config: improve diagnostic for "set" with missing value

"git config set pull.rebase=false" currently fails with "wrong
number of arguments", and the implicit form "git config
pull.rebase=false" fails with "invalid key". Neither points at
the real problem: the value is missing.

Report that directly, and when the argument has the shape
"<valid-key>=<value>", also suggest the split form:

    $ git config set pull.rebase=false
    error: missing value to set to the variable 'pull.rebase=false'
    hint: did you mean "git config set pull.rebase false"?

When the prefix before "=" is not a valid key, drop the hint:

    $ git config set foo=bar
    error: missing value to set to a variable with an invalid name 'foo=bar'

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>

Author: HaraldNordgren

builtin/config.c

@@ -1,6 +1,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 #include "builtin.h"
 #include "abspath.h"
+#include "advice.h"
 #include "config.h"
 #include "color.h"
 #include "date.h"
@@ -210,6 +211,33 @@ static void check_argc(int argc, int min, int max)
 	exit(129);
 }
 
+static int is_valid_key(const char *key)
+{
+	const char *last_dot = strrchr(key, '.');
+
+	return last_dot && isalpha(last_dot[1]);
+}
+
+static NORETURN void die_missing_set_value(const char *arg)
+{
+	const char *last_dot = strrchr(arg, '.');
+	const char *eq = last_dot ? strchr(last_dot + 1, '=') : NULL;
+	char *prefix = eq ? xstrndup(arg, eq - arg) : NULL;
+
+	if (prefix && is_valid_key(prefix)) {
+		error(_("missing value to set to the variable '%s'"), arg);
+		advise(_("did you mean \"git config set %s %s\"?"),
+		       prefix, eq + 1);
+	} else if (is_valid_key(arg)) {
+		error(_("missing value to set to the variable '%s'"), arg);
+	} else {
+		error(_("missing value to set to a variable with an invalid name '%s'"),
+		      arg);
+	}
+	free(prefix);
+	exit(129);
+}
+
 static void show_config_origin(const struct config_display_options *opts,
 			       const struct key_value_info *kvi,
 			       struct strbuf *buf)
@@ -1133,6 +1161,8 @@ static int cmd_config_set(int argc, const char **argv, const char *prefix,
 
 	argc = parse_options(argc, argv, prefix, opts, builtin_config_set_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
+	if (argc == 1)
+		die_missing_set_value(argv[0]);
 	check_argc(argc, 2, 2);
 
 	if ((flags & CONFIG_FLAGS_FIXED_VALUE) && !value_pattern)
@@ -1371,6 +1401,7 @@ static int cmd_config_actions(int argc, const char **argv, const char *prefix)
 	};
 	char *value = NULL, *comment = NULL;
 	int ret = 0;
+	int actions_implicit;
 	struct key_value_info default_kvi = KVI_INIT;
 
 	argc = parse_options(argc, argv, prefix, opts,
@@ -1385,7 +1416,8 @@ static int cmd_config_actions(int argc, const char **argv, const char *prefix)
 		exit(129);
 	}
 
-	if (actions == 0)
+	actions_implicit = (actions == 0);
+	if (actions_implicit)
 		switch (argc) {
 		case 1: actions = ACTION_GET; break;
 		case 2: actions = ACTION_SET; break;
@@ -1394,6 +1426,11 @@ static int cmd_config_actions(int argc, const char **argv, const char *prefix)
 			error(_("no action specified"));
 			exit(129);
 		}
+	if (actions_implicit && argc == 1) {
+		const char *last_dot = strrchr(argv[0], '.');
+		if (last_dot && strchr(last_dot + 1, '='))
+			die_missing_set_value(argv[0]);
+	}
 	if (display_opts.omit_values &&
 	    !(actions == ACTION_LIST || actions == ACTION_GET_REGEXP)) {
 		error(_("--name-only is only applicable to --list or --get-regexp"));

t/t1300-config.sh

@@ -469,6 +469,49 @@ test_expect_success 'invalid key' '
 	test_must_fail git config inval.2key blabla
 '
 
+test_expect_success 'set with 1 arg of "key=value": valid key suggests split form' '
+	test_must_fail git config set pull.rebase=false 2>err &&
+	test_grep "missing value to set to the variable .pull\\.rebase=false." err &&
+	test_grep "did you mean .git config set pull\\.rebase false." err
+'
+
+test_expect_success 'set with 1 arg of "key=value": implicit form suggests split form' '
+	test_must_fail git config pull.rebase=false 2>err &&
+	test_grep "missing value to set to the variable .pull\\.rebase=false." err &&
+	test_grep "did you mean .git config set pull\\.rebase false." err
+'
+
+test_expect_success 'set with 1 arg of "key=value": invalid key does not suggest split form' '
+	test_must_fail git config set foo=bar 2>err &&
+	test_grep "missing value to set to a variable with an invalid name .foo=bar." err &&
+	test_grep ! "did you mean" err
+'
+
+test_expect_success 'set with 1 arg: variable name starting with digit is invalid' '
+	test_must_fail git config set foo.1bar=baz 2>err &&
+	test_grep "missing value to set to a variable with an invalid name .foo\\.1bar=baz." err &&
+	test_grep ! "did you mean" err
+'
+
+test_expect_success 'set with 1 arg of valid key reports missing value' '
+	test_must_fail git config set pull.rebase 2>err &&
+	test_grep "missing value to set to the variable .pull\\.rebase." err &&
+	test_grep ! "did you mean" err
+'
+
+test_expect_success 'set with 2 args including "=" in invalid key does not suggest' '
+	test_must_fail git config set pull.rebase=false true 2>err &&
+	test_grep ! "did you mean" err
+'
+
+test_expect_success '"=" inside subsection is valid' '
+	test_when_finished "rm -f subsection.cfg" &&
+	git config set -f subsection.cfg foo.bar=baz.boo qux &&
+	echo qux >expect &&
+	git config get -f subsection.cfg foo.bar=baz.boo >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'correct key' '
 	git config 123456.a123 987
 '