From 412b902b0edeef4f02f6b3548227fc2d25084abb Mon Sep 17 00:00:00 2001
From: David James <davidjames@chromium.org>
Date: Fri, 15 Jul 2011 19:54:16 -0700
Subject: [PATCH] Fix race condition when starting env_syncer in
 enter_chroot.sh.

Sometimes, two processes might start the env_syncer at the same time.
If this occurs, then the pid file will get overwritten and one of the
processes won't be killed.

I've also fixed the env_syncer to properly daemonize, such that
cbuildbot won't wait on the env_syncer to be killed before exiting.
This requires closing stdin, stderr, and stdout.

BUG=chromium-os:17680
TEST=Reproduce hang, and verify that this fixes the hang. Verify that
     multiple chroots can be entered at the same time and that the
     env_syncer is killed when the last chroot exits. Verify that
     cbuildbot is not kept hanging even if the env_syncer doesn't
     exit.

Change-Id: I3cb9fbdf47a406e818e12cf91593b8e3481aa578
Reviewed-on: http://gerrit.chromium.org/gerrit/4232
Reviewed-by: Ryan Cui <rcui@chromium.org>
Tested-by: David James <davidjames@chromium.org>
---
 enter_chroot.sh | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/enter_chroot.sh b/enter_chroot.sh
index 8fa593a6ea..79e2f49627 100755
--- a/enter_chroot.sh
+++ b/enter_chroot.sh
@@ -136,6 +136,10 @@ function env_sync_proc {
     sudo chown ${USER} ${FLAGS_chroot}/${file} 1>&2
   done
 
+  # Drop stdin, stderr, stdout, and chroot lock.
+  # This is needed for properly daemonizing the process.
+  exec 0>&- 1>&- 2>&- 200>&-
+
   while true; do
     # Sync files
     for file in ${sync_files}; do
@@ -186,19 +190,20 @@ function setup_env {
   # Don't use sudo -v since that has issues on machines w/ no password.
   sudo echo "" > /dev/null
 
-  # Syncer proc, but only the first time
-  if ! [ -f "${SYNCERPIDFILE}" ] || \
-     ! [ -d /proc/$(cat "${SYNCERPIDFILE}") ]; then
-    debug "Starting sync process"
-    env_sync_proc &
-    echo $! > "${SYNCERPIDFILE}"
-    disown $!
-  fi
-
   (
     flock 200
     echo $$ >> "$LOCKFILE"
 
+    # If there isn't a syncer daemon started already, start one. This daemon
+    # is killed by the enter_chroot that exits last.
+    if ! [ -f "${SYNCERPIDFILE}" ] || \
+       ! [ -d /proc/$(cat "${SYNCERPIDFILE}") ]; then
+      debug "Starting sync process"
+      env_sync_proc &
+      echo $! > "${SYNCERPIDFILE}"
+      disown $!
+    fi
+
     debug "Mounting chroot environment."
     ensure_mounted none "-t proc" /proc
     ensure_mounted none "-t sysfs" /sys