[PATCH] git-compat-util.h: introduce CALLOC(x)

From: Taylor Blau <me@ttaylorr.com>
Date: Dec 5, 2022, 18:54 UTC
Message-ID: <6694c52b38674859eb0390c7f62da1209a8d8ec3.1670266373.git.me@ttaylorr.com>

When zero-initializing a struct without the use of static initializers like "{ 0 }", it is common to write one of:

    T *ptr = xcalloc(1, sizeof(T));
    T *ptr = xcalloc(1, sizeof(*ptr));

These correctly initialize "*ptr" to the all-zeros value, but have a couple of drawbacks. Notably, both initializations are verbose, but the former is a foot-gun. If "*ptr"'s type changes to something other than "T", the programmer has to remember to update not only the type of the variable, but the right-hand side of its initialization, too.

In git.git, it is sometimes common to write something like:

    T *ptr;
    CALLOC_ARRAY(ptr, 1);

...but that is confusing, since we're not initializing an array. Rather, we're using the CALLOC_ARRAY() macro to pretend like we are. But since the array length is 1, the effect is zero initializing a single element.

Introduce a new variant, CALLOC(x), which initializes "x" to the all-zeros value, without exposing the confusing use of CALLOC_ARRAY(), or the foot-guns available when using xcalloc() directly.

Write a Coccinelle patch which codifies these rules, but mark it as "pending" since the resulting diff is too large to include in a single patch:

    $ git apply .build/contrib/coccinelle/xcalloc.pending.cocci.patch
    $ git diff --shortstat
     89 files changed, 221 insertions(+), 178 deletions(-)

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
This is a follow-up on [1], where introducing CALLOC(x) as the preferred
alternative to CALLOC_ARRAY(x, 1) was first suggested.

The patch is straightforward, and I am pretty sure that the Coccinelle rules are right, too ;-). But as a first-time Coccinelle user, any extra scrutiny on those bits would be appreciated.

The main point of discussion I think is whether we should consider adopting this rule. I am biased, of course, but I think that we should.

In any case, we should focus our efforts on polishing v2.39.0, but I wanted to send this out to the list before I forgot about it.

[1]: https://lore.kernel.org/git/Y1MrXoobkVKngYL1@coredump.intra.peff.net/

 contrib/coccinelle/xcalloc.pending.cocci | 21 +++++++++++++++++++++
 git-compat-util.h                        |  8 ++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 contrib/coccinelle/xcalloc.pending.cocci

diff --git a/contrib/coccinelle/xcalloc.pending.cocci b/contrib/coccinelle/xcalloc.pending.cocci
new file mode 100644
index 0000000000..83e4ca1a68
--- /dev/null
+++ b/contrib/coccinelle/xcalloc.pending.cocci
@@ -0,0 +1,21 @@
+@@
+type T;
+T *ptr;
+@@
+- ptr = xcalloc(1, \( sizeof(T) \| sizeof(*ptr) \) )
++ CALLOC(ptr)
+
+@@
+type T;
+identifier ptr;
+@@
+- T ptr = xcalloc(1, \( sizeof(T) \| sizeof(*ptr) \) );
++ T ptr;
++ CALLOC(ptr);
+
+@@
+type T;
+T *ptr;
+@@
+- CALLOC_ARRAY(ptr, 1)
++ CALLOC(ptr)
diff --git a/git-compat-util.h b/git-compat-util.h
index a76d0526f7..827e5be89c 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1107,6 +1107,14 @@ static inline void move_array(void *dst, const void *src, size_t n, size_t size)
 		memmove(dst, src, st_mult(size, n));
 }

+/*
+ * Like CALLOC_ARRAY, but the argument is treated as a pointer to a
+ * single struct.
+ *
+ * Preferable over xcalloc(1, sizeof(...)), or CALLOC_ARRAY(..., 1).
+ */
+#define CALLOC(x) (x) = CALLOC_ARRAY((x), 1)
+
 /*
  * These functions help you allocate structs with flex arrays, and copy
  * the data directly into the array. For example, if you had:
--
2.38.0.16.g393fd4c6db