process.env: coerce to string on every execution of an assignment, not just the first

[robobun]

Problem

• Only the first assignment to a process.env key from a given statement is coerced to a string; later executions store the raw value. Node stores a string every time.

  for (let i = 0; i < 3; i++) { process.env.X = i; console.log(typeof process.env.X); }
  // bun 1.4.0: string string number        node: string string string

  An object value ends up stored as the object itself, and Bun.env, {...process.env}, JSON.stringify(process.env) and structuredClone(process.env) all expose the raw value.
• The same caching aborts the process when the assigned value's toString() touches process.env. put() ToStrings the value before it stores it, so that code runs inside the store and can add or delete a key. JSC's put caches cannot model a structure transition in a store to an existing property: slow_path_put_by_id hits RELEASE_ASSERT(newStructure == oldStructure) (LLIntSlowPaths.cpp:1148) and tryCachePutBy has the same assert (Repatch.cpp:1105). One line is enough, on its first execution, in release builds too. It ships since 1.4.0. Bun 1.3.14 stores the raw object and never calls toString(), so it cannot abort. process: port Node.js v26.3.0 process compatibility tests and fix the gaps they surface (env exotic-object/TZ semantics, warnings pipeline + CLI flags, uncaught origin/exit codes, execve throw, threadCpuUsage/finalization/loadEnvFile, native-module identity; +26 tests) #31831 (45eda51) replaced the plain object with this class and its coercing put().

  process.env.X = { toString() { process.env.X = "inner"; return "outer" } };
  // bun 1.4.0, 1.4.1, 1.4.2, 1.4.3-canary.1: panic(main thread): abort() called, exit 134
  // bun 1.3.14: exit 0

• Cause 1, named keys: JSEnvironmentVariableMap (POSIX process.env) only set OverridesPut (src/jsc/bindings/JSEnvironmentVariableMap.h:21). Its put() ToStrings the value and stores it (JSEnvironmentVariableMap.cpp:133-167), but the put caching paths decide from the PutPropertySlot and the structure, never from OverridesPut: the LLInt put_by_id cache (LLIntSlowPaths.cpp, slow_path_put_by_id), the baseline/DFG/FTL put ICs (Repatch.cpp, tryCachePutBy) and the DFG's structure-only PutByStatus::computeFor used by tryFoldAsPutByOffset. Execution 1 of a site goes through put(), execution 2 gets cached as a plain store, execution 3 (2 if the key already existed) stores the raw value.
• TZ and NODE_TLS_REJECT_UNAUTHORIZED are stored with putDirect inside put(), which keeps them out of the LLInt/JIT ICs but not out of the DFG path: once a read IC has watched the property and one write has replaced it, PutByStatus::computeFor reports a plain Replace and the DFG folds the write. On the unfixed build env.TZ = zone in a hot function stops changing the timezone at about the 100th call while the stored string still looks right (call 99: stored UTC, offset -540 in the new test), and a non-string assigned to NODE_TLS_REJECT_UNAUTHORIZED is stored raw.
• Cause 2, integer-like keys (process.env[700] = 1, process.env["701"] = 1): these go through the putByIndex override, but the first store gave the object ordinary indexed storage, and JSObject::putByIndexInline stores into that vector directly (trySetIndexQuickly) without consulting the method table. So the second store to an index, and even the first store to a neighbouring index, skipped putByIndex. Independent of the JIT; fails on the first loop iteration for the neighbouring key.
• Windows is not affected by either: its process.env is a Proxy whose set trap coerces on every write.

