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Identical lookups issued while one is in flight are chained onto its pending-cache entry and settled from one reply. The five drain loops converted the reply once per global and resolved every waiter's promise with that one array, so a caller's sort(), shift() or length = 0 showed in the answers of the others, and dns.lookup() returned the order of whichever caller sorted first. Outcome is no longer Copy: settle() consumes it, so one converted answer can settle one promise. Each drain now converts the reply once per waiter, in that waiter's own global. The per-global bookkeeping and the keep_alive calls around the shared value are gone with it. Once a conversion meets the VM's termination, no later waiter of that chain is converted or settled.
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Status Reproduced on main (3f1765a) and on 1.4.3-canary with a loopback DNS server. Two The same run with With this change, both scripts print what Node prints. The three new tests in the |
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Review comments at @test/js/node/dns/node-dns.test.js:
- Around line 1303-1338: In the “after a termination the rest of a chain settles
nobody” test, assert the settled counters and queries first, then assert the
worker’s exitCode separately as the final assertion.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked the removal of the keep_alive/ensure_still_alive calls for a GC window: each for_waiter conversion result is moved straight into on_complete -> settle -> promise resolve, with only a mem::take of the promise and a global_this() read in between (no allocation), so no raw JSValue is held across a settle in any of the five drains. The pre-computed head outcome in drain_pending_host_native is likewise consumed before heap::take runs, and stopped is seeded from it so the termination short-circuit matches the other four loops.
Extended reasoning...
The change touches src/runtime/dns_jsc/dns.rs (the five pending-cache drain loops and the Outcome enum) and test/js/node/dns/node-dns.test.js; it touches no auth, crypto, or injection surface. It removes all keep-alive calls on a previously shared JSValue, which is the memory-safety category REVIEW.md flags most, so a human look alongside the inline findings is still worthwhile. The GC-window and host-native head-path concerns were examined and ruled out by reading the settle path.
| while let Some(value) = pending { | ||
| let new_global = (*value.as_ptr()).global_this(); | ||
| if !core::ptr::eq(prev_global, new_global) { | ||
| array = Outcome::of(new_global, addr.to_js_response(new_global, T::TYPE_NAME)); | ||
| prev_global = new_global; | ||
| } | ||
| let global = (*value.as_ptr()).global_this(); | ||
| pending = (*value.as_ptr()).next; | ||
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| keep_alive(&array); | ||
| CAresLookup::<T>::on_complete(value.as_ptr(), array); | ||
| keep_alive(&array); | ||
| CAresLookup::<T>::on_complete( | ||
| value.as_ptr(), | ||
| Outcome::for_waiter(&mut stopped, global, || { | ||
| addr.to_js_response(global, T::TYPE_NAME) | ||
| }), |
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🟡 (optional) Any process that opens many connections to one host at once now stalls its event loop for the whole burst, where the base settled the burst in one conversion. Every chained waiter runs addr.to_js_response again inside the single c-ares callback at dns.rs:4343-4345 (same shape at dns.rs:4405, 4465, 4529, 4580), and PendingCacheKey::append puts no cap on the chain. Fix: bound the per-callback work for a shared query, e.g. convert once per global and give each later waiter a structural copy (new array and record objects, shared strings), or cap the chain and start a new wire query past it, so a 10,000-waiter burst stays near the base's 4 ms rather than 157 ms.
Why this was flagged
The trigger is N overlapping identical lookups on one resolver: node:net connect, node:http without keep-alive, a pool warm-up or a Bun.ModuleGraph fan-out all route through dns.lookup and chain on one pending-cache slot. On the base the drain converted once per global (old prev_global branch at dns.rs:4331-4342 of the base) and settled all N waiters with that value, so the callback cost was O(records). After the change each waiter's for_waiter closure at dns.rs:4343 and siblings rebuilds the full answer (per PR: 7,566 instructions, 9 mallocs and 5 JS cells per extra 2-address waiter; 157 ms vs 4 ms for 10,000 waiters on 50 records), all inside one c-ares poll callback with no yield. Every other timer, socket and promise on that loop waits for it. The dismissal accepted the author's cost table as a trade-off; the author's own description asks whether to accept it or add a structural copy, which is the remedy.
Verification: Triggering condition: N overlapping identical lookups on one resolver that chain onto one pending-cache slot before the reply arrives. Outcome::for_waiter (src/runtime/dns_jsc/dns.rs:1884-1895) runs convert() for every waiter; the base converted once per distinct global. PendingCacheKey::append (dns.rs:1022-1030) only links the node onto the tail; there is no cap on chain length.
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This is the trade-off that the Downsides section puts to the reviewer. The numbers for it are in the Notes. Two points on the framing:
- A burst of
net.connect()to one hostname does chain on one lookup (lookupAndConnectinsrc/js/node/net.tscallsdns.lookup), and each connection now gets its own conversion. That cost is small beside the connects. 500 connections tolocalhostfor each batch, release builds, 6 alternating runs: 61.9 ms on main (59.3 to 64.1) and 63.3 ms here (59.4 to 68.7). The 157 ms case is 10,000 waiters on one 50-record answer. - A cap on the chain sends more wire queries for one name. This PR keeps the existing behavior that identical in-flight lookups share one query.
