[PATCH] config: introduce an Operating System-specific `includeIf` condition

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

It is relatively common for users to maintain identical `~/.gitconfig`
files across all of their setups, using the `includeIf` construct
liberally to adjust the settings to the respective setup as needed.

In case of Operating System-specific adjustments, Git currently offers
no support to the users and they typically use a work-around like this:

	[includeIf "gitdir:/home/"]
		path = ~/.gitconfig-linux
	[includeIf "gitdir:/Users/"]
		path = ~/.gitconfig-mac
	[includeIf "gitdir:C:"]
		path = ~/.gitconfig-windows

However, this is fragile, as it would not even allow to discern between
Operating Systems that happen to host their home directories in
`/home/`, such as Linux and the BSDs.

Let's introduce a new condition: `os:<uname-s>` where `<uname-s>` is the
system name, i.e. the output of `uname -s`.

This addresses https://github.com/git-for-windows/git/issues/4125

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    config: introduce an Operating System-specific includeIf condition
    
    I was about to write up guidelines how to write this patch, but it
    turned out that it was much faster to write the patch instead.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1429%2Fdscho%2Finclude-if-os-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1429/dscho/include-if-os-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1429

 Documentation/config.txt |  5 +++++
 config.c                 | 11 +++++++++++
 t/t1309-early-config.sh  | 16 ++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index e376d547ce0..2db73d8228e 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -186,6 +186,11 @@ As for the naming of this keyword, it is for forwards compatibiliy with
 a naming scheme that supports more variable-based include conditions,
 but currently Git only supports the exact keyword described above.
 
+`os`::
+	The data that follows this keyword is taken as the name of an
+	Operating System; If it matches the output of `uname -s`, the
+	include condition is met.
+
 A few more notes on matching via `gitdir` and `gitdir/i`:
 
  * Symlinks in `$GIT_DIR` are not resolved before matching.
diff --git a/config.c b/config.c
index 9b0e9c93285..9ab311ae99b 100644
--- a/config.c
+++ b/config.c
@@ -394,6 +394,15 @@ static int include_by_remote_url(struct config_include_data *inc,
 					     inc->remote_urls);
 }
 
+static int include_by_os(const char *cond, size_t cond_len)
+{
+	struct utsname uname_info;
+
+	return !uname(&uname_info) &&
+		!strncasecmp(uname_info.sysname, cond, cond_len) &&
+		!uname_info.sysname[cond_len];
+}
+
 static int include_condition_is_true(struct config_include_data *inc,
 				     const char *cond, size_t cond_len)
 {
@@ -408,6 +417,8 @@ static int include_condition_is_true(struct config_include_data *inc,
 	else if (skip_prefix_mem(cond, cond_len, "hasconfig:remote.*.url:", &cond,
 				   &cond_len))
 		return include_by_remote_url(inc, cond, cond_len);
+	else if (skip_prefix_mem(cond, cond_len, "os:", &cond, &cond_len))
+		return include_by_os(cond, cond_len);
 
 	/* unknown conditionals are always false */
 	return 0;
diff --git a/t/t1309-early-config.sh b/t/t1309-early-config.sh
index 537435b90ae..7da67af06e7 100755
--- a/t/t1309-early-config.sh
+++ b/t/t1309-early-config.sh
@@ -100,4 +100,20 @@ test_expect_success 'onbranch config outside of git repo' '
 	nongit git help
 '
 
+test_expect_success '[includeIf "os:..."]' '
+	test_config x.y 0 &&
+	echo "[x] y = z" >.git/xyz &&
+
+	if test_have_prereq MINGW
+	then
+		uname_s=Windows
+	else
+		uname_s="$(uname -s)"
+	fi &&
+	test_config "includeIf.os:not-$uname_s.path" xyz &&
+	test 0 = "$(git  config x.y)" &&
+	test_config "includeIf.os:$uname_s.path" xyz &&
+	test z = "$(git config x.y)"
+'
+
 test_done

base-commit: e4a4b31577c7419497ac30cebe30d755b97752c5
-- 
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