Fix

• JSEnvironmentVariableMap::StructureFlags gains two flags (the only src/ change):
  • ProhibitsPropertyCaching: makes Structure::propertyAccessesAreCacheable() false, which is the predicate all three put caching paths above check, so every [[Set]] takes the generic path and OverridesPut dispatches it to put(). Same flag the SHARE_ENV process.env (JSSharedEnvMap, same file) and JSC::ProxyObject use; require.extensions: assignments from a repeated statement no longer bypass the loader table #38854 applies it to require.extensions, the only other OverridesPut class in src/ without it (the remaining three, JSSharedEnvMap, NodeVM.h, NodeSqlite.h, already have it). Both abort sites are behind that same predicate, so the flag removes the crash with the stale value.
  • InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero: JSObject::indexingShouldBeSparse() then puts indexed properties in the sparse map (putByIndexBeyondVectorLength, blank case), which has no quick-store path, so every indexed store reaches putByIndex(). Also what JSSharedEnvMap sets. Enumeration order, structuredClone, JSON.stringify, spread, defineProperty and delete of index keys were checked against node and are unchanged apart from the values now being strings (probe below).
• slot.disableCaching() in put() is not a fix: it only reaches the IC paths. I built that variant and the DFG still bypassed put() after 1100 to 2100 calls in every shape tried, including a function that only writes (probe below). On a WebKit with [JSC] A store to Error.stackTraceLimit from JIT code keeps updating the stack trace limit WebKit#640 that changes: the DFG's structure-only fold then respects OverridesPut, so slot.disableCaching() plus the indexed flag fixes every case and keeps the read cache. Measured with a stand-in for Quoting Code does not work for bun install in the "Not implemented yet" section in README.md #640 in this comment. Which shape to land is an open maintainer decision.
• Cost: reads of process.env lose their inline cache too, since JSC has no put-only flag. Release bun 1.4.0 with BUN_JSC_forceICFailure=1 as a stand-in for the uncached path: a process.env.X read goes from about 2.6 ns to about 44 ns, a write from the (incorrect) 4.7 ns direct store to about 43 ns, which is what put() already cost from any uncached site. Node reads in about 200 ns and writes in about 500 ns on the same machine. Bun's own built-ins read process.env at module init or per connection, not per operation.
• delete ICs consult the same predicate, but there was no observable bypass to test: each set-then-delete cycle leaves the object in a new structure, so a delete IC never hit.
• Verification, all in test/js/node/process/process.test.js; each test fails on bun 1.4.0 (the first and third also checked against a debug build of main) and passes with this diff:
  • "coerces to string on every execution of the same assignment": fresh key, pre-existing key, computed key, object with toString, four passes each (fails from pass 3).
  • "coerces integer-like keys to string on every assignment": repeated index store, string-form index, first store to a neighbouring index (fails on pass 1 of the string-form key).
  • "reads are never stale and writes always coerce across JIT tiers": the existing IC-soundness test, now writing numbers; its functions reach Baseline, DFG and FTL within the loop (BUN_JSC_reportCompileTimes).
  • "TZ and NODE_TLS_REJECT_UNAUTHORIZED writes still reach put() once optimized": asserts the timezone offset and the stored string on every call with BUN_JSC_useConcurrentJIT=0; BUN_JSC_reportCompileTimes shows writeTZ DFG-compiled before the unfixed build fails at call 99, and both functions DFG-compiled on the fixed build. The default tier-up thresholds are load-bearing: with jitPolicyScale=0 the DFG compiles before the replacement watchpoint exists and the unfixed build passes (noted in the test).
  • "survives a value whose toString() mutates process.env": the same-key, add-key and delete-key forms, plus a Symbol.toPrimitive value written 5000 times from one site. Each form exits 134 (SIGABRT) on release bun 1.4.3-canary.1 and on a debug build of main at 4ff9193; all four print their coerced string with this diff.
  • Also run with the debug build: the rest of process.test.js, test/cli/run/env.test.ts, the worker_threads env tests and the eight vendored test/js/node/test/parallel/test-process-env*.js files (one of which covers structuredClone(process.env)).

Background

• put() / putByIndex() are JSC's [[Set]] hooks on a class's method table. OverridesPut makes JSObject::putInline dispatch named stores to the class's put(); it says nothing about caching, and indexed stores are dispatched separately.
• Inline cache (IC): after a property access site runs once, JSC patches the site to repeat the outcome directly ("object has structure S, store at offset N") whenever the structure matches again, without calling into the runtime. The LLInt has a per-site cache, the JITs have ICs, and the DFG additionally folds accesses at compile time from what it knows about the structure (PutByStatus).
• PutPropertySlot is the record a put() fills in describing what it did; isCacheablePut() is how the LLInt and JIT caches learn whether a site may be cached. Base::put fills it in as cacheable; putDirect does not touch it. The DFG path does not look at it at all.
• Replacement watchpoint: when a read IC caches a property, JSC starts watching that property slot for replacement; the DFG will only fold a write into a plain store once such a watchpoint exists and has fired, which is why the TZ bypass needs a read plus one earlier write before it shows up.
• ProhibitsPropertyCaching is a structure flag meaning "never cache property accesses on this structure"; Structure::propertyAccessesAreCacheable() returns false for it.
• Indexed storage: a plain object stores integer-like keys in a vector (butterfly) once it has any; stores into that vector bypass the method table. InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero is the flag JSC uses to mark objects that must see every indexed access; it switches them to the sparse map, where every indexed get/put goes through the method table.