The structural copy stays the follow-up if a maintainer wants the lower cost for each waiter: convert once into a value that no caller receives, then give each waiter a new array and new records with the strings shared. Outcome stays move-only in that shape too.
The worker waited for the fake server's replies with no bound. A reply that never leaves the server kept the test until the file's 5 minute timeout. The wait now ends after 30 seconds and the worker exits with 2, which the assertion reports.
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I re-reviewed after the second commit and found no bugs; the gate wait in the termination test is now bounded (30s, worker exits 2, caught by the exitCode assertion), which closes the hang concern. A human look is still worthwhile: this rewrites all five unsafe pending-cache drain loops in src/runtime/dns_jsc/dns.rs, deletes the keep_alive calls from #37075, and the description leaves open a design call (per-waiter conversion cost on large chains vs a structural copy) that a maintainer should decide.
What was reviewed:
- Each
Outcomenow moves from its conversion closure straight intosettle->promise.resolve/reject, with nothing GC-visible held across a settle that can run user JS (thethengetter), so the deletedkeep_alive/ensure_still_alivecalls have no remaining referent. for_waitershort-circuit: once a conversion yieldsStopped, later waiters are completed and freed without converting; the libinfo path seedsstoppedfrom the precomputed head, matching the other four drains. Error drains (process_resolve/process_get_addr_info_nativewitherr) were already per-waiter and are untouched.pending = (*value).nextis still read beforeon_completefrees the node, and the five drains plus the per-requeston_completepaths are the onlyOutcomeproducers; no sibling site was missed.
Extended reasoning...
The PR changes src/runtime/dns_jsc/dns.rs (removes Copy from Outcome, adds Outcome::for_waiter, rewrites five pending-cache drain loops to convert the c-ares/libinfo reply once per waiter, deletes the keep_alive helpers) and adds four tests plus a richer fake DNS server in test/js/node/dns/node-dns.test.js. It touches no auth, crypto or injection surface; the sensitive surface is GC-safety and node-lifetime in unsafe Rust callback code. The second commit addressed the only nit from the prior review; the remaining open point is the author's own question about per-waiter conversion cost on large chains, which is a design decision rather than a defect. Because the change is unsafe-Rust drain-loop code that removes existing GC keep-alives and has an unresolved design question, it is not a candidate for approve-without-human-look.
Still open from earlier reviews (1):
- Unresolved: 1 minor or pre-existing.
Problem
sort()orshift()by one caller changed the others' answers, and the in-placeordersort ofdns.lookup()reached averbatimlookup. Node gives each caller its own.drain_pending_*loops (src/runtime/dns_jsc/dns.rs:4285) settled every waiter with oneCopyOutcome.Fix
Outcomeis notCopy:settleconsumes it. Each drain converts the reply once per waiter, in that waiter's global.keep_alivecalls around the shared value (Worker / worker_threads: WebCore-shaped lifetimes, joined threads, one ordered VM teardown #37075) go with it: each answer moves from its conversion intopromise.resolve.test/js/node/dns/node-dns.test.js(pending cache): 3 new tests fail on main, pass here. Also the other DNS suites,rust:check-all. Self-reviewed: 21 concerns raised, 21 addressed.Background
src/js/node/dns.ts: it missesBun.dns.*and charges lone callers.Downsides
lookup). A lone caller pays nothing new.bench/snippets/dns.ts(50 callers, one lookup): 133 to 258 µs at 2 records, 157 to 564 µs at 12. 10,000 waiters on a 50-record answer: 157 ms stall (main 4 ms).Notes
Source. Found by internal fuzzing. There is no issue and no user report. The shared array dates from the first DNS module (#1691).
Repro (loopback only, also runs on Node):
false false,1true true,0false false,1Code that hits it. Each row ran against a loopback DNS server (the Vite row against the hosts file), with the calls in flight together.
resolveExchange()of EmailEngine (lib/autodetect-imap-settings.js:resolveMx, thensort().shift()), two requests, domain with one MXmx10,mx10mx10, throwsNo MX record found for domainmx10,mx10mx10,mx10mx10,mx20mx10,mx10getLocalhostAddressIfDiffersFromDNS()of Vite (lookup("localhost")besidelookup("localhost", { verbatim: true })), default orderipv4first, on a host wherelocalhostis::1then127.0.0.1::1127.0.0.1::1Bun.ModuleGraphinstances of one app that doesresolveMx().sort().shift()at start, imported togethermx10, onemx20, six get nothing. Which instance is right changes per runmx10eight timesThe Vite row needs no mutation by any caller.
