compiler will catch any such invalid
    -+// code, i.e. that our constructors/destructors don't take a "void *".
    -+//
     +// This rule also isn't capable of finding cases where &buf is used,
     +// but only to e.g. pass that variable to a static function which
     +// doesn't use it. The analysis is only function-local.
-:  ----------- > 2:  6324d3956ed cocci: catch unused "strbuf" using an xmalloc() pattern
2:  4130dc15287 ! 3:  9a5e7208dec cocci: remove "when strict" from unused.cocci
    @@ builtin/merge.c: static void restore_state(const struct object_id *head,
      }
      
     
    - ## builtin/repack.c ##
    -@@ builtin/repack.c: int cmd_repack(int argc, const char **argv, const char *prefix)
    - 	struct child_process cmd = CHILD_PROCESS_INIT;
    - 	struct string_list_item *item;
    - 	struct string_list names = STRING_LIST_INIT_DUP;
    --	struct string_list rollback = STRING_LIST_INIT_NODUP;
    - 	struct string_list existing_nonkept_packs = STRING_LIST_INIT_DUP;
    - 	struct string_list existing_kept_packs = STRING_LIST_INIT_DUP;
    - 	struct pack_geometry *geometry = NULL;
    -@@ builtin/repack.c: int cmd_repack(int argc, const char **argv, const char *prefix)
    - 	}
    - 
    - 	string_list_clear(&names, 0);
    --	string_list_clear(&rollback, 0);
    - 	string_list_clear(&existing_nonkept_packs, 0);
    - 	string_list_clear(&existing_kept_packs, 0);
    - 	clear_pack_geometry(geometry);
    -
      ## contrib/coccinelle/unused.cocci ##
    -@@ contrib/coccinelle/unused.cocci: identifier REL2 =~ "^(release|clear|free)_[a-z_]*$";
    +@@ contrib/coccinelle/unused.cocci: identifier REL1 =~ "^strbuf_release$";
      // ... Optionally followed by lines that make no use of "buf", "&buf"
      // etc., but which ...
      <... when != \( I \| &I \)
     -     when strict
    + (
      // .. (only) make use of "buf" or "&buf" to call something like
      // "strbuf_init(&buf, ...)" ...
    - (
    -@@ contrib/coccinelle/unused.cocci: identifier REL2 =~ "^(release|clear|free)_[a-z_]*$";
    +@@ contrib/coccinelle/unused.cocci: identifier REL1 =~ "^strbuf_release$";
      // init/strbuf_release() patterns, where "&buf" could be used
      // afterwards.
        ... when != \( I \| &I \)
     -      when strict
    - // Note that we're intentionally loose in accepting e.g. a
    - // "strbuf_init(&buf)" followed by a "string_list_clear(&buf,
    - // 0)". It's assumed that the compiler will catch any such invalid
    + // This rule also isn't capable of finding cases where &buf is used,
    + // but only to e.g. pass that variable to a static function which
    + // doesn't use it. The analysis is only function-local.
-:  ----------- > 4:  45a429b9cc9 cocci: generalize "unused" rule to cover more than "strbuf"