version: introduce osversion.command config for os-version output

Currently by default, the new `os-version` capability only exchange the
operating system name between servers and clients i.e "Linux" or
"Windows".

Let's introduce a new configuration option, `osversion.command`, to handle
the string exchange between servers and clients. This option allows
customization of the exchanged string by leveraging the output of the
specified command. This customization might be especially useful on some
quite uncommon platforms like NonStop where interesting OS information is
available from other means than uname(2).

If this new configuration option is not set, the `os-version` capability
exchanges just the operating system name.

Helped-by: Randall S. Becker <rsbecker@nexbridge.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/transfer.txt

@@ -131,4 +131,13 @@ transfer.advertiseOSVersion::
 	servers. It makes clients and servers send to each other a string
 	representing the operating system name, like "Linux" or "Windows".
 	This string is retrieved from the `sysname` field of the struct returned
-	by the uname(2) system call. Defaults to true.
+	by the uname(2) system call. If the `osVersion.command` is set, the
+	output of the command specified will be the string exchanged by the clients
+	and the servers. Defaults to true.
+
+osVersion.command::
+	If this variable is set, the specified command will be run and the output
+	will be used as the value `X` for `os-version` capability (in the form
+	`os-version=X`). `osVersion.command` is only used if `transfer.advertiseOSVersion`
+	is true. Refer to the linkgit:git-config[1] documentation to learn more about
+	`transfer.advertiseOSVersion` config option.

Documentation/gitprotocol-v2.txt

@@ -193,20 +193,19 @@ the presence or absence of particular features.
 os-version
 ~~~~~~~~~~
 
-In the same way as the `agent` capability above, the server can
-advertise the `os-version` capability to notify the client the
-kind of operating system it is running on. The client may optionally
-send its own `os-version` capability, to notify the server the kind of
-operating system it is also running on in its request to the server
-(but it MUST NOT do so if the server did not advertise the os-version
-capability). The value of this capability may consist of ASCII printable
+In the same way as the `agent` capability above, the server can advertise
+the `os-version` capability to notify the client the kind of operating system
+it is running on. The client may optionally send its own `os-version` capability,
+to notify the server the kind of operating system it is also running on in its
+request to the server (but it MUST NOT do so if the server did not advertise the
+os-version capability). The value of this capability may consist of ASCII printable
 characters(from 33 to 126 inclusive) and are typically made from the result of
-`uname -s`(OS name e.g Linux). The os-version capability can be disabled
-entirely by setting the `transfer.advertiseOSVersion` config option
-to `false`. The `os-version` strings are purely informative for
-statistics and debugging purposes, and MUST NOT be used to
-programmatically assume the presence or absence of particular
-features.
+`uname -s`(OS name e.g Linux). If the `osVersion.command` is set, the value of this
+capability are made from the ouput of the command specified. The os-version capability
+can be disabled entirely by setting the `transfer.advertiseOSVersion` config option
+to `false`. The `os-version` strings are purely informative for statistics and
+debugging purposes, and MUST NOT be used to programmatically assume the presence or
+absence of particular features.
 
 ls-refs
 ~~~~~~~

t/t5555-http-smart-common.sh

@@ -150,6 +150,34 @@ test_expect_success 'git upload-pack --advertise-refs: v2' '
 	test_cmp actual expect
 '
 
+test_expect_success 'git upload-pack --advertise-refs: v2 with osVersion.command config set' '
+	test_config osVersion.command "uname -srvm" &&
+	printf "agent=FAKE" >agent_and_long_osversion &&
+
+	if test_have_prereq !WINDOWS
+	then
+		printf "\nos-version=%s\n" $(uname -srvm | test_redact_non_printables) >>agent_and_long_osversion
+	fi &&
+
+	cat >expect <<-EOF &&
+	version 2
+	$(cat agent_and_long_osversion)
+	ls-refs=unborn
+	fetch=shallow wait-for-done
+	server-option
+	object-format=$(test_oid algo)
+	0000
+	EOF
+
+	GIT_PROTOCOL=version=2 \
+	GIT_USER_AGENT=FAKE \
+	git upload-pack --advertise-refs . >out 2>err &&
+
+	test-tool pkt-line unpack <out >actual &&
+	test_must_be_empty err &&
+	test_cmp actual expect
+'
+
 test_expect_success 'git receive-pack --advertise-refs: v2' '
 	# There is no v2 yet for receive-pack, implicit v0
 	cat >expect <<-EOF &&

