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>

2 files changed, 23 insertions, 1 deletions

diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index 4c24a188b9..678a346ed0 100755
--- a/t/t5701-git-serve.sh
+++ b/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 &&
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]" "."
+}