Probe: slot.disableCaching() alone vs the structure flag (debug builds)

Each line runs a function 200000 times with process.env.PROBE_B starting as "0" and reports the first call after which a number is stored.

== slot.disableCaching() in put(), no structure flag ==
closure read+write: BYPASS at call 1122 (stored a number)
process.env write+read: BYPASS at call 1591 (stored a number)
write-only fn, read elsewhere: BYPASS at call 2147 (stored a number)
write-only, typeof after loop: BYPASS at call 1832 (stored a number)

== ProhibitsPropertyCaching (this PR) ==
all four shapes: no bypass in 200000 calls

== unfixed ==
all four shapes: BYPASS at call 2

Probe: integer-like keys with the sparse flag, compared with node

Run with an OS env var literally named 5 set to five, then env[42] = 1; env[42] = 2;.

                      bun 1.4.0                      this PR                       node 26
env[42]               2                              "2"                           "2"
descriptor.value      2                              "2"                           "2"
structuredClone       2                              "2"                           "2"
JSON.stringify        2                              "2"                           "2"
{...env}[42]          2                              "2"                           "2"
Object.entries        [["5","five"],["42",2]]        [["5","five"],["42","2"]]     [["5","five"],["42","2"]]
defineProperty 43     "3"                            "3"                           "3"
delete env[42]        gone                           gone                          gone
Object.keys order     index keys first (unchanged)   index keys first (unchanged)  env order

The key order difference is pre-existing JSC object behavior and is the same before and after.

Cost measurement

Hot function reading / writing one process.env key, 1M iterations after warm-up, same linux x64 container.

bun 1.4.0 release (unfixed)                     read 2.6 ns   write 4.7 ns (direct store, the bug)
bun 1.4.0 release, BUN_JSC_forceICFailure=1     read 44 ns    write 43 ns
node v26.3.0                                    read 197 ns   write 530 ns

Earlier version of this PR

The first revision only added ProhibitsPropertyCaching and claimed TZ / NODE_TLS_REJECT_UNAUTHORIZED were unaffected because put() stores them with putDirect. Self-review showed that claim only holds for the LLInt/JIT ICs, not for the DFG path, and found the unrelated-to-JIT indexed-storage bypass for integer-like keys; the second revision adds the indexed flag and the two extra tests described above.

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no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/process/process.test.js

[robobun]

^{Updated 3:13 AM PT - Sep 10th, 2026}

❌ @robobun, your commit 66706cc has 1 failures in Build #113787 (All Failures):

• test/js/bun/http/serve-pending-promise-abort-leak.test.ts - code 1 on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 38871

That installs a local version of the PR into your bun-38871 executable, so you can run:

bun-38871 --bun

[robobun]

Status: reproduced on bun 1.4.0 with for (let i = 0; i < 3; i++) process.env.X = i (third pass stores a number; second when the key already existed), with process.env[700] = i in a loop (raw from the second store), and with a hot env.TZ = zone function (timezone stops changing at call 99). Fix is the two structure flags in JSEnvironmentVariableMap.h; the tests in test/js/node/process/process.test.js fail without them and pass with them.

Rebased onto main at 66706cc (build 113787). The diff is unchanged. Every lane that ran is green except one: test/js/bun/http/serve-pending-promise-abort-leak.test.ts on the x64 ASAN lane, a WeakRef GC test for streaming Response bodies from #41080 that this change does not touch (reported separately). The other four entries are flakes that passed on retry. Ready for review.

