[PATCH i-g-t] lib/igt_core: buffer signal-safe log output per line
From: Jeevan B <hidden>
Date: 2026-09-07 05:21:09
Subsystem:
library code, the rest · Maintainers:
Andrew Morton, Linus Torvalds
xputch() wrote one byte at a time via write(STDERR_FILENO, &c, 1) from the async-signal-safe backtrace printer. When a fatal signal hits multiple processes/threads at once (e.g. forked test children all dumping a backtrace after the same signal), their single-byte writes interleave on the shared stderr/runner fd, producing garbled or truncated output. This showed up as tests (e.g. igt@kms_cursor_legacy@*) being reported as incomplete with no useful logs. Buffer output in xputch() and flush it with a single write() (or log_to_runner_sig_safe() call) per line via a new xflush(), instead of one syscall per character. xprintfmt() now also flushes on reaching the end of the format string, so a trailing partial line without a newline is still emitted. Assisted-by: Claude:claude-sonnet-4-5 Signed-off-by: Jeevan B <redacted> --- lib/igt_core.c | 31 ++++++++++++++++++++++++++++--- 1 file changed, 28 insertions(+), 3 deletions(-)
diff --git a/lib/igt_core.c b/lib/igt_core.c
index af9d93762..e376a972c 100644
--- a/lib/igt_core.c
+++ b/lib/igt_core.c@@ -2105,13 +2105,37 @@ static void write_stderr(const char *str) #ifdef HAVE_LIBUNWIND static const char hex[] = "0123456789abcdef"; +/* + * Buffer output and flush it in one write() per line, instead of one + * write() per character. Concurrent single-byte writes from multiple + * processes (e.g. forked children all dumping a backtrace after + * receiving the same fatal signal) interleave on the shared stderr/ + * runner fd and produce unreadable, garbled logs. + */ +static char xputch_buf[256]; +static size_t xputch_buf_len; + static void -xputch(int c) +xflush(void) { + if (!xputch_buf_len) + return; + if (runner_connected()) - log_to_runner_sig_safe((const void *) &c, 1); + log_to_runner_sig_safe(xputch_buf, xputch_buf_len); else - igt_ignore_warn(write(STDERR_FILENO, (const void *) &c, 1)); + igt_ignore_warn(write(STDERR_FILENO, xputch_buf, xputch_buf_len)); + + xputch_buf_len = 0; +} + +static void +xputch(int c) +{ + xputch_buf[xputch_buf_len++] = c; + + if (c == '\n' || xputch_buf_len == sizeof(xputch_buf)) + xflush(); } static int
@@ -2166,6 +2190,7 @@ xprintfmt(const char *fmt, va_list ap) while (1) { while ((ch = *(const unsigned char *) fmt++) != '%') { if (ch == '\0') { + xflush(); return; } xputch(ch);
--
2.43.0