[PATCH v4 6/6] agent: advertise OS name via agent capability

[Usman Akinyemi <usmanakinyemi202@gmail.com>]

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.

Add the `transfer.advertiseOSInfo` config option to address privacy
concerns. It defaults to `true` and can be changed to `false`.
When `true`, both the client and server independently append their
operating system name(os) to the `agent` capability value. The `agent`
capability will now be in form of "package/version os" (e.g.,
"git/1.8.3.1 Linux"). When `false`, the `agent` capability will be
in the form of "package/version" e.g "git/1.8.3.1". The server's
configuration is independent of the client's. Defaults to `true`.
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>
---
 Documentation/config/transfer.txt |  8 +++++++
 Documentation/gitprotocol-v2.txt  | 15 ++++++++-----
 t/t5555-http-smart-common.sh      | 10 ++++++++-
 t/t5701-git-serve.sh              |  9 +++++++-
 t/test-lib-functions.sh           |  8 +++++++
 version.c                         | 37 +++++++++++++++++++++++++++++++
 version.h                         | 15 +++++++++++++
 7 files changed, 95 insertions(+), 7 deletions(-)

diff --git a/Documentation/config/transfer.txt b/Documentation/config/transfer.txt
index f1ce50f4a6..1e1dc849ef 100644
--- a/Documentation/config/transfer.txt
+++ b/Documentation/config/transfer.txt
@@ -125,3 +125,11 @@ transfer.bundleURI::
 transfer.advertiseObjectInfo::
 	When `true`, the `object-info` capability is advertised by
 	servers. Defaults to false.
+
+transfer.advertiseOSInfo::
+	When `true`, both the client and server independently append their
+	operating system name (os) to the `agent` capability value. The `agent`
+	capability will now be in form of "package/version os" (e.g.,
+	"git/1.8.3.1 Linux"). When `false`, the `agent` capability will be
+	in the form of "package/version" e.g "git/1.8.3.1". The server's
+	configuration is independent of the client's. Defaults to `true`.
diff --git a/Documentation/gitprotocol-v2.txt b/Documentation/gitprotocol-v2.txt
index 1652fef3ae..8fab7d7d52 100644
--- a/Documentation/gitprotocol-v2.txt
+++ b/Documentation/gitprotocol-v2.txt
@@ -184,11 +184,16 @@ 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
-and debugging purposes, and MUST NOT be used to programmatically assume
-the presence or absence of particular features.
+printable ASCII characters (i.e., the byte range 32 < x < 127), 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. If `transfer.advertiseOSInfo` is `false` on the server, the server
+omits the `os` from X. If it is `false` on the client, the client omits the
+`os` from `Y`. 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.
 
 ls-refs
 ~~~~~~~
diff --git a/t/t5555-http-smart-common.sh b/t/t5555-http-smart-common.sh
index e47ea1ad10..140a7f0ffb 100755
--- a/t/t5555-http-smart-common.sh
+++ b/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_capability &&
+	if test_have_prereq WINDOWS
+	then
+		printf "\n" >>agent_capability &&
+		git config transfer.advertiseOSInfo false
+	else
+		printf " %s\n" $(uname -s | test_redact_non_printables) >>agent_capability
+	fi &&
 	cat >expect <<-EOF &&
 	version 2
-	agent=FAKE
+	$(cat agent_capability)
 	ls-refs=unborn
 	fetch=shallow wait-for-done
 	server-option
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index 4c24a188b9..a4c12372f8 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -8,13 +8,20 @@ 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 "\n" >>agent_capability &&
+		git config transfer.advertiseOSInfo false
+	else
+		printf " %s\n" $(uname -s | test_redact_non_printables) >>agent_capability
+	fi &&
 	cat >expect.base <<-EOF &&
 	version 2
 	$(cat agent_capability)
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 78e054ab50..3465904323 100644
--- a/t/test-lib-functions.sh
+++ b/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]" "."
+}
diff --git a/version.c b/version.c
index d95221a72a..f0f936a75e 100644
--- a/version.c
+++ b/version.c
@@ -1,9 +1,12 @@
+#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"
+#include "config.h"
 
 const char git_version_string[] = GIT_VERSION;
 const char git_built_from_commit_string[] = GIT_BUILT_FROM_COMMIT;
@@ -43,6 +46,12 @@ const char *git_user_agent_sanitized(void)
 
 		strbuf_addstr(&buf, git_user_agent());
 		redact_non_printables(&buf);
+		/* Add os name if the transfer.advertiseosinfo config is true */
+		if (advertise_os_info()) {
+			/* Add space to space character after git version string */
+			strbuf_addch(&buf, ' ');
+			strbuf_addstr(&buf, os_info_sanitized());
+		}
 		agent = strbuf_detach(&buf, NULL);
 	}
 
@@ -69,3 +78,31 @@ int get_uname_info(struct strbuf *buf, unsigned int full)
 	     strbuf_addf(buf, "%s\n", uname_info.sysname);
 	return 0;
 }
+
+const char *os_info_sanitized(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;
+}
+
+int advertise_os_info(void)
+{
+	static int transfer_advertise_os_info= -1;
+
+	if (transfer_advertise_os_info == -1) {
+		repo_config_get_bool(the_repository, "transfer.advertiseosinfo", &transfer_advertise_os_info);
+		/* enabled by default */
+		transfer_advertise_os_info = !!transfer_advertise_os_info;
+	}
+	return transfer_advertise_os_info;
+}
diff --git a/version.h b/version.h
index 5eb586c0bd..b2325865d7 100644
--- a/version.h
+++ b/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 +16,17 @@ const char *git_user_agent_sanitized(void);
 */
 int get_uname_info(struct strbuf *buf, unsigned int full);
 
+/*
+  Retrieve, sanitize and cache operating system info for subsequent
+  calls. Return a pointer to the sanitized operating system info
+  string.
+*/
+const char *os_info_sanitized(void);
+
+/*
+  Retrieve and cache transfer.advertiseosinfo config value. Return 1
+  if true, 0 if false.
+*/
+int advertise_os_info(void);
+
 #endif /* VERSION_H */
-- 
2.48.1