Add support for db locking in zsh (via flock)

This commit ensures the database is locked before reading and updating
when used in zsh. We load the zsh/system module which provides the
zsystem flock command to perform advisory file locking.

A separate code path is needed when updating the database entry because
replacing the file (via mv) would cause any process waiting for a lock
to fail. Here we simply clobber the database with the contents of the
tempfile.

z.sh

@@ -57,6 +57,13 @@ _z() {
 
         # maintain the data file
         local tempfile="$(mktemp "${datafile}.XXXXXXXX")"
+        if [ "$_Z_USE_ZSYSTEM_FLOCK" -eq 1 ]; then
+            # make sure datafile exists (for locking)
+            [ -f "$datafile" ] || touch "$datafile"
+            local lockfd
+            # grab exclusive lock (released when function exits)
+            zsystem flock -f lockfd "$datafile" || return
+        fi
         awk < <(_z_dirs 2>/dev/null) -v path="$*" -v now="$(date +%s)" -F"|" '
             BEGIN {
                 rank[path] = 1
@@ -80,12 +87,20 @@ _z() {
                 } else for( x in rank ) print x "|" rank[x] "|" time[x]
             }
         ' 2>/dev/null >| "$tempfile"
-        # do our best to avoid clobbering the datafile in a race condition.
-        if [ $? -ne 0 -a -f "$datafile" ]; then
+        local ret=$?
+        if [ "$_Z_USE_ZSYSTEM_FLOCK" -eq 1 ]; then
+            # replace contents of datafile with tempfile
+            env cat "$tempfile" >| "$datafile"
+            [ "$_Z_OWNER" ] && chown $_Z_OWNER:$(id -ng $_Z_OWNER) "$datafile"
             env rm -f "$tempfile"
         else
-            [ "$_Z_OWNER" ] && chown $_Z_OWNER:$(id -ng $_Z_OWNER) "$tempfile"
-            env mv -f "$tempfile" "$datafile" || env rm -f "$tempfile"
+            # do our best to avoid clobbering the datafile in a race condition.
+            if [ $ret -ne 0 -a -f "$datafile" ]; then
+                env rm -f "$tempfile"
+            else
+                [ "$_Z_OWNER" ] && chown $_Z_OWNER:$(id -ng $_Z_OWNER) "$tempfile"
+                env mv -f "$tempfile" "$datafile" || env rm -f "$tempfile"
+            fi
         fi
 
     # tab completion
@@ -212,6 +227,14 @@ alias ${_Z_CMD:-z}='_z 2>&1'
 
 [ "$_Z_NO_RESOLVE_SYMLINKS" ] || _Z_RESOLVE_SYMLINKS="-P"
 
+if type zmodload >/dev/null 2>&1; then
+    # load zsh/system for the zsystem flock builtin
+    zmodload zsh/system &>/dev/null
+    if zsystem supports flock &>/dev/null; then
+        _Z_USE_ZSYSTEM_FLOCK=1
+    fi
+fi
+
 if type compctl >/dev/null 2>&1; then
     # zsh
     [ "$_Z_NO_PROMPT_COMMAND" ] || {