[PATCH v2 1/4] builtin/config: introduce `--default`

[Taylor Blau <me@ttaylorr.com>]

For some use cases, callers of the `git-config(1)` builtin would like to
fallback to default values when the slot asked for does not exist. In
addition, users would like to use existing type specifiers to ensure
that values are parsed correctly when they do exist in the
configuration.

For example, to fetch a value without a type specifier and fallback to
`$fallback`, the following is required:

  $ git config core.foo || echo "$fallback"

This is fine for most values, but can be tricky for difficult-to-express
`$fallback`'s, like ANSI color codes.

This motivates `--get-color`, which is a one-off exception to the normal
type specifier rules wherein a user specifies both the configuration
slot and an optional fallback. Both are formatted according to their
type specifier, which eases the burden on the user to ensure that values
are correctly formatted.

This commit (and those following it in this series) aim to eventually
replace `--get-color` with a consistent alternative. By introducing
`--default`, we allow the `--get-color` action to be promoted to a
`--color` type specifier, retaining the "fallback" behavior via the
`--default` flag introduced in this commit.

For example, we aim to replace:

  $ git config --get-color slot [default] [...]

with:

  $ git config --default default --color slot [...]

Values filled by `--default` behave exactly as if they were present in
the affected configuration file; they will be parsed by type specifiers
without the knowledge that they are not themselves present in the
configuraion.

Specifically, this means that the following will work:

  $ git config --int --default 1M does.not.exist
  1048576

In subsequent commits, we will offer `--color`, which (in conjunction
with `--default`) will be sufficient to replace `--get-color`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-config.txt |   6 +-
 builtin/config.c             |  17 +++++
 t/t1310-config-default.sh    | 125 +++++++++++++++++++++++++++++++++++
 3 files changed, 146 insertions(+), 2 deletions(-)
 create mode 100755 t/t1310-config-default.sh

diff --git a/Documentation/git-config.txt b/Documentation/git-config.txt
index e09ed5d7d..d9e389a33 100644
--- a/Documentation/git-config.txt
+++ b/Documentation/git-config.txt
@@ -233,8 +233,10 @@ See also <<FILES>>.
 	using `--file`, `--global`, etc) and `on` when searching all
 	config files.
 
-CONFIGURATION
--------------
+--default value::
+  When using `--get`, behave as if value were the value assigned to the given
+  slot.
+
 `pager.config` is only respected when listing configuration, i.e., when
 using `--list` or any of the `--get-*` which may return multiple results.
 The default is to use a pager.
diff --git a/builtin/config.c b/builtin/config.c
index 01169dd62..1410be850 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -26,6 +26,7 @@ static char term = '\n';
 static int use_global_config, use_system_config, use_local_config;
 static struct git_config_source given_config_source;
 static int actions, types;
+static char *default_value;
 static int end_null;
 static int respect_includes_opt = -1;
 static struct config_options config_options;
@@ -94,6 +95,7 @@ static struct option builtin_config_options[] = {
 	OPT_BOOL(0, "name-only", &omit_values, N_("show variable names only")),
 	OPT_BOOL(0, "includes", &respect_includes_opt, N_("respect include directives on lookup")),
 	OPT_BOOL(0, "show-origin", &show_origin, N_("show origin of config (file, standard input, blob, command line)")),
+	OPT_STRING(0, "default", &default_value, N_("value"), N_("with --get, use default value when missing entry")),
 	OPT_END(),
 };
 
@@ -258,6 +260,16 @@ static int get_value(const char *key_, const char *regex_)
 	config_with_options(collect_config, &values,
 			    &given_config_source, &config_options);
 
+	if (!values.nr && default_value) {
+		struct strbuf *item;
+		ALLOC_GROW(values.items, values.nr + 1, values.alloc);
+		item = &values.items[values.nr++];
+		strbuf_init(item, 0);
+		if (format_config(item, key_, default_value) < 0) {
+			values.nr = 0;
+		}
+	}
+
 	ret = !values.nr;
 
 	for (i = 0; i < values.nr; i++) {
@@ -601,6 +613,11 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		usage_with_options(builtin_config_usage, builtin_config_options);
 	}
 
+	if (default_value && !(actions & ACTION_GET)) {
+		error("--default is only applicable to --get");
+		usage_with_options(builtin_config_usage, builtin_config_options);
+	}
+
 	if (actions & PAGING_ACTIONS)
 		setup_auto_pager("config", 1);
 
diff --git a/t/t1310-config-default.sh b/t/t1310-config-default.sh
new file mode 100755
index 000000000..0e464c206
--- /dev/null
+++ b/t/t1310-config-default.sh
@@ -0,0 +1,125 @@
+#!/bin/sh
+
+test_description='Test git config in different settings (with --default)'
+
+. ./test-lib.sh
+
+test_expect_success 'clear default config' '
+	rm -f .git/config
+'
+
+test_expect_success 'uses default when missing entry' '
+	echo quux >expect &&
+	git config --default quux core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'uses entry when available' '
+	echo bar >expect &&
+	git config --add core.foo bar &&
+	git config --default baz core.foo >actual &&
+	git config --unset core.foo &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshals default value as bool' '
+	echo true >expect &&
+	git config --default true --bool core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshals default value as int' '
+	echo 810 >expect &&
+	git config --default 810 --int core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshals default value as int (storage unit)' '
+	echo 1048576 >expect &&
+	git config --default 1M --int core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshals default value as bool-or-int' '
+	{
+		echo 1 &&
+		echo true
+	} >expect &&
+	git config --default 1 --bool-or-int core.foo >actual &&
+	git config --default true --bool-or-int core.foo >>actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshal default value as path' '
+	echo /path/to/file >expect &&
+	git config --default /path/to/file --path core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'marshal default value as expiry-date' '
+	echo 0 >expect &&
+	git config --default never --expiry-date core.foo >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'does not allow --default with --get-all' '
+	test_must_fail git config --default quux --get-all a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --get-regexp' '
+	test_must_fail git config --default quux --get-regexp a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --replace-all' '
+	test_must_fail git config --default quux --replace-all a b >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --add' '
+	test_must_fail git config --default quux --add a b >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --unset' '
+	test_must_fail git config --default quux --unset a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --unset-all' '
+	test_must_fail git config --default quux --unset-all a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --rename-section' '
+	test_must_fail git config --default quux --rename-section a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --remove-section' '
+	test_must_fail git config --default quux --remove-section a >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --list' '
+	test_must_fail git config --default quux --list >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --edit' '
+	test_must_fail git config --default quux --edit >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --get-color' '
+	test_must_fail git config --default quux --get-color >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_expect_success 'does not allow --default with --get-colorbool' '
+	test_must_fail git config --default quux --get-colorbool >output 2>&1 &&
+	test_i18ngrep "\-\-default is only applicable to" output
+'
+
+test_done
-- 
2.16.2.440.gc6284da4f