[coderabbitai]

Walkthrough

JSEnvironmentVariableMap now prohibits property caching. Process tests verify current string-coerced values across repeated assignments, computed keys, object values, and JIT tiers.

Changes

process.env write semantics

Layer / File(s)	Summary
Route writes through put() src/jsc/bindings/JSEnvironmentVariableMap.h	JSEnvironmentVariableMap::StructureFlags now includes JSC::ProhibitsPropertyCaching.
Validate repeated and optimized writes test/js/node/process/process.test.js	Tests cover string coercion, temporary variable cleanup, optimization tiers, hot reads, and post-warmup writes.

Possibly related PRs

• oven-sh/bun#38821: Both PRs modify JSEnvironmentVariableMap and process.env tests, but address different concerns.

Suggested reviewers: jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary change: coercing process.env assignment values to strings on every execution.
Description check	✅ Passed	The description explains the problem, implementation, risks, and verification in detail. It does not use the template headings exactly, but it includes the required information for what the PR does an…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/node/process/process.test.js`:
- Around line 435-461: Update the environment setup around the repeated
assignment test to capture each target key’s prior process.env value before
overwriting it, then restore those values in the finally block instead of
unconditionally deleting the keys. Preserve deletion only for keys that were
previously absent, including the computed key and all keys used by the test.

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• src/jsc/bindings/JSEnvironmentVariableMap.h
• test/js/node/process/process.test.js

[claude]

LGTM — one-line structure-flag addition with thorough tests.

What was reviewed:

• Confirmed ProhibitsPropertyCaching matches the sibling JSSharedEnvMap (same file, line 603) and the pattern in NodeVM.h/NodeSqlite.h.
• The new in-process test covers fresh/pre-existing/computed keys and object toString; the reworked JIT-tier subprocess test now writes numbers on every call and still reaches FTL at N=30000.
• Checked that the alternative (slot.disableCaching()) was empirically ruled out and the read-side perf cost is measured and acceptable (still well under Node).

Extended reasoning...

Overview

This PR adds JSC::ProhibitsPropertyCaching to JSEnvironmentVariableMap::StructureFlags — a single flag on one line in the header. The rest of the diff is a four-line explanatory comment, one new test, and a strengthening rewrite of the existing JIT IC-soundness test in test/js/node/process/process.test.js. The bug: repeated executions of the same process.env.X = <non-string> site got cached as a plain offset store by the LLInt/JIT put IC or DFG PutByStatus fold, bypassing put() and its ToString.

Security risks

None. This is a Node-compat correctness fix to process.env write semantics; no auth, crypto, or untrusted-input parsing is touched. If anything the change is strictly safer — it guarantees the overridden put() (which handles TZ/NODE_TLS_REJECT_UNAUTHORIZED side-effects and ToString) is always reached rather than sometimes bypassed.

Level of scrutiny

Low-to-medium. The functional change is one well-known JSC structure flag, applied to bring this class in line with its file-local sibling JSSharedEnvMap (which already sets it at JSEnvironmentVariableMap.cpp:603) and with JSC::ProxyObject, NodeVMGlobalObject, and NodeSqlite's namespace object. The PR description traces the mechanism through every put-caching path (LLInt, Repatch ICs, DFG PutByStatus::computeFor), empirically demonstrates why slot.disableCaching() alone is insufficient (DFG fold bypass at ~1100–2100 calls), and measures the read-side perf cost (2.6 ns → ~44 ns; Node is ~200 ns). This is exactly the depth REVIEW.md asks for when making a non-obvious choice.

Other factors

• Tests: The new test covers the variant matrix (fresh key, pre-existing key, computed put_by_val key, object with toString) over 4 iterations each with a single combined-object assertion and finally cleanup. The existing JIT-tier subprocess test was strengthened to write a number on every one of 30k calls (previously it only did one non-looped coerce check that wouldn't have caught the IC bypass), and its iteration count was reduced from 100k+200k to 30k+30k while still reaching FTL per BUN_JSC_reportCompileTimes — faster and stronger. Both tests are hermetic, deterministic, and fail-for-the-right-reason verified per the PR description.
• Pattern consistency: The fix follows the exact approach used by the SHARE_ENV variant in the same file and by #38854 for the same bug class on require.extensions.
• No outstanding review comments; only robobun build-status noise on the timeline.

