[PATCH v2] usage: trace2 BUG() invocations

[Jonathan Tan <jonathantanmy@google.com>]

die() messages are traced in trace2, but BUG() messages are not. Anyone
tracking die() messages would have even more reason to track BUG().
Therefore, write to trace2 when BUG() is invoked.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
Thanks everyone for your suggestions. I've used an in_bug static
variable (which does prevent the infinite loop - I injected the bug that
Peff described and verified that it indeed loops without the mitigation
and doesn't loop with the mitigation) and moved the trace2 to be below
the vreportf (necessitating a va_copy).
---
 t/helper/test-trace2.c   |  9 +++++++++
 t/t0210-trace2-normal.sh | 19 +++++++++++++++++++
 usage.c                  | 11 +++++++++++
 3 files changed, 39 insertions(+)

diff --git a/t/helper/test-trace2.c b/t/helper/test-trace2.c
index 823f33ceff..f93633f895 100644
--- a/t/helper/test-trace2.c
+++ b/t/helper/test-trace2.c
@@ -198,6 +198,14 @@ static int ut_006data(int argc, const char **argv)
 	return 0;
 }
 
+static int ut_007bug(int argc, const char **argv)
+{
+	/*
+	 * Exercise BUG() to ensure that the message is printed to trace2.
+	 */
+	BUG("the bug message");
+}
+
 /*
  * Usage:
  *     test-tool trace2 <ut_name_1> <ut_usage_1>
@@ -214,6 +222,7 @@ static struct unit_test ut_table[] = {
 	{ ut_004child,    "004child",  "[<child_command_line>]" },
 	{ ut_005exec,     "005exec",   "<git_command_args>" },
 	{ ut_006data,     "006data",   "[<category> <key> <value>]+" },
+	{ ut_007bug,      "007bug",    "" },
 };
 /* clang-format on */
 
diff --git a/t/t0210-trace2-normal.sh b/t/t0210-trace2-normal.sh
index ce7574edb1..81af180c4c 100755
--- a/t/t0210-trace2-normal.sh
+++ b/t/t0210-trace2-normal.sh
@@ -147,6 +147,25 @@ test_expect_success 'normal stream, error event' '
 	test_cmp expect actual
 '
 
+# Verb 007bug
+#
+# Check that BUG writes to trace2
+
+test_expect_success 'normal stream, exit code 1' '
+	test_when_finished "rm trace.normal actual expect" &&
+	test_must_fail env GIT_TRACE2="$(pwd)/trace.normal" test-tool trace2 007bug &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 007bug
+		cmd_name trace2 (trace2)
+		error the bug message
+		exit elapsed:_TIME_ code:99
+		atexit elapsed:_TIME_ code:99
+	EOF
+	test_cmp expect actual
+'
+
 sane_unset GIT_TRACE2_BRIEF
 
 # Now test without environment variables and get all Trace2 settings
diff --git a/usage.c b/usage.c
index 1868a24f7a..1b206de36d 100644
--- a/usage.c
+++ b/usage.c
@@ -266,6 +266,10 @@ int BUG_exit_code;
 static NORETURN void BUG_vfl(const char *file, int line, const char *fmt, va_list params)
 {
 	char prefix[256];
+	va_list params_copy;
+	static int in_bug;
+
+	va_copy(params_copy, params);
 
 	/* truncation via snprintf is OK here */
 	if (file)
@@ -274,6 +278,13 @@ static NORETURN void BUG_vfl(const char *file, int line, const char *fmt, va_lis
 		snprintf(prefix, sizeof(prefix), "BUG: ");
 
 	vreportf(prefix, fmt, params);
+
+	if (in_bug)
+		abort();
+	in_bug = 1;
+
+	trace2_cmd_error_va(fmt, params_copy);
+
 	if (BUG_exit_code)
 		exit(BUG_exit_code);
 	abort();
-- 
2.30.0.365.g02bc693789-goog