Mutable state outside FindProxyForURL
low
A top-level variable that FindProxyForURL writes to (a cache, a counter, a "first call" flag) assumes one persistent JavaScript context. Engines may run several contexts or recreate them, so the state is unreliable and behaviour becomes non-reproducible.
Why it matters
Engines are free to evaluate the PAC on more than one thread, to recreate the context after
a re-fetch, or to keep it for the whole session. Code that memoises dnsResolve results in a
top-level object or counts calls therefore works by accident, differs between engines and
makes support cases impossible to reproduce. FindProxyForURL should be a pure function of
its two arguments; hoisting within one call (PAC-P004) is fine, cross-call state is not.
Draft: per-engine context lifetime has not been measured in the lab.
How to fix
Keep top-level declarations constant (lists, proxy strings) and do all per-request work inside the function.
Examples
Bad
var cache = {};
function FindProxyForURL(url, host) {
if (!(host in cache)) {
cache[host] = isInNet(dnsResolve(host), "10.0.0.0", "255.0.0.0");
}
return cache[host] ? "DIRECT" : "PROXY proxy.corp.example:8080";
}
Good
var internalSuffixes = [".corp.example", ".lab.example"];
function FindProxyForURL(url, host) {
for (var i = 0; i < internalSuffixes.length; i++) {
if (dnsDomainIs(host, internalSuffixes[i])) {
return "DIRECT";
}
}
return "PROXY proxy.corp.example:8080";
}