[github-actions]

This PR may be a duplicate of:

1. process.env: Node-semantics exotic object on POSIX; coerce/validate/setenv-sync, first-wins dup load, typed-cache invalidate #35882 - Replaces POSIX process.env with a JSProcessEnvMap whose StructureFlags already include JSC::OverridesPut | JSC::ProhibitsPropertyCaching plus ToString in put(), containing this PR's one-line fix as a subset of the same file.
2. process: back main-thread process.env by live libc environ on POSIX #35270 - Adds JSRealEnvMap for main-thread process.env with the same OverridesPut | ProhibitsPropertyCaching flags and string coercion in put(), so it subsumes this fix.
3. process.env: drop '='/empty keys, truncate NUL in key/value like Node #35264 - Diagnoses the identical put-IC bypass on process.env and fixes it with slot.disableCaching() — the alternative approach this PR evaluates and rejects as insufficient.
4. process.env: coerce assigned values to strings across all construction paths #34728 - Same goal (every process.env assignment coerces to string) across the same two files, via a JSProcessEnvMap with OverridesPut.

🤖 Generated with Claude Code

[robobun]

Not duplicates. All four were opened in July (July 19 to 26), before #31831 landed on Aug 7 and made JSEnvironmentVariableMap the exotic object this PR fixes; each of them introduces its own new env class instead, none of them touches JSEnvironmentVariableMap::StructureFlags, and all four currently conflict with main (mergeable_state: dirty). They are also much larger, multi-concern changes (+298 to +1058 lines across 7 to 13 files). Per PR:

• process.env: Node-semantics exotic object on POSIX; coerce/validate/setenv-sync, first-wins dup load, typed-cache invalidate #35882 and process: back main-thread process.env by live libc environ on POSIX #35270: their new classes (JSProcessEnvMap / JSRealEnvMap) do set ProhibitsPropertyCaching, but as part of replacing the object wholesale (native-store backed reads, live environ, key normalization, ...). The class that exists on main today came from process: port Node.js v26.3.0 process compatibility tests and fix the gaps they surface (env exotic-object/TZ semantics, warnings pipeline + CLI flags, uncaught origin/exit codes, execve throw, threadCpuUsage/finalization/loadEnvFile, native-module identity; +26 tests) #31831, after both of these.
• process.env: drop '='/empty keys, truncate NUL in key/value like Node #35264: its new class uses OverridesPut plus slot.disableCaching() in put(), with the stated goal of keeping reads on the IC path. That is the variant this PR's description rules out: built against main, the DFG still folds the put and stores the raw value after about 1100 to 2100 executions in every shape I probed (output in the PR body), because PutByStatus::computeFor works from the structure and never sees the slot. So it would not fix this bug even if it applied.
• process.env: coerce assigned values to strings across all construction paths #34728: its new class sets only OverridesPut, i.e. the same state main is in now; superseded by process: port Node.js v26.3.0 process compatibility tests and fix the gaps they surface (env exotic-object/TZ semantics, warnings pipeline + CLI flags, uncaught origin/exit codes, execve throw, threadCpuUsage/finalization/loadEnvFile, native-module identity; +26 tests) #31831.

This PR is the one-flag fix for the bug as it exists on main after #31831.

[claude]

I reviewed this PR and didn't find any bugs. The one-flag fix is correct and matches the pattern already used by JSSharedEnvMap in the same file (and by NodeVM / NodeSqlite). Because it trades a documented ~17x read slowdown on process.env for correctness — a call a maintainer should sign off on — and the comment-cop bot is still flagging the header comment on the latest commit, a human look would still be worthwhile.

What was reviewed:

• Verified ProhibitsPropertyCaching is the established pattern for OverridesPut classes in this codebase (JSSharedEnvMap at JSEnvironmentVariableMap.cpp:603, NodeVM.h, NodeSqlite.h).
• Checked the reworked JIT-tier test still asserts the read-staleness property the original protected (it does — readHot() after warmup + post-write).
• Confirmed the new in-process test's four env keys are test-owned and cleanup follows the file's convention.

