Add upgrade locking for macs (#15038)

bin/flutter

@@ -33,7 +33,13 @@ function path_uri() {
   echo "$1" | sed -E -e "s,^/+,/,"
 }
 
+function _rmlock () {
+  [ -n "$FLUTTER_UPGRADE_LOCK" ] && rm -f "$FLUTTER_UPGRADE_LOCK"
+}
+
 function upgrade_flutter () {
+  mkdir -p "$FLUTTER_ROOT/bin/cache"
+
   # This function is executed with a redirect that pipes the source of
   # this script into file descriptor 3.
   #
@@ -47,14 +53,14 @@ function upgrade_flutter () {
   # file, having inherited the file descriptor from the shell.
   #
   # Complicating matters, there are two major scenarios where this
-  # will not work. The first is if the platform doesn't have "flock",
-  # for example on Mac. There does not appear to be any sort of other
-  # equivalent, so on platforms that don't have flock, we just don't
-  # do anything.
+  # will not work.
   #
-  # To determine if we have "flock" available, we abuse the "hash"
-  # shell built-in to determine if "flock" resolves to anything. If it
-  # does, we call it, otherwise we forget the whole thing.
+  # The first is if the platform doesn't have "flock", for example on Mac.
+  # There is not a direct equivalent, so on platforms that don't have flock,
+  # we fall back to using a lockfile and spinlock with "shlock".  This
+  # doesn't work as well over NFS as it relies on PIDs. Any platform
+  # without either of these tools has no locking at all. To determine if we
+  # have "flock" or "shlock" available, we abuse the "hash" shell built-in.
   #
   # The second complication is NFS. On NFS, to obtain an exclusive
   # lock you need a file descriptor that is open for writing, because
@@ -65,16 +71,20 @@ function upgrade_flutter () {
   # users will typically not care about errors from flock and are
   # more likely to be confused by them than helped.
   #
-  # The lock is released when the file descriptor goes out of scope,
-  # i.e. when this function returns.
+  # For "flock", the lock is released when the file descriptor goes out of
+  # scope,  i.e. when this function returns.  The lock is released via
+  # a trap when using "shlock".
   if hash flock 2>/dev/null; then
     flock 3 2>/dev/null || true
+  elif hash shlock 2>/dev/null; then
+    FLUTTER_UPGRADE_LOCK="$FLUTTER_ROOT/bin/cache/.upgrade_lock"
+    while ! shlock -f "$FLUTTER_UPGRADE_LOCK" -p $$ ; do sleep .1 ; done
+    trap _rmlock EXIT
   fi
 
   local revision=`(cd "$FLUTTER_ROOT"; git rev-parse HEAD)`
   if [ ! -f "$SNAPSHOT_PATH" ] || [ ! -s "$STAMP_PATH" ] || [ `cat "$STAMP_PATH"` != "$revision" ] || [ "$FLUTTER_TOOLS_DIR/pubspec.yaml" -nt "$FLUTTER_TOOLS_DIR/pubspec.lock" ]; then
     rm -f "$FLUTTER_ROOT/version"
-    mkdir -p "$FLUTTER_ROOT/bin/cache"
     touch "$FLUTTER_ROOT/bin/cache/.dartignore"
     "$FLUTTER_ROOT/bin/internal/update_dart_sdk.sh"
 
@@ -97,6 +107,11 @@ function upgrade_flutter () {
     "$DART" --snapshot="$SNAPSHOT_PATH" --packages="$FLUTTER_TOOLS_DIR/.packages" "$SCRIPT_PATH"
     echo "$revision" > "$STAMP_PATH"
   fi
+  # The exit here is duplicitous since the function is run in a subshell,
+  # but this serves as documentation that running the function in a
+  # subshell is required to make sure any lockfile created by shlock
+  # is cleaned up.
+  exit $?
 }
 
 PROG_NAME="$(path_uri "$(follow_links "$BASH_SOURCE")")"