Fix/logging snprintf negative time 4556 (#4593)

Closes #4556

## What's new?

1. Updated input_timestamp() to correctly handle future timestamps.
2. Replaced the buggy modulo logic—which produced confusing "double
negative" strings like -100.-500ms ago—with absolute duration
calculations.
3. Added a contextual suffix (ago vs in future) to make the logs
human-readable regardless of clock drift or event timing. (No negative
sign would be printed for future timestamps).

Past values will be printed like -
`500 ms ago`
Future values will be printed like -
`100 ms in future`

## How to test
Added unit tests for the function, run `./bin/mir_unit_tests
--gtest_filter=TimestampTest.*` and verify.

## Checklist

- [ ✅] Tests added and pass
- [ ✅] Adequate documentation added

Author: tarek-y-ismail

src/common/logging/input_timestamp.cpp

@@ -16,6 +16,7 @@
 
 #include <mir/logging/input_timestamp.h>
 #include <cstdio>
+#include <cstdlib>
 
 using namespace std::chrono;
 std::string mir::logging::input_timestamp(nanoseconds when)
@@ -26,9 +27,15 @@ std::string mir::logging::input_timestamp(nanoseconds when)
     long long const now_ns = duration_cast<nanoseconds>(now).count();
     long long const age_ns = now_ns - when_ns;
 
+    long long const abs_age_ns = std::llabs(age_ns);
+    long long const milliseconds = abs_age_ns / 1000000LL;
+    long long const sub_milliseconds = abs_age_ns % 1000000LL;
+
+    char const* sign_suffix = (age_ns < 0) ? "in the future" : "ago";
+
     char str[64];
-    snprintf(str, sizeof str, "%lld (%lld.%06lldms ago)",
-             when_ns, age_ns / 1000000LL, age_ns % 1000000LL);
+    snprintf(str, sizeof str, "%lld (%lld.%06lldms %s)",
+             when_ns, milliseconds, sub_milliseconds, sign_suffix);
 
     return std::string(str);
 }

tests/unit-tests/logging/CMakeLists.txt

@@ -1,6 +1,7 @@
 list(APPEND UNIT_TEST_SOURCES
   ${CMAKE_CURRENT_SOURCE_DIR}/test_display_report.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test_compositor_report.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/test_input_timestamp.cpp
 )
 
 set(UNIT_TEST_SOURCES ${UNIT_TEST_SOURCES} PARENT_SCOPE)

tests/unit-tests/logging/test_input_timestamp.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright © Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 or 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <mir/logging/input_timestamp.h>
+
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+#include <chrono>
+#include <string>
+
+using namespace std::chrono;
+using namespace std::chrono_literals;
+using namespace testing;
+namespace ml = mir::logging;
+
+TEST(TimestampTest, past_time_is_correctly_formatted)
+{
+    auto const past_event = steady_clock::now().time_since_epoch() - 500ms;
+
+    std::string out = ml::input_timestamp(past_event);
+
+    EXPECT_THAT(out, Not(HasSubstr(".-")));
+    EXPECT_THAT(out, HasSubstr("500.0"));
+    EXPECT_THAT(out, HasSubstr("ms ago"));
+}
+
+TEST(TimestampTest, future_time_is_correctly_formatted)
+{
+    auto const future_event = steady_clock::now().time_since_epoch() + 1000ms + 10us;
+
+    std::string out = ml::input_timestamp(future_event);
+
+    EXPECT_THAT(out, Not(HasSubstr(".-")));
+    EXPECT_THAT(out, HasSubstr("in the future"));
+    EXPECT_THAT(out, HasSubstr("1000.0"));
+    EXPECT_THAT(out, HasSubstr("ms"));
+
+}