tests/procdesc: Fix race in pdopenpid_pdwait_only_one

The child exited immediately after pdfork(), so the parent's pdopenpid() could
catch it mid-exit (P_WEXIT) and fail with EBUSY.
Block the child on a pipe until the parent has opened the second descriptor,
then release it

Approved by:	markj
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D58546

1 files changed, 15 insertions, 2 deletions

diff --git a/tests/sys/kern/procdesc.c b/tests/sys/kern/procdesc.c
index 541f15b49338..1feaef34e2cb 100644
--- a/tests/sys/kern/procdesc.c
+++ b/tests/sys/kern/procdesc.c
@@ -519,16 +519,29 @@ ATF_TC_WITHOUT_HEAD(pdopenpid_pdwait_only_one);
 ATF_TC_BODY(pdopenpid_pdwait_only_one, tc)
 {
 	pid_t child;
-	int fd1, fd2, status;
+	int fd1, fd2, pip[2], status;
+
+	ATF_REQUIRE_EQ(pipe(pip), 0);
 
 	child = pdfork(&fd1, PD_DAEMON);
 	ATF_REQUIRE_MSG(child >= 0, "pdfork: %s", strerror(errno));
-	if (child == 0)
+	if (child == 0) {
+		char c;
+
+		close(pip[1]);
+		/* Block until the parent has opened the second fd. */
+		(void)read(pip[0], &c, 1);
 		_exit(42);
+	}
+	ATF_REQUIRE(close(pip[0]) == 0);
 
+	/* Open the second fd while the child is still alive. */
 	fd2 = pdopenpid(child, 0);
 	ATF_REQUIRE_MSG(fd2 >= 0, "pdopenpid: %s", strerror(errno));
 
+	/* Release the child so that it exits. */
+	ATF_REQUIRE(close(pip[1]) == 0);
+
 	/* Collect via the first fd. */
 	ATF_REQUIRE_MSG(pdwait(fd1, &status, WEXITED, NULL, NULL) == 0,
 	    "pdwait(fd1): %s", strerror(errno));