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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     linux-kernel@vger.kernel.org
Cc:     gwml@vger.gnuweeb.org, kernel-team@meta.com, w@lwt.eu,
        Zhangjin Wu <falcon@tinylab.org>,
        =?UTF-8?q?Thomas=20Wei=C3=9Fschuh?= <linux@weissschuh.net>,
        Willy Tarreau <w@1wt.eu>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH v2 nolibc 40/53] selftests/nolibc: test_fork: fix up duplicated print
Date:   Mon, 12 Jun 2023 13:45:01 -0700
Message-Id: <20230612204514.292087-40-paulmck@kernel.org>
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References: <8b757cc0-3719-4e63-a755-9710384137bc@paulmck-laptop>
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From: Zhangjin Wu <falcon@tinylab.org>

running nolibc-test with glibc on x86_64 got such print issue:

    29 execve_root = -1 EACCES                                       [OK]
    30 fork30 fork = 0                                                      [OK]
    31 getdents64_root = 712                                         [OK]

The fork test case has three printf calls:

    (1) llen += printf("%d %s", test, #name);
    (2) llen += printf(" = %d %s ", expr, errorname(errno));
    (3) llen += pad_spc(llen, 64, "[FAIL]\n"); --> vfprintf()

In the following scene, the above issue happens:

    (a) The parent calls (1)
    (b) The parent calls fork()
    (c) The child runs and shares the print buffer of (1)
    (d) The child exits, flushs the print buffer and closes its own stdout/stderr
        * "30 fork" is printed at the first time.
    (e) The parent calls (2) and (3), with "\n" in (3), it flushs the whole buffer
        * "30 fork = 0 ..." is printed

Therefore, there are two "30 fork" in the stdout.

Between (a) and (b), if flush the stdout (and the sterr), the child in
stage (c) will not be able to 'see' the print buffer.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 tools/testing/selftests/nolibc/nolibc-test.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index d417ca5d976f..fa18e7dc35c8 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -486,7 +486,13 @@ static int test_getpagesize(void)
 static int test_fork(void)
 {
 	int status;
-	pid_t pid = fork();
+	pid_t pid;
+
+	/* flush the printf buffer to avoid child flush it */
+	fflush(stdout);
+	fflush(stderr);
+
+	pid = fork();
 
 	switch (pid) {
 	case -1:
-- 
2.40.1