Extended reasoning...

Overview

One-line native change: adds JSC::ProhibitsPropertyCaching to JSEnvironmentVariableMap::StructureFlags so every process.env.X = value reaches put() (and its ToString) instead of being served by an LLInt/JIT put IC or DFG PutByStatus fold after the first execution. Tests: one new in-process test (fresh key, pre-existing key, computed key, object with toString, 4 iterations each) and a rewrite of the existing JIT IC-soundness subprocess test to write numbers so each tier's write path is exercised.

Security risks

None. This tightens behavior toward Node's (values always coerced to string). No new user-controlled input paths, no auth/crypto/permissions.

Level of scrutiny

Medium-high. The flag itself is a well-known JSC structure flag already used on the sibling JSSharedEnvMap class in the same file, on NodeVM and on NodeSqlite — so the mechanism is established and low-risk. But it changes property-access semantics on a process-global object every package can touch, and the PR body itself documents that reads go from ~2.6ns to ~44ns because JSC has no put-only prohibit flag. That is a correctness-over-perf tradeoff on process.env reads that a maintainer should explicitly acknowledge (per .claude/docs/landing-prs.md Performance section).

Other factors

• The fix is exceptionally well-argued: the description traces the bypass through all three JSC put-caching paths, empirically rules out slot.disableCaching() (DFG bypass at ~1100–2100 calls), and benchmarks the cost against Node.
• The comment-cop bot fired again on the latest commit (0745ae6, after the author already shortened the comment once); that thread is unresolved.
• The initial CI build (582ebc1) reported failures and there's no updated status visible on the latest commit — worth confirming green before merge.
• The rewritten JIT test drops N from 100000 to 30000 iterations and now uses named functions; the PR body says FTL lands around call ~10000, so 30000 still covers all tiers, and the test still guards the original read-staleness invariant plus the new write-coercion one.

[robobun]

Note: #40571 carries the same two StructureFlags and all four process.test.js cells from this PR, because its OS-environment write-through needs every assignment to reach put() (without the flags the third process.env.X = i in a loop never reaches it, so the OS value stays at "1"). This PR is the smaller, independent change and can land first; #40571 then rebases onto it. If #40571 lands first, this PR becomes empty and can be closed.

[robobun]

One correction to the walkthrough above: the header change adds two structure flags, not one. ProhibitsPropertyCaching keeps named stores out of the put caches. InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero keeps integer-like keys (process.env[700] = 1) on putByIndex(). The PR body describes both under Fix. No code change is needed for this comment.

[robobun]

The put caching this PR opts out of also aborts the process. The new commit covers that case.

put() ToStrings the value before it stores it, so the value's toString() runs inside the store. If that code adds or deletes a key, the structure transitions while the store targets an existing property. JSC's put caches cannot model that: slow_path_put_by_id hits RELEASE_ASSERT(newStructure == oldStructure) (LLIntSlowPaths.cpp:1148), and tryCachePutBy has the same assert (Repatch.cpp:1105). Both sites sit behind oldStructure->propertyAccessesAreCacheable(), which ProhibitsPropertyCaching turns off, so this diff removes the abort together with the stale value.

Exit codes, release bun 1.4.3-canary.1 (5f55496) against a debug build of main at 4ff9193 with this PR's header change applied:

form                                                              unfixed   with the flags
process.env.X = { toString() { process.env.X = "i"; ... } }         134            0
toString() adds a different key                                     134            0
toString() deletes a different key                                  134            0
Symbol.toPrimitive, 5000 writes from one site                       134            0
toString() only reads process.env                                     0            0

134 is SIGABRT. The read-only form never transitions the structure, so it never reaches the assert. The first form needs no warm-up: the assert fires on the first execution of the statement, in the LLInt, in release builds too.

The new test is "process.env survives a value whose toString() mutates process.env" in test/js/node/process/process.test.js. It spawns one child that runs the four crashing forms in order, so the output names the first form that dies.

What was run: this PR's diff applies cleanly to today's main (4ff9193). With it applied there, the whole of process.test.js passes, the new test included (173 pass, 4 skip, 1 failure that is pre-existing in my container because USER is unset in the environment). The released binary exits 134 on the new test. The branch itself is not rebased.

