Atomic rehash

libexec/rbenv-rehash

@@ -1,31 +1,67 @@
 #!/usr/bin/env bash
 set -e
 
+SHIM_PATH="${HOME}/.rbenv/shims"
+PROTOTYPE_SHIM_PATH="${SHIM_PATH}/.rbenv-shim"
+
+# Create the shims directory if it doesn't already exist.
+mkdir -p "$SHIM_PATH"
+
+# Ensure only one instance of rbenv-rehash is running at a time by
+# setting the shell's `noclobber` option and attempting to write to
+# the prototype shim file. If the file already exists, print a warning
+# to stderr and exit with a non-zero status.
+set -o noclobber
+{ echo > "$PROTOTYPE_SHIM_PATH"
+} 2>/dev/null ||
+{ echo "rbenv: cannot rehash: $PROTOTYPE_SHIM_PATH exists"
+  exit 1
+} >&2
+set +o noclobber
+
+# If we were able to obtain a lock, register a trap to clean up the
+# prototype shim when the process exits.
+trap remove_prototype_shim SIGINT SIGTERM EXIT
+
+remove_prototype_shim() {
+  rm -f "$PROTOTYPE_SHIM_PATH"
+}
+
+# The prototype shim file is a script that re-execs itself, passing
+# its filename and any arguments to `rbenv exec`. This file is
+# hard-linked for every binary and then removed. The linking technique
+# is fast, uses less disk space than unique files, and also serves as
+# a locking mechanism.
 create_prototype_shim() {
-  cat > .rbenv-shim <<SH
+  cat > "$PROTOTYPE_SHIM_PATH" <<SH
 #!/usr/bin/env bash
 set -e
 exec rbenv exec "\${0##*/}" "\$@"
 SH
-  chmod +x .rbenv-shim
+  chmod +x "$PROTOTYPE_SHIM_PATH"
 }
 
+# Make shims by iterating over every filename argument and creating a
+# hard link from the prototype shim to a file of the same name in the
+# shims directory, if one does not already exist.
 make_shims() {
   local glob="$@"
 
   for file in $glob; do
     local shim="${file##*/}"
-    [ -e "$shim" ] || ln -f .rbenv-shim "$shim"
+    [ -e "$shim" ] || ln -f "$PROTOTYPE_SHIM_PATH" "$shim"
   done
 }
 
-mkdir -p "${HOME}/.rbenv/shims"
-cd "${HOME}/.rbenv/shims"
-rm -f *
+# Empty out the shims directory and make it the working directory.
+rm -f "$SHIM_PATH"/*
+cd "$SHIM_PATH"
 
+# Create the prototype shim, then make shims for all known binaries.
 create_prototype_shim
 make_shims ../versions/*/bin/*
 
+# Find and run any plugins that might want to make shims too.
 shopt -s nullglob
 RBENV_REHASH_PLUGINS=(/etc/rbenv.d/rehash/*.bash ${HOME}/.rbenv/rbenv.d/rehash/*.bash)
 shopt -u nullglob
@@ -33,5 +69,3 @@ shopt -u nullglob
 for script in ${RBENV_REHASH_PLUGINS[@]}; do
   source $script
 done
-
-rm -f .rbenv-shim