Fix race condition with zed pidfile creation

When the zed is started as a forking daemon (by default),
a race-condition exists where the parent process can terminate before
the pidfile has been created by the grandchild process.  When invoked
as a Type=forking systemd service, this can result in the following:

  systemd[1]: Starting ZFS Event Daemon (zed)...
  systemd[1]: PID file /var/run/zed.pid not readable (yet?) after start.

This commit adds a daemonize pipe to allow the grandchild process to
signal the parent process that initialization is complete (and the
pidfile has been created).  The parent process will wait for this
notification before exiting.

Signed-off-by: Chris Dunlap <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue #2252

1 files changed, 95 insertions, 0 deletions

diff --git a/cmd/zed/zed_log.c b/cmd/zed/zed_log.c
index bc432bc21..2a787e357 100644
--- a/cmd/zed/zed_log.c
+++ b/cmd/zed/zed_log.c
@@ -25,6 +25,7 @@
  */
 
 #include <assert.h>
+#include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -40,6 +41,7 @@ static struct {
 	unsigned do_syslog:1;
 	int level;
 	char id[ZED_LOG_MAX_ID_LEN];
+	int pipe_fd[2];
 } _ctx;
 
 void
@@ -53,6 +55,8 @@ zed_log_init(const char *identity)
 	} else {
 		_ctx.id[0] = '\0';
 	}
+	_ctx.pipe_fd[0] = -1;
+	_ctx.pipe_fd[1] = -1;
 }
 
 void
@@ -63,6 +67,97 @@ zed_log_fini()
 	}
 }
 
+/*
+ * Create pipe for communicating daemonization status between the parent and
+ *   child processes across the double-fork().
+ */
+void
+zed_log_pipe_open(void)
+{
+	if ((_ctx.pipe_fd[0] != -1) || (_ctx.pipe_fd[1] != -1))
+		zed_log_die("Invalid use of zed_log_pipe_open in PID %d",
+		    (int) getpid());
+
+	if (pipe(_ctx.pipe_fd) < 0)
+		zed_log_die("Failed to create daemonize pipe in PID %d: %s",
+		    (int) getpid(), strerror(errno));
+}
+
+/*
+ * Close the read-half of the daemonize pipe.
+ * This should be called by the child after fork()ing from the parent since
+ *   the child will never read from this pipe.
+ */
+void
+zed_log_pipe_close_reads(void)
+{
+	if (_ctx.pipe_fd[0] < 0)
+		zed_log_die(
+		    "Invalid use of zed_log_pipe_close_reads in PID %d",
+		    (int) getpid());
+
+	if (close(_ctx.pipe_fd[0]) < 0)
+		zed_log_die(
+		    "Failed to close reads on daemonize pipe in PID %d: %s",
+		    (int) getpid(), strerror(errno));
+
+	_ctx.pipe_fd[0] = -1;
+}
+
+/*
+ * Close the write-half of the daemonize pipe.
+ * This should be called by the parent after fork()ing its child since the
+ *   parent will never write to this pipe.
+ * This should also be called by the child once initialization is complete
+ *   in order to signal the parent that it can safely exit.
+ */
+void
+zed_log_pipe_close_writes(void)
+{
+	if (_ctx.pipe_fd[1] < 0)
+		zed_log_die(
+		    "Invalid use of zed_log_pipe_close_writes in PID %d",
+		    (int) getpid());
+
+	if (close(_ctx.pipe_fd[1]) < 0)
+		zed_log_die(
+		    "Failed to close writes on daemonize pipe in PID %d: %s",
+		    (int) getpid(), strerror(errno));
+
+	_ctx.pipe_fd[1] = -1;
+}
+
+/*
+ * Block on reading from the daemonize pipe until signaled by the child
+ *   (via zed_log_pipe_close_writes()) that initialization is complete.
+ * This should only be called by the parent while waiting to exit after
+ *   fork()ing the child.
+ */
+void
+zed_log_pipe_wait(void)
+{
+	ssize_t n;
+	char c;
+
+	if (_ctx.pipe_fd[0] < 0)
+		zed_log_die("Invalid use of zed_log_pipe_wait in PID %d",
+		    (int) getpid());
+
+	for (;;) {
+		n = read(_ctx.pipe_fd[0], &c, sizeof (c));
+		if (n < 0) {
+			if (errno == EINTR)
+				continue;
+			zed_log_die(
+			    "Failed to read from daemonize pipe in PID %d: %s",
+			    (int) getpid(), strerror(errno));
+		}
+		if (n == 0) {
+			break;
+		}
+	}
+}
+
 void
 zed_log_stderr_open(int level)
 {