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[PATCH] sequencer: clarify intention to break out of loop




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 through these steps:

  1. `len = i`

  2. `i = i + 1`

  3. Is `i < len`? No, so break out.

Since `i` is signed, step 2 is undefined if `i` has the value `INT_MAX`.
It can't actually have that value, but that doesn't stop my copy of gcc
7.3.0 from throwing the following:

> sequencer.c:2853:3: error: assuming signed overflow does not occur when
> assuming that (X + c) < X is always false [-Werror=strict-overflow]
>    for (i = 0; i < len; i++)
>    ^~~

That is, the compiler has realized that the code is essentially
evaluating "(len + 1) < len" and that for `len = INT_MAX`, this is
undefined behavior. What it hasn't figured out is that if `i` and `len`
are both `INT_MAX` after step 1, then `len` must have had a value larger
than `INT_MAX` before that step, which it can't have had.

Let's be explicit about breaking out of the loop. This helps the
compiler grok our intention. As a bonus, it might make it (even) more
obvious to human readers that the loop stops at the first space.

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

Signed-off-by: Martin Ågren <martin.agren@xxxxxxxxx>
---
 sequencer.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 0c164d5f98..a351638ad9 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 +2849,14 @@ 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]))
+		for (i = 0; i < len; i++) {
+			if (isspace(name[i])) {
 				len = i;
+				break;
+			}
+		}
 
 		strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 		if (get_oid(ref_name.buf, &oid) &&
-- 
2.19.1.593.gc670b1f876.dirty