Chroot: allow multiple simultaneous entry.

Change enter_chroot.sh to keep a list of open PIDs (enter_chroot.sh
instances) in chroot/var/lock, and use flock on that file to support
atomic operations to it.

When entering chroot, append the PID to the lock file.

When exiting chroot, clear out current PID and any dead
PIDs/duplicates. If the lock file is empty, only then clean up the
mounts.

Note that I have a little bit of code duplication: the number 200,
which is a manually numbered fd in bash, seemed impossible to place
inside a variable. If you know how to do it, please let me know.

Review URL: http://codereview.chromium.org/584010

enter_chroot.sh

@@ -59,7 +59,15 @@ fi
 # TODO: replace shflags with something less error-prone, or contribute a fix.
 set -e
 
+sudo chmod 0777 "$FLAGS_chroot/var/lock"
+
+LOCKFILE="$FLAGS_chroot/var/lock/enter_chroot"
+
 function setup_env {
+  (
+    flock 200
+    echo $$ >> "$LOCKFILE"
+
     echo "Mounting chroot environment."
 
     # Mount only if not already mounted
@@ -80,12 +88,44 @@ function setup_env {
     then
       sudo mount --bind "$FLAGS_trunk" "$MOUNTED_PATH"
     fi
+  ) 200>>"$LOCKFILE"
 }
 
 function teardown_env {
+  # Only teardown if we're the last enter_chroot to die
+
+  (
+    flock 200
+
+    # check each pid in $LOCKFILE to see if it's died unexpectedly
+    TMP_LOCKFILE="$LOCKFILE.tmp"
+
+    echo -n > "$TMP_LOCKFILE"  # Erase/reset temp file
+    cat "$LOCKFILE" | while read PID; do
+      if [ "$PID" = "$$" ]; then
+        # ourself, leave PROC_NAME empty
+        PROC_NAME=""
+      else
+        PROC_NAME=$(ps --pid $PID -o comm=)
+      fi
+
+      if [ ! -z "$PROC_NAME" ]; then
+        # All good, keep going
+        echo "$PID" >> "$TMP_LOCKFILE"
+      fi
+    done
+    # Remove any dups from lock file while installing new one
+    sort -n "$TMP_LOCKFILE" | uniq > "$LOCKFILE"
+
+    if [ -s "$LOCKFILE" ]; then
+      echo "At least one other pid is running in the chroot, so not"
+      echo "tearing down env."
+    else
       echo "Unmounting chroot environment."
       mount | grep "on $(readlink -f "$FLAGS_chroot")" | awk '{print $3}' \
         | xargs -r -L1 sudo umount
+    fi
+  ) 200>>"$LOCKFILE"
 }
 
 if [ $FLAGS_mount -eq $FLAGS_TRUE ]