Try to work better on NFS (#13955)

(untested)

bin/flutter

@@ -34,9 +34,41 @@ function path_uri() {
 }
 
 function upgrade_flutter () {
+  # This function is executed with a redirect that pipes the source of
+  # this script into file descriptor 3.
+  #
+  # To ensure that we don't simultaneously update Dart in multiple
+  # parallel instances, we try to obtain an exclusive lock on this
+  # file descriptor (and thus this script's source file) while we are
+  # updating Dart and compiling the script. To do this, we try to use
+  # the command line program "flock", which is available on many
+  # Unix-like platforms, in particular on most Linux distributions.
+  # You give it a file descriptor, and it locks the corresponding
+  # file, having inherited the file descriptor from the shell.
+  #
+  # Complicating matters, there are two major scenarios where this
+  # will not work. The first is if the platform doesn't have "flock",
+  # for example on Mac. There does not appear to be any sort of other
+  # equivalent, so on platforms that don't have flock, we just don't
+  # do anything.
+  #
+  # To determine if we have "flock" available, we abuse the "hash"
+  # shell built-in to determine if "flock" resolves to anything. If it
+  # does, we call it, otherwise we forget the whole thing.
+  #
+  # The second complication is NFS. On NFS, to obtain an exclusive
+  # lock you need a file descriptor that is open for writing, because
+  # NFS implements exclusive locks by writing, or some such. Thus, we
+  # ignore errors from flock. We do so by using the '|| true' trick,
+  # since we are running in a 'set -e' environment wherein all errors
+  # are fatal, and by redirecting all output to /dev/null, since
+  # users will typically not care about errors from flock and are
+  # more likely to be confused by them than helped.
+  #
+  # The lock is released when the file descriptor goes out of scope,
+  # i.e. when this function returns.
   if hash flock 2>/dev/null; then
-    flock 3 # ensures that we don't simultaneously update Dart in multiple parallel instances
-    # some platforms (e.g. Mac) don't have flock or any reliable alternative
+    flock 3 2>/dev/null || true
   fi
 
   local revision=`(cd "$FLUTTER_ROOT"; git rev-parse HEAD)`