wrapper: add function to compare strings with different NUL termination

bk2204 · gitster · commit 14570dc67d2a · 2020-05-27T10:07:06.000-07:00
When parsing capabilities for the pack protocol, there are times we'll
want to compare the value of a capability to a NUL-terminated string.
Since the data we're reading will be space-terminated, not
NUL-terminated, we need a function that compares the two strings, but
also checks that they're the same length.  Otherwise, if we used strncmp
to compare these strings, we might accidentally accept a parameter that
was a prefix of the expected value.

Add a function, xstrncmpz, that takes a NUL-terminated string and a
non-NUL-terminated string, plus a length, and compares them, ensuring
that they are the same length.

Signed-off-by: brian m. carlson &lt;sandals@crustytoothpaste.net&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git a/git-compat-util.h b/git-compat-util.h
@@ -868,6 +868,12 @@ char *xgetcwd(void);
 FILE *fopen_for_writing(const char *path);
 FILE *fopen_or_warn(const char *path, const char *mode);
 
+/*
+ * Like strncmp, but only return zero if s is NUL-terminated and exactly len
+ * characters long.  If it is not, consider it greater than t.
+ */
+int xstrncmpz(const char *s, const char *t, size_t len);
+
 /*
  * FREE_AND_NULL(ptr) is like free(ptr) followed by ptr = NULL. Note
  * that ptr is used twice, so don't pass e.g. ptr++.
diff --git a/wrapper.c b/wrapper.c
@@ -105,6 +105,14 @@ char *xstrndup(const char *str, size_t len)
 	return xmemdupz(str, p ? p - str : len);
 }
 
+int xstrncmpz(const char *s, const char *t, size_t len)
+{
+	int res = strncmp(s, t, len);
+	if (res)
+		return res;
+	return s[len] == '\0' ? 0 : 1;
+}
+
 void *xrealloc(void *ptr, size_t size)
 {
 	void *ret;

Original file line number	Diff line number	Diff line change
`@@ -105,6 +105,14 @@ char xstrndup(const char str, size_t len)`
`105`	`105`	`return xmemdupz(str, p ? p - str : len);`
`106`	`106`	`}`
`107`	`107`
	`108`	`+int xstrncmpz(const char s, const char t, size_t len)`
	`109`	`+{`
	`110`	`+ int res = strncmp(s, t, len);`
	`111`	`+ if (res)`
	`112`	`+ return res;`
	`113`	`+ return s[len] == '\0' ? 0 : 1;`
	`114`	`+}`
	`115`	`+`
`108`	`116`	`void xrealloc(void ptr, size_t size)`
`109`	`117`	`{`
`110`	`118`	`void *ret;`