From acfde3da7a7399ffd35014481908419b33b315b9 Mon Sep 17 00:00:00 2001
From: Alexandre Rulleau <alexandre.rulleau@datadoghq.com>
Date: Wed, 11 Feb 2026 12:52:34 +0100
Subject: [PATCH 1/2] fix(logging): fsync crash logs before _Exit() to prevent
 data loss

When a SIGSEGV occurs, the signal handler logs "Segmentation fault encountered"
and then calls _Exit() which terminates the process immediately. Without fsync(),
kernel write buffers may not be flushed to disk before termination, causing
a race condition where the error log file is sometimes not created.

This fix adds fsync() on Unix/Linux and _commit() on Windows after write() in
ddtrace_log_with_time() to ensure crash logs persist to disk before process
termination.

The issue affects production (rare but possible during power loss, kernel panic,
or I/O errors) and causes consistent test failures where tests check for log
files immediately after crashes (before kernel writeback completes).

Fixes flaky test_metrics SigSegVTest::testGet failures on Kubernetes where
dd_php_error.log was not being created consistently.
---
 ext/logging.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/ext/logging.c b/ext/logging.c
index c0e2f9fb717..deaa957e96b 100644
--- a/ext/logging.c
+++ b/ext/logging.c
@@ -160,6 +160,15 @@ int ddtrace_log_with_time(int fd, const char *msg, int msg_len) {
 
     int ret = write(fd, msgbuf, p - msgbuf);
 
+    // Flush to disk to ensure data persists even if process terminates immediately
+    if (ret > 0) {
+#ifndef _WIN32
+        fsync(fd);
+#else
+        _commit(fd);
+#endif
+    }
+
     free(msgbuf);
     return ret;
 }

From db4f3450d2e09456dc8824fc9dd6bb6978501867 Mon Sep 17 00:00:00 2001
From: Alexandre Rulleau <alexandre.rulleau@datadoghq.com>
Date: Wed, 11 Feb 2026 14:34:40 +0100
Subject: [PATCH 2/2] fix(signals): move flush in sigsegv handler

Signed-off-by: Alexandre Rulleau <alexandre.rulleau@datadoghq.com>
---
 ext/logging.c | 9 ---------
 ext/signals.c | 9 +++++++++
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/ext/logging.c b/ext/logging.c
index deaa957e96b..c0e2f9fb717 100644
--- a/ext/logging.c
+++ b/ext/logging.c
@@ -160,15 +160,6 @@ int ddtrace_log_with_time(int fd, const char *msg, int msg_len) {
 
     int ret = write(fd, msgbuf, p - msgbuf);
 
-    // Flush to disk to ensure data persists even if process terminates immediately
-    if (ret > 0) {
-#ifndef _WIN32
-        fsync(fd);
-#else
-        _commit(fd);
-#endif
-    }
-
     free(msgbuf);
     return ret;
 }
diff --git a/ext/signals.c b/ext/signals.c
index 0c2de33c6a0..096bf28b20b 100644
--- a/ext/signals.c
+++ b/ext/signals.c
@@ -100,6 +100,15 @@ static void dd_sigsegv_handler(int sig) {
 #endif
     }
 
+    int error_log_fd = atomic_load(&ddtrace_error_log_fd);
+    if (error_log_fd != -1) {
+#ifndef _WIN32
+        fsync(error_log_fd);
+#else
+        _commit(error_log_fd);
+#endif
+    }
+
     // _Exit to avoid atexit() handlers, they may crash in this SIGSEGV signal handler...
     _Exit(128 + sig);
 }