[v2] sequencer: break out of loop explicitly
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Message ID 20181030080937.11118-1-martin.agren@gmail.com
State New
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  • [v2] sequencer: break out of loop explicitly
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Martin Ågren Oct. 30, 2018, 8:09 a.m. UTC
It came up in review [1, 2] that this non-idiomatic loop is a bit tricky.
When we find a space, we set `len = i`, which gives us the answer we are
looking for, but which also breaks out of the loop.

It turns out that this loop can confuse compilers as well. My copy of
gcc 7.3.0 realizes that we are essentially evaluating `(len + 1) < len`
and warns that the behavior is undefined if `len` is `INT_MAX`. (Because
the assignment `len = i` is guaranteed to decrease `len`, such undefined
behavior is not actually possible.)

Rewrite the loop to a more idiomatic variant which doesn't muck with
`len` in the loop body. That should help compilers and human readers
figure out what is going on here. But do note that we need to update
`len` since it is not only used just after this loop (where we could
have used `i` directly), but also later in this function.

While at it, reduce the scope of `i`.

[1] https://public-inbox.org/git/CAPig+cQbG2s-LrAo9+7C7=dXifbWFJ3SzuNa-QePHDk7egK=jg@mail.gmail.com/

[2] https://public-inbox.org/git/CAPig+cRjU6niXpT2FrDWZ0x1HmGf1ojVZj3uk2qXEGe-S7i_HQ@mail.gmail.com/

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
 Thanks for the comments on v1. Based on them, I decided to go for
 Eric's option 2, and to make the log message less technical in favor of
 "compilers and humans alike can get this wrong".

 sequencer.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Comments

Johannes Schindelin Oct. 31, 2018, 2:54 p.m. UTC | #1
Hi Martin,

On Tue, 30 Oct 2018, Martin Ågren wrote:

> It came up in review [1, 2] that this non-idiomatic loop is a bit tricky.
> When we find a space, we set `len = i`, which gives us the answer we are
> looking for, but which also breaks out of the loop.
> 
> It turns out that this loop can confuse compilers as well. My copy of
> gcc 7.3.0 realizes that we are essentially evaluating `(len + 1) < len`
> and warns that the behavior is undefined if `len` is `INT_MAX`. (Because
> the assignment `len = i` is guaranteed to decrease `len`, such undefined
> behavior is not actually possible.)
> 
> Rewrite the loop to a more idiomatic variant which doesn't muck with
> `len` in the loop body. That should help compilers and human readers
> figure out what is going on here. But do note that we need to update
> `len` since it is not only used just after this loop (where we could
> have used `i` directly), but also later in this function.
> 
> While at it, reduce the scope of `i`.
> 
> [1] https://public-inbox.org/git/CAPig+cQbG2s-LrAo9+7C7=dXifbWFJ3SzuNa-QePHDk7egK=jg@mail.gmail.com/
> 
> [2] https://public-inbox.org/git/CAPig+cRjU6niXpT2FrDWZ0x1HmGf1ojVZj3uk2qXEGe-S7i_HQ@mail.gmail.com/
> 
> Helped-by: Eric Sunshine <sunshine@sunshineco.com>
> Signed-off-by: Martin Ågren <martin.agren@gmail.com>
> ---

ACK. Thanks for cleaning up after me,
Dscho

>  Thanks for the comments on v1. Based on them, I decided to go for
>  Eric's option 2, and to make the log message less technical in favor of
>  "compilers and humans alike can get this wrong".
> 
>  sequencer.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/sequencer.c b/sequencer.c
> index 0c164d5f98..e7aa4d5020 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -2829,7 +2829,7 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
>  	struct tree_desc desc;
>  	struct tree *tree;
>  	struct unpack_trees_options unpack_tree_opts;
> -	int ret = 0, i;
> +	int ret = 0;
>  
>  	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
>  		return -1;
> @@ -2849,10 +2849,13 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
>  		}
>  		oidcpy(&oid, &opts->squash_onto);
>  	} else {
> +		int i;
> +
>  		/* Determine the length of the label */
>  		for (i = 0; i < len; i++)
>  			if (isspace(name[i]))
> -				len = i;
> +				break;
> +		len = i;
>  
>  		strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
>  		if (get_oid(ref_name.buf, &oid) &&
> -- 
> 2.19.1.593.gc670b1f876.dirty
> 
>
Eric Sunshine Oct. 31, 2018, 5:28 p.m. UTC | #2
On Wed, Oct 31, 2018 at 10:54 AM Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> On Tue, 30 Oct 2018, Martin Ågren wrote:
> > Rewrite the loop to a more idiomatic variant which doesn't muck with
> > `len` in the loop body. That should help compilers and human readers
> > figure out what is going on here. But do note that we need to update
> > `len` since it is not only used just after this loop (where we could
> > have used `i` directly), but also later in this function.
> >
> > Signed-off-by: Martin Ågren <martin.agren@gmail.com>
> > ---
>
> ACK. Thanks for cleaning up after me,

Looks good to me, as well. Thanks for working on it.
Martin Ågren Oct. 31, 2018, 6:41 p.m. UTC | #3
On Wed, 31 Oct 2018 at 18:28, Eric Sunshine <sunshine@sunshineco.com> wrote:
>
> On Wed, Oct 31, 2018 at 10:54 AM Johannes Schindelin
> <Johannes.Schindelin@gmx.de> wrote:

> > ACK. Thanks for cleaning up after me,
>
> Looks good to me, as well. Thanks for working on it.

Thanks, both of you.

Martin

Patch
diff mbox series

diff --git a/sequencer.c b/sequencer.c
index 0c164d5f98..e7aa4d5020 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2829,7 +2829,7 @@  static int do_reset(const char *name, int len, struct replay_opts *opts)
 	struct tree_desc desc;
 	struct tree *tree;
 	struct unpack_trees_options unpack_tree_opts;
-	int ret = 0, i;
+	int ret = 0;
 
 	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
 		return -1;
@@ -2849,10 +2849,13 @@  static int do_reset(const char *name, int len, struct replay_opts *opts)
 		}
 		oidcpy(&oid, &opts->squash_onto);
 	} else {
+		int i;
+
 		/* Determine the length of the label */
 		for (i = 0; i < len; i++)
 			if (isspace(name[i]))
-				len = i;
+				break;
+		len = i;
 
 		strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 		if (get_oid(ref_name.buf, &oid) &&