Avoid file creation/deletion races in emsdk_env.sh by using a temporary file(#307)

Fixes https://github.com/emscripten-core/emscripten/issues/9090.

emsdk

@@ -2603,13 +2603,17 @@ def main():
 
     return 0
   elif cmd == 'construct_env':
-    silentremove(EMSDK_SET_ENV) # Clean up old temp file up front, in case of failure later before we get to write out the new one.
+    if len(sys.argv) == 2:
+      outfile = EMSDK_SET_ENV
+      silentremove(EMSDK_SET_ENV) # Clean up old temp file up front, in case of failure later before we get to write out the new one.
+    else:
+      outfile = sys.argv[2]
     tools_to_activate = currently_active_tools()
     tools_to_activate = process_tool_list(tools_to_activate, log_errors=True)
     env_string = construct_env(tools_to_activate, len(sys.argv) >= 3 and 'perm' in sys.argv[2])
-    open(EMSDK_SET_ENV, 'w').write(env_string)
+    open(outfile, 'w').write(env_string)
     if LINUX or OSX:
-      os.chmod(EMSDK_SET_ENV, 0o755)
+      os.chmod(outfile, 0o755)
     return 0
   elif cmd == 'update':
     update_emsdk()

emsdk_env.sh

@@ -14,7 +14,6 @@
 #     ./emsdk_env.sh
 #
 # which won't have any effect.
-
 SRC="$BASH_SOURCE"
 if [ "$SRC" = "" ]; then
   SRC="$0"
@@ -23,7 +22,9 @@ CURDIR="$(pwd)"
 cd "$(dirname "$SRC")"
 unset SRC
 
-./emsdk construct_env "$@"
-. ./emsdk_set_env.sh
+tmpfile=`mktemp` || exit 1
+./emsdk construct_env "$@" $tmpfile
+. $tmpfile
+rm -f $tmpfile
 
 cd "$CURDIR"