#40571 carries the same two flags, so it removes the abort as well.

[claude]

I re-reviewed after the rebase and didn't find any bugs. My earlier concern about the TZ/TLS test's iteration count is addressed — the test comment now explains why the default tier-up thresholds (with useConcurrentJIT=0) are load-bearing and why jitPolicyScale=0 would let the unfixed build pass. Given the documented read-path perf tradeoff from ProhibitsPropertyCaching, a maintainer sign-off on that tradeoff is still worthwhile.

What was reviewed:

• The two new StructureFlags match JSSharedEnvMap in the same file line-for-line, so this brings the POSIX process.env in line with the SHARE_ENV variant.
• The five tests follow harness conventions (spawned with bunExe()/bunEnv, await using, pipes drained via Promise.all, single composite .toEqual, test.concurrent for the subprocess cases, cleanup in finally).
• Checked that the in-process tests don't leak env keys into later tests — all four/three keys are deleted in finally.

Extended reasoning...

Overview

The PR adds JSC::ProhibitsPropertyCaching and JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero to JSEnvironmentVariableMap::StructureFlags in src/jsc/bindings/JSEnvironmentVariableMap.h, so that every write to process.env reaches the overridden put()/putByIndex() and gets ToString-coerced (matching Node), and so that a value whose toString() mutates process.env mid-store no longer trips JSC's slow_path_put_by_id structure-mismatch RELEASE_ASSERT. The existing IC-soundness test in test/js/node/process/process.test.js is replaced with five focused tests covering the LLInt put cache, fast indexed storage, the SIGABRT crash, cross-tier read/write soundness, and the DFG fold of TZ/NODE_TLS_REJECT_UNAUTHORIZED writes.

Security risks

None identified. The change tightens behavior (forces the slow path so side effects and coercion always run) rather than loosening it. No new user input is parsed and no trust boundary is crossed.

Level of scrutiny

Medium. The src/ change is three lines and mirrors JSSharedEnvMap's flags in the same translation unit exactly, so the pattern is established in-file. However, disabling property caching on process.env is a deliberate perf regression on reads (~2.6 ns → ~44 ns per the PR body), and while the author's justification (still ~5× faster than Node, built-ins read process.env at init/per-connection rather than per-op) is reasonable, that's a product tradeoff a maintainer should ratify rather than an automated review.

Other factors

My prior inline concern (that N=200 with only useConcurrentJIT=0 might not reach the DFG) was addressed by commit 129104e, which adds a comment explaining that with non-concurrent JIT the default thresholds compile writeTZ at ~call 100 (unfixed build fails at call 99) and that jitPolicyScale=0 would compile before the replacement watchpoint exists and let the unfixed build pass — i.e., raising N or lowering thresholds would defeat the test. The tests otherwise follow the repo's harness conventions closely: test.concurrent for the three independent subprocess tests, await using on spawns, concurrent pipe draining, composite-object .toEqual assertions, and try/finally cleanup of the env keys the in-process tests set. No outstanding third-party CHANGES_REQUESTED; the only unresolved-by-non-author threads are author-self-resolved bot comments on the header, which the current one-line comment appears to have settled.

[robobun]

Independent check of the abort that the new spawned test covers, on this branch (66706cc) and on a debug ASAN build of main. The abort is:

ASSERTION FAILED: newStructure == oldStructure
vendor/WebKit/Source/JavaScriptCore/llint/LLIntSlowPaths.cpp(1148) : llint_slow_path_put_by_id

Release builds exit 134 with the crash banner. A debug build prints the assert.

The number of variables the process starts with decides whether two of the four doors fire. Each cell is one process, env -i with N variables, debug build of main (unfixed), exit code:

door	N=10	N=49	N=200	N=400
SAME: toString() creates the key being assigned	134	134	0	0
ADD: toString() adds another key	134	134	0	0
DEL: toString() deletes another key	134	134	134	134
HOT: Symbol.toPrimitive adds and deletes, 5000 calls	134	134	134	134

So the test as written discriminates at any environment size, because the last two doors do not depend on it. Keep them if the test is ever trimmed. Above about 120 starting variables process.env gets a dictionary structure, