[PATCH RESEND 1/5] virCommand: Actually acquire pidfile instead of just writing it

[Michal Privoznik]

Our virCommand module allows us to set a pidfile for commands we
want to spawn. The caller constructs the string of pidfile path
and then uses virCommandSetPidFile() to tell the module to write
the pidfile once the command is ran. This usually works, but has
two flaws:

1) the child process does not hold the pidfile open & locked.
Therefore, the caller (or anybody else) can't use our fancy
virPidFileForceCleanupPath() function to kill the command
afterwards. Also, for everybody else on the system it's
needlessly harder to check if the pid from the pidfile is still
alive or not.

2) if the caller ever makes a mistake and passes the same pidfile
path for two different commands, the start of the second command
will overwrite the pidfile even though the first command might
still be running.

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 docs/internals/command.html.in |  4 ++-
 src/util/vircommand.c          | 56 +++++++++++++++++++++++++++++-----
 tests/commanddata/test4.log    |  1 +
 3 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/docs/internals/command.html.in b/docs/internals/command.html.in
index 8a9061e70f..823d89cc71 100644
--- a/docs/internals/command.html.in
+++ b/docs/internals/command.html.in
@@ -226,7 +226,9 @@ virCommandSetPidFile(cmd, "/var/run/dnsmasq.pid");
 
     <p>
       This PID file is guaranteed to be written before
-      the intermediate process exits.
+      the intermediate process exits. Moreover, the daemonized
+      process will inherit the FD of the opened and locked PID
+      file.
     </p>
 
     <h3><a id="privs">Reducing command privileges</a></h3>
diff --git a/src/util/vircommand.c b/src/util/vircommand.c
index 9ffa0cf49b..77078d09fb 100644
--- a/src/util/vircommand.c
+++ b/src/util/vircommand.c
@@ -612,6 +612,7 @@ virExec(virCommandPtr cmd)
     int null = -1;
     int pipeout[2] = {-1, -1};
     int pipeerr[2] = {-1, -1};
+    int pipesync[2] = {-1, -1};
     int childin = cmd->infd;
     int childout = -1;
     int childerr = -1;
@@ -745,9 +746,15 @@ virExec(virCommandPtr cmd)
     /* Initialize full logging for a while */
     virLogSetFromEnv();
 
+    if (cmd->pidfile &&
+        virPipe(pipesync) < 0)
+        goto fork_error;
+
     /* Daemonize as late as possible, so the parent process can detect
      * the above errors with wait* */
     if (cmd->flags & VIR_EXEC_DAEMON) {
+        char c;
+
         if (setsid() < 0) {
             virReportSystemError(errno,
                                  "%s", _("cannot become session leader"));
@@ -768,12 +775,13 @@ virExec(virCommandPtr cmd)
         }
 
         if (pid > 0) {
-            if (cmd->pidfile && (virPidFileWritePath(cmd->pidfile, pid) < 0)) {
-                if (virProcessKillPainfully(pid, true) >= 0)
-                    virReportSystemError(errno,
-                                         _("could not write pidfile %s for %d"),
-                                         cmd->pidfile, pid);
-                goto fork_error;
+            /* The parent expect us to have written the pid file before
+             * exiting. Wait here for the child to write it and signal us. */
+            if (cmd->pidfile &&
+                saferead(pipesync[0], &c, sizeof(c)) != sizeof(c)) {
+                virReportSystemError(errno, "%s",
+                                     _("Unable to wait for child process"));
+                _exit(EXIT_FAILURE);
             }
             _exit(EXIT_SUCCESS);
         }
@@ -788,6 +796,37 @@ virExec(virCommandPtr cmd)
     if (cmd->setMaxCore &&
         virProcessSetMaxCoreSize(0, cmd->maxCore) < 0)
         goto fork_error;
+    if (cmd->pidfile) {
+        VIR_AUTOCLOSE pidfilefd = -1;
+        int newpidfilefd = -1;
+        char c;
+
+        pidfilefd = virPidFileAcquirePath(cmd->pidfile, false, getpid());
+        if (pidfilefd < 0)
+            goto fork_error;
+        if (virSetInherit(pidfilefd, true) < 0) {
+            virReportSystemError(errno, "%s",
+                                 _("Cannot disable close-on-exec flag"));
+            goto fork_error;
+        }
+
+        c = '1';
+        if (safewrite(pipesync[1], &c, sizeof(c)) != sizeof(c)) {
+            virReportSystemError(errno, "%s", _("Unable to notify child process"));
+            goto fork_error;
+        }
+        VIR_FORCE_CLOSE(pipesync[0]);
+        VIR_FORCE_CLOSE(pipesync[1]);
+
+        /* This is here only to move the pidfilefd
+         * to the lowest possible number. */
+        if ((newpidfilefd = dup(pidfilefd)) < 0) {
+            virReportSystemError(errno, "%s", _("Unable to dup FD"));
+            goto fork_error;
+        }
+
+        /* newpidfilefd is intentionally leaked. */
+    }
 
     if (cmd->hook) {
         VIR_DEBUG("Run hook %p %p", cmd->hook, cmd->opaque);
@@ -1080,8 +1119,9 @@ virCommandPassFDGetFDIndex(virCommandPtr cmd, int fd)
  * @cmd: the command to modify
  * @pidfile: filename to use
  *
- * Save the child PID in a pidfile.  The pidfile will be populated
- * before the exec of the child.
+ * Save the child PID in a pidfile. The pidfile will be populated before the
+ * exec of the child and the child will inherit opened and locked FD to the
+ * pidfile.
  */
 void
 virCommandSetPidFile(virCommandPtr cmd, const char *pidfile)
diff --git a/tests/commanddata/test4.log b/tests/commanddata/test4.log
index f170be204e..5820f28307 100644
--- a/tests/commanddata/test4.log
+++ b/tests/commanddata/test4.log
@@ -9,6 +9,7 @@ ENV:USER=test
 FD:0
 FD:1
 FD:2
+FD:3
 DAEMON:yes
 CWD:/
 UMASK:0022
-- 
2.24.1