git-compat-util: introduce skip_to_optional_arg()

We often accept both a "--key" option and a "--key=<val>" option.

These options currently are parsed using something like:

if (!strcmp(arg, "--key")) {
	/* do something */
} else if (skip_prefix(arg, "--key=", &arg)) {
	/* do something with arg */
}

which is a bit cumbersome compared to just:

if (skip_to_optional_arg(arg, "--key", &arg)) {
	/* do something with arg */
}

This also introduces skip_to_optional_arg_default() for the few
cases where something different should be done when the first
argument is exactly "--key" than when it is exactly "--key=".

In general it is better for UI consistency and simplicity if
"--key" and "--key=" do the same thing though, so that using
skip_to_optional_arg() should be encouraged compared to
skip_to_optional_arg_default().

Note that these functions can be used to parse any "key=value"
string where "key" is also considered as valid, not just
command line options.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -484,6 +484,29 @@ static inline int skip_prefix(const char *str, const char *prefix,
 	return 0;
 }
 
+/*
+ * If the string "str" is the same as the string in "prefix", then the "arg"
+ * parameter is set to the "def" parameter and 1 is returned.
+ * If the string "str" begins with the string found in "prefix" and then a
+ * "=" sign, then the "arg" parameter is set to "str + strlen(prefix) + 1"
+ * (i.e., to the point in the string right after the prefix and the "=" sign),
+ * and 1 is returned.
+ *
+ * Otherwise, return 0 and leave "arg" untouched.
+ *
+ * When we accept both a "--key" and a "--key=<val>" option, this function
+ * can be used instead of !strcmp(arg, "--key") and then
+ * skip_prefix(arg, "--key=", &arg) to parse such an option.
+ */
+int skip_to_optional_arg_default(const char *str, const char *prefix,
+				 const char **arg, const char *def);
+
+static inline int skip_to_optional_arg(const char *str, const char *prefix,
+				       const char **arg)
+{
+	return skip_to_optional_arg_default(str, prefix, arg, "");
+}
+
 /*
  * Like skip_prefix, but promises never to read past "len" bytes of the input
  * buffer, and returns the remaining number of bytes in "out" via "outlen".

strbuf.c

@@ -11,6 +11,28 @@ int starts_with(const char *str, const char *prefix)
 			return 0;
 }
 
+int skip_to_optional_arg_default(const char *str, const char *prefix,
+				 const char **arg, const char *def)
+{
+	const char *p;
+
+	if (!skip_prefix(str, prefix, &p))
+		return 0;
+
+	if (!*p) {
+		if (arg)
+			*arg = def;
+		return 1;
+	}
+
+	if (*p != '=')
+		return 0;
+
+	if (arg)
+		*arg = p + 1;
+	return 1;
+}
+
 /*
  * Used as the default ->buf value, so that people can always assume
  * buf is non NULL and ->buf is NUL terminated even for a freshly