download stdlib to file before sourcing

the curl_retry_on_18 function re-starts the entire download, as -C - (continue) is not supported by all servers

if a download restarts this way, some file contents would be duplicate from the previous attempt, all sourced from stdout, breaking the code sourcing

CHANGELOG.md

@@ -10,6 +10,10 @@
 - Trap unhandled build errors with dedicated message [David Zuelke]
 - Summarize all emitted warnings if subsequent build error occurs [David Zuelke]
 
+### FIX
+
+- stdlib download during build init may theoretically fail on download restart [David Zuelke]
+
 ## v139 (2018-07-20)
 
 ### ADD

bin/compile

@@ -27,7 +27,18 @@ source $bp_dir/bin/util/common.sh
 trap 'err_trap' ERR
 
 # stdlib
-source /dev/stdin <<< "$(curl_retry_on_18 --silent https://lang-common.s3.amazonaws.com/buildpack-stdlib/v8/stdlib.sh)"
+# download to a file first, as the function restarts the entire download on code 18 connection reset (not all servers support -C)
+curl_retry_on_18 --fail --silent --location -o $bp_dir/bin/util/stdlib.sh https://lang-common.s3.amazonaws.com/buildpack-stdlib/v8/stdlib.sh || {
+	error <<-EOF
+		Failed to download standard library for bootstrapping!
+		
+		This is most likely a temporary internal error. If the problem
+		persists, make sure that you are not running a custom or forked
+		version of the Heroku PHP buildpack which may need updating.
+	EOF
+}
+source $bp_dir/bin/util/stdlib.sh
+rm $bp_dir/bin/util/stdlib.sh
 
 # failure counting
 source $bp_dir/bin/util/failures.sh