agent: advertise OS name via agent capability

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.

Our current agent capability is in the form of "package/version" (e.g.,
"git/1.8.3.1"). Let's extend it to include the operating system name (os)
i.e in the form "package/version-os" (e.g., "git/1.8.3.1-Linux").

Including OS details in the agent capability simplifies implementation,
maintains backward compatibility, avoids introducing a new capability,
encourages adoption across Git-compatible software, and enhances
debugging by providing complete environment information without affecting
functionality. The operating system name is retrieved using the 'sysname'
field of the `uname(2)` system call or its equivalent.

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/gitprotocol-v2.txt

@@ -184,9 +184,13 @@ form `agent=X`) to notify the client that the server is running version
 the `agent` capability with a value `Y` (in the form `agent=Y`) in its
 request to the server (but it MUST NOT do so if the server did not
 advertise the agent capability). The `X` and `Y` strings may contain any
-printable ASCII characters except space (i.e., the byte range 32 < x <
-127), and are typically of the form "package/version" (e.g.,
-"git/1.8.3.1"). The agent strings are purely informative for statistics
+printable ASCII characters except space (i.e., the byte range 33 <= x <=
+126), and are typically of the form "package/version-os" (e.g.,
+"git/1.8.3.1-Linux") where `os` is the operating system name (e.g.,
+"Linux"). `X` and `Y` can be configured using the GIT_USER_AGENT
+environment variable and it takes priority. The `os` is
+retrieved using the 'sysname' field of the `uname(2)` system call
+or its equivalent. The agent strings are purely informative for statistics
 and debugging purposes, and MUST NOT be used to programmatically assume
 the presence or absence of particular features.
 

connect.c

@@ -625,7 +625,7 @@ const char *parse_feature_value(const char *feature_list, const char *feature, s
 					*offset = found + len - orig_start;
 				return value;
 			}
-			/* feature with a value (e.g., "agent=git/1.2.3") */
+			/* feature with a value (e.g., "agent=git/1.2.3-Linux") */
 			else if (*value == '=') {
 				size_t end;
 

t/t5701-git-serve.sh

@@ -8,13 +8,19 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 . ./test-lib.sh
 
 test_expect_success 'setup to generate files with expected content' '
-	printf "agent=git/%s\n" "$(git version | cut -d" " -f3)" >agent_capability &&
+	printf "agent=git/%s" "$(git version | cut -d" " -f3)" >agent_capability &&
 
 	test_oid_cache <<-EOF &&
 	wrong_algo sha1:sha256
 	wrong_algo sha256:sha1
 	EOF
 
+	if test_have_prereq WINDOWS
+	then
+		printf "agent=FAKE\n" >agent_capability
+	else
+		printf -- "-%s\n" $(uname -s | test_redact_non_printables) >>agent_capability
+	fi &&
 	cat >expect.base <<-EOF &&
 	version 2
 	$(cat agent_capability)
@@ -31,6 +37,10 @@ test_expect_success 'setup to generate files with expected content' '
 test_expect_success 'test capability advertisement' '
 	cat expect.base expect.trailer >expect &&
 
+	if test_have_prereq WINDOWS
+	then
+		GIT_USER_AGENT=FAKE && export GIT_USER_AGENT
+	fi &&
 	GIT_TEST_SIDEBAND_ALL=0 test-tool serve-v2 \
 		--advertise-capabilities >out &&
 	test-tool pkt-line unpack <out >actual &&
@@ -361,6 +371,10 @@ test_expect_success 'test capability advertisement with uploadpack.advertiseBund
 	    expect.extra \
 	    expect.trailer >expect &&
 
+	if test_have_prereq WINDOWS
+	then
+		GIT_USER_AGENT=FAKE && export GIT_USER_AGENT
+	fi &&
 	GIT_TEST_SIDEBAND_ALL=0 test-tool serve-v2 \
 		--advertise-capabilities >out &&
 	test-tool pkt-line unpack <out >actual &&

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
+# correspond to decimal intervals 1-32 and 127-255
+test_redact_non_printables () {
+    tr -d "\n\r" | tr "[\001-\040][\177-\377]" "."
+}

version.c

@@ -1,8 +1,9 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
 #include "git-compat-util.h"
 #include "version.h"
 #include "version-def.h"
 #include "strbuf.h"
-#include "sane-ctype.h"
 #include "gettext.h"
 
 const char git_version_string[] = GIT_VERSION;
@@ -34,6 +35,27 @@ const char *git_user_agent(void)
 	return agent;
 }
 
+/*
+  Retrieve, sanitize and cache operating system info for subsequent
+  calls. Return a pointer to the sanitized operating system info
+  string.
+*/
+static const char *os_info(void)
+{
+	static const char *os = NULL;
+
+	if (!os) {
+		struct strbuf buf = STRBUF_INIT;
+
+		get_uname_info(&buf, 0);
+		/* Sanitize the os information immediately */
+		redact_non_printables(&buf);
+		os = strbuf_detach(&buf, NULL);
+	}
+
+	return os;
+}
+
 const char *git_user_agent_sanitized(void)
 {
 	static const char *agent = NULL;
@@ -42,6 +64,11 @@ const char *git_user_agent_sanitized(void)
 		struct strbuf buf = STRBUF_INIT;
 
 		strbuf_addstr(&buf, git_user_agent());
+
+		if (!getenv("GIT_USER_AGENT")) {
+			strbuf_addch(&buf, '-');
+			strbuf_addstr(&buf, os_info());
+		}
 		redact_non_printables(&buf);
 		agent = strbuf_detach(&buf, NULL);
 	}

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 +16,5 @@ const char *git_user_agent_sanitized(void);
 */
 int get_uname_info(struct strbuf *buf, unsigned int full);
 
+
 #endif /* VERSION_H */