t/t5701-git-serve.sh

@@ -53,6 +53,35 @@ test_expect_success 'test capability advertisement' '
 	test_cmp expect actual
 '
 
+test_expect_success 'test capability advertisement with osVersion.command config set' '
+	test_config osVersion.command "uname -srvm" &&
+	printf "agent=git/$(git version | cut -d" " -f3)" >agent_and_long_osversion &&
+
+	if test_have_prereq !WINDOWS
+	then
+		printf "\nos-version=%s\n" $(uname -srvm | test_redact_non_printables) >>agent_and_long_osversion
+	fi &&
+
+	test_oid_cache <<-EOF &&
+	wrong_algo sha1:sha256
+	wrong_algo sha256:sha1
+	EOF
+	cat >expect_long.base <<-EOF &&
+	version 2
+	$(cat agent_and_long_osversion)
+	ls-refs=unborn
+	fetch=shallow wait-for-done
+	server-option
+	object-format=$(test_oid algo)
+	EOF
+	cat expect_long.base expect.trailer >expect &&
+
+	GIT_TEST_SIDEBAND_ALL=0 test-tool serve-v2 \
+		--advertise-capabilities >out &&
+	test-tool pkt-line unpack <out >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'stateless-rpc flag does not list capabilities' '
 	# Empty request
 	test-tool pkt-line pack >in <<-EOF &&

version.c

@@ -1,9 +1,13 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
 #include "git-compat-util.h"
 #include "version.h"
 #include "version-def.h"
 #include "strbuf.h"
 #include "gettext.h"
 #include "config.h"
+#include "run-command.h"
+#include "alias.h"
 
 const char git_version_string[] = GIT_VERSION;
 const char git_built_from_commit_string[] = GIT_BUILT_FROM_COMMIT;
@@ -71,14 +75,59 @@ int get_uname_info(struct strbuf *buf, unsigned int full)
 	return 0;
 }
 
+/*
+ * Return -1 if unable to retrieve the osversion.command config or
+ * if the command is malformed; otherwise, return 0 if successful.
+ */
+static int fill_os_version_command(struct child_process *cmd)
+{
+	const char *os_version_command;
+	const char **argv;
+	char *os_version_copy;
+	int n;
+
+	if (git_config_get_string_tmp("osversion.command", &os_version_command))
+		return -1;
+
+	os_version_copy = xstrdup(os_version_command);
+	n = split_cmdline(os_version_copy, &argv);
+
+	if (n < 0) {
+		warning(_("malformed osVersion.command config option: %s"),
+			_(split_cmdline_strerror(n)));
+		free(os_version_copy);
+		return -1;
+	}
+
+	for (int i = 0; i < n; i++)
+		strvec_push(&cmd->args, argv[i]);
+	free(os_version_copy);
+	free(argv);
+
+	return 0;
+}
+
+static int capture_os_version(struct strbuf *buf)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+
+	if (fill_os_version_command(&cmd))
+		return -1;
+	if (capture_command(&cmd, buf, 0))
+		return -1;
+
+	return 0;
+}
+
 const char *os_version(void)
 {
 	static const char *os = NULL;
 
 	if (!os) {
 		struct strbuf buf = STRBUF_INIT;
 
-		get_uname_info(&buf, 0);
+		if (capture_os_version(&buf))
+			get_uname_info(&buf, 0);
 		os = strbuf_detach(&buf, NULL);
 	}