dns.tssorts the native array in place before it maps it, andorderis not part of the key of the shared query.What shared an answer on main. Every API that returns the native value:
resolve4/resolve6withttl,resolveMx,resolveTxt,resolveNs,resolveCname,resolvePtr,resolveSrv,resolveCaa,resolveNaptr,resolveSoa,resolveAny,resolve(name, type),reverse, in the callback and the promise form, onResolverinstances and on the default resolver, andBun.dns.lookup,Bun.dns.lookupService,Bun.dns.reverse.dns.lookup,resolve4/resolve6withoutttlandlookupServicemap the native value into a new one indns.ts. They returned distinct objects, but they read a list that another caller could already have sorted.When it happens. The calls must overlap on one resolver. On the c-ares paths that is the first overlapping burst for a name: c-ares keeps an answer cache, and a cached name completes inside the call with its own array. OS
lookuphas no such cache and shares on every overlapping burst. So===between two answers on the c-ares paths already depended on that cache.The five drains.
drain_pending_cares<T>(the 12 record types),drain_pending_host_cares(lookup, c-ares backend),drain_pending_host_native(lookup, OS backends: libc, macOS dns_sd, Windows libuv),drain_pending_addr_cares(reverse),drain_pending_name_info_cares(lookupService).Termination. On main, a conversion that met the VM's termination settled none of the waiters that shared it.
Outcome::for_waiterkeeps that: after such a conversion the rest of the chain is completed and freed withOutcome::Stopped, and no more conversions run. One difference: main converted again when the next waiter lived in another global, this PR does not. A conversion that throws an ordinary error now rejects only the waiter it ran for. The testafter a termination the rest of a chain settles nobodyfails when the latch is removed: two more answers are then settled in a worker that asked for its exit.The removed
keep_alivecalls. They kept one shared value alive across the completions of all waiters. There is no shared value now: eachOutcomeis built and moved intosettle, which passes the value topromise.resolve. The five per-request paths do the same and never had the call. GC stress on the debug ASAN build: athengetter onArray.prototypeandObject.prototypethat runsBun.gc(true)inside every settle, 200 batches of 25 coalesced calls for each of 11 call kinds (all five drains, 57,200 collections): 0 wrong answers, 0 ASAN reports.BUN_JSC_validateExceptionChecks=1on the repro and on a probe of all five drains: 0 failures. The testa collection inside every settle leaves each caller's answer intactkeeps a small version of this.Cost. linux-x64, release builds of main 3f1765a and of this change from one worktree. Instruction counts are gdb single-step counts with the JIT off, because perf and valgrind are not installed here. Times come from alternating runs of the two builds, with the range of the runs beside each value.
Per extra waiter, and for a lone caller:
lookuplookupmi_malloccalls per extra waiterlookupThe 16 drain-carrying functions are the only text symbols whose size differs between the two builds.
Whole shapes:
bench/snippets/dns.ts, rowlibc ((cached) dns.lookup remote x 50), 2-record answer (2 pairs)dns.lookup("localhost", { all: true }), per batch (5 pairs, median)dgram.send()tolocalhost, 1,000 per tick, per send (3 pairs)ttl(3 pairs)The bench rows ran with the unmodified bench file against a fake DNS server in a private network namespace. Its other rows do not chain callers on an answer.
What the numbers say:
node:dgramsocket does onedns.lookupfor eachsend()to a hostname, so a burst of sends is a chain of waiters.socket.connect(), an IP literal or a cachinglookupoption avoid it.The choice for the reviewer. This PR runs the whole conversion for each extra waiter. The cheaper shape converts once into a value that no caller receives and gives each waiter a structural copy of it: a new array and new records, with the strings shared. That needs a C++ helper, a rooted template value for each drain and a branch on the lone path.
Outcomestays move-only either way. It can follow this PR if the numbers above are too high.Meeting #39504, #40190 and #32023. All three are open, rewrite these loops and keep
OutcomeCopywith one value for all waiters: #39504 indrain_pending_cares, #40190 in all five drains, #32023 in all five through itsdrain_chain_okhelper. With only#[derive(Clone, Copy)]removed,cargo check -p bun_runtimereports 10 errors E0382, all in the five drains, so a rebase that keeps the shared loop does not compile. The resolution: keepOutcomewithoutCopyorCloneand callOutcome::for_waiter(&mut stopped, global, || convert(global))once per waiter. In #32023 that meansdrain_chain_okdrops itsarrayandprev_globalparameters and converts for each node. Text-level merge ofdns.rs(git merge-file): 2 more conflict hunks against #39504 (9 to 11), 6 more against #40190 (67 to 73), 7 more against #32023 (7 to 14). The three tests that fail on main carry the fix across a rebase.Tests. In the sandbox
node-dns.test.jshas 28 tests that need outside DNS. They fail the same way with and without the change. With the change: 147 pass, 28 fail. Without it: 144 pass, 31 fail (the same 28 and the 3 new tests). Two more new tests pass on main too: they guard the removedkeep_alivecalls and the termination latch. The order test is skipped wherelocalhosthas one address family, because the two orders are then the same.resolve-dns.test.ts: the same 25 outside-DNS failures both ways. The other files intest/js/node/dns/andtest/js/bun/dns/pass.Not in this PR. Bun reports
ENOTFOUNDwhere Node reportsENODATAfor an empty answer. #32816 had a fix for that and was closed as stale.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/dns/node-dns.test.js