connect: advertise OS version

As some issues that can happen with a Git client can be operating system
specific, it can be useful for a server to know which OS a client is
using. In the same way it can be useful for a client to know which OS
a server is using.

Let's introduce a new protocol (`os-version`) allowing Git clients and
servers to exchange operating system information. The protocol is
controlled by the new `transfer.advertiseOSVersion` config option.

Add the `transfer.advertiseOSVersion` config option to address
privacy concerns. It defaults to `true` and can be changed to
`false`. When enabled, this option makes clients and servers send each
other the OS name (e.g., "Linux" or "Windows"). The information is
retrieved using the 'sysname' field of the `uname(2)` system call.

However, there are differences between `uname(1)` (command-line utility)
and `uname(2)` (system call) outputs on Windows. These discrepancies
complicate testing on Windows platforms. For example:
  - `uname(1)` output: MINGW64_NT-10.0-20348.3.4.10-87d57229.x86_64\
  .2024-02-14.20:17.UTC.x86_64
  - `uname(2)` output: Windows.10.0.20348

On Windows, uname(2) is not actually system-supplied but is instead
already faked up by Git itself. We could have overcome the test issue
on Windows by implementing a new `uname` subcommand in `test-tool`
using uname(2), but except uname(2), which would be tested against
itself, there would be nothing platform specific, so it's just simpler
to disable the tests on Windows.

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

@@ -125,3 +125,10 @@ transfer.bundleURI::
 transfer.advertiseObjectInfo::
 	When `true`, the `object-info` capability is advertised by
 	servers. Defaults to false.
+
+transfer.advertiseOSVersion::
+	When `true`, the `os-version` capability is advertised by clients and
+	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.

Documentation/gitprotocol-v2.txt

@@ -190,6 +190,24 @@ printable ASCII characters except space (i.e., the byte range 32 < x <
 and debugging purposes, and MUST NOT be used to programmatically assume
 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
+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.
+
 ls-refs
 ~~~~~~~
 

connect.c

@@ -492,6 +492,9 @@ static void send_capabilities(int fd_out, struct packet_reader *reader)
 	if (server_supports_v2("agent"))
 		packet_write_fmt(fd_out, "agent=%s", git_user_agent_sanitized());
 
+	if (server_supports_v2("os-version") && advertise_os_version(the_repository))
+		packet_write_fmt(fd_out, "os-version=%s", os_version_sanitized());
+
 	if (server_feature_v2("object-format", &hash_name)) {
 		int hash_algo = hash_algo_by_name(hash_name);
 		if (hash_algo == GIT_HASH_UNKNOWN)

serve.c

@@ -31,6 +31,16 @@ static int agent_advertise(struct repository *r UNUSED,
 	return 1;
 }
 
+static int os_version_advertise(struct repository *r,
+			   struct strbuf *value)
+{
+	if (!advertise_os_version(r))
+		return 0;
+	if (value)
+		strbuf_addstr(value, os_version_sanitized());
+	return 1;
+}
+
 static int object_format_advertise(struct repository *r,
 				   struct strbuf *value)
 {
@@ -123,6 +133,10 @@ static struct protocol_capability capabilities[] = {
 		.name = "agent",
 		.advertise = agent_advertise,
 	},
+	{
+		.name = "os-version",
+		.advertise = os_version_advertise,
+	},
 	{
 		.name = "ls-refs",
 		.advertise = ls_refs_advertise,

t/t5555-http-smart-common.sh

@@ -123,9 +123,17 @@ test_expect_success 'git receive-pack --advertise-refs: v1' '
 '
 
 test_expect_success 'git upload-pack --advertise-refs: v2' '
+	printf "agent=FAKE" >agent_and_osversion &&
+	if test_have_prereq WINDOWS
+	then
+		git config transfer.advertiseOSVersion false
+	else
+		printf "\nos-version=%s\n" $(uname -s | test_redact_non_printables) >>agent_and_osversion
+	fi &&
+
 	cat >expect <<-EOF &&
 	version 2
-	agent=FAKE
+	$(cat agent_and_osversion)
 	ls-refs=unborn
 	fetch=shallow wait-for-done
 	server-option

t/t5701-git-serve.sh

@@ -26,10 +26,26 @@ test_expect_success 'setup to generate files with expected content' '
 	cat >expect.trailer <<-EOF &&
 	0000
 	EOF
+
+	if test_have_prereq WINDOWS
+	then
+		git config transfer.advertiseOSVersion false
+	else
+		printf "\nos-version=%s\n" $(uname -s | test_redact_non_printables) >>agent_and_osversion
+	fi &&
+
+	cat >expect_osversion.base <<-EOF
+	version 2
+	$(cat agent_and_osversion)
+	ls-refs=unborn
+	fetch=shallow wait-for-done
+	server-option
+	object-format=$(test_oid algo)
+	EOF
 '
 
 test_expect_success 'test capability advertisement' '
-	cat expect.base expect.trailer >expect &&
+	cat expect_osversion.base expect.trailer >expect &&
 
 	GIT_TEST_SIDEBAND_ALL=0 test-tool serve-v2 \
 		--advertise-capabilities >out &&
@@ -357,7 +373,7 @@ test_expect_success 'test capability advertisement with uploadpack.advertiseBund
 	cat >expect.extra <<-EOF &&
 	bundle-uri
 	EOF
-	cat expect.base \
+	cat expect_osversion.base \
 	    expect.extra \
 	    expect.trailer >expect &&
 

t/test-lib-functions.sh

@@ -2007,3 +2007,11 @@ test_trailing_hash () {
 		test-tool hexdump |
 		sed "s/ //g"
 }
+
+# Trim and replace each character with ascii code below 32 or above
+# 127 (included) using a dot '.' character.
+# Octal intervals \001-\040 and \177-\377
+# corresponds to decimal intervals 1-32 and 127-255
+test_redact_non_printables () {
+    tr -d "\n\r" | tr "[\001-\040][\177-\377]" "."
+}

version.c

@@ -3,6 +3,7 @@
 #include "version-def.h"
 #include "strbuf.h"
 #include "gettext.h"
+#include "config.h"
 
 const char git_version_string[] = GIT_VERSION;
 const char git_built_from_commit_string[] = GIT_BUILT_FROM_COMMIT;
@@ -69,3 +70,44 @@ int get_uname_info(struct strbuf *buf, unsigned int full)
 	     strbuf_addf(buf, "%s\n", uname_info.sysname);
 	return 0;
 }
+
+const char *os_version(void)
+{
+	static const char *os = NULL;
+
+	if (!os) {
+		struct strbuf buf = STRBUF_INIT;
+
+		get_uname_info(&buf, 0);
+		os = strbuf_detach(&buf, NULL);
+	}
+
+	return os;
+}
+
+const char *os_version_sanitized(void)
+{
+	static const char *os_sanitized = NULL;
+
+	if (!os_sanitized) {
+		struct strbuf buf = STRBUF_INIT;
+
+		strbuf_addstr(&buf, os_version());
+		redact_non_printables(&buf);
+		os_sanitized = strbuf_detach(&buf, NULL);
+	}
+
+	return os_sanitized;
+}
+
+int advertise_os_version(struct repository *r)
+{
+	static int transfer_advertise_os_version = -1;
+
+	if (transfer_advertise_os_version == -1) {
+		repo_config_get_bool(r, "transfer.advertiseosversion", &transfer_advertise_os_version);
+		/* enabled by default */
+		transfer_advertise_os_version = !!transfer_advertise_os_version;
+	}
+	return transfer_advertise_os_version;
+}

version.h

@@ -1,6 +1,8 @@
 #ifndef VERSION_H
 #define VERSION_H
 
+struct repository;
+
 extern const char git_version_string[];
 extern const char git_built_from_commit_string[];
 
@@ -14,4 +16,23 @@ const char *git_user_agent_sanitized(void);
 */
 int get_uname_info(struct strbuf *buf, unsigned int full);
 
+/*
+  Retrieve and cache system information for subsequent calls.
+  Return a pointer to the cached system information string.
+*/
+const char *os_version(void);
+
+/*
+  Retrieve system information string from os_version(). Then
+  sanitize and cache it. Return a pointer to the sanitized
+  system information string.
+*/
+const char *os_version_sanitized(void);
+
+/*
+  Retrieve and cache whether os-version capability is enabled.
+  Return 1 if enabled, 0 if disabled.
+*/
+int advertise_os_version(struct repository *r);
+
 #endif /* VERSION_H */