Re: [PATCH v2 3/4] grep: copy struct in one fell swoop

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 24, 2020, 22:34 UTC
Message-ID: <xmqqsg8ygza7.fsf@gitster.c.googlers.com>
In-Reply-To: <359355fb4eff6d99cb1baad9b72ff96e7dcda51d.1606251358.git.martin.agren@gmail.com>

Martin Ågren <martin.agren@gmail.com> writes:

Show 8 quoted lines

> We have a `struct grep_opt` with our defaults which we then copy into
> the caller's struct. Rather than zeroing the target struct and copying
> each element one by one, just copy everything at once. This leaves the
> code simpler and more maintainable.
>
> We don't have any ownership issues with what we're copying now and can
> just greedily copy the whole thing. If and when we do need to handle
> such elements (`char *`?), we must and can handle it appropriately.

That is correct, but ...

> This
> commit doesn't really change that.

... I suspect this is not.

In the original code, those who are adding a new field would notice that it is not copied over to the new instance (because they didn't add anything to grep_init() to copy the field) and at that point they must stop and think how the new field need to be copied.

The structure assignment of the outer shell done in this patch means they are robbed of the opportunity to stop and think, because most of the time it "works" out of the box. I'd feel safer if we left a clue to future developers if we were to do your clean-up, perhaps like:

diff --git c/grep.h w/grep.h
index b5c4e223a8..388d226da3 100644
--- c/grep.h
+++ w/grep.h
@@ -115,6 +115,14 @@ struct grep_expr {
 	} u;
 };
 
+/*
+ * grep_config() initializes one "default" instance of this type, and
+ * it is copied by grep_init() to be used by each individual
+ * invocation.  When adding a new field to this structure that is
+ * populated from the configuration, be sure to think about ownership
+ * (i.e. a shallow copy may not be what you want for the type of your
+ * newly added field).
+ */
 struct grep_opt {
 	struct grep_pat *pattern_list;
 	struct grep_pat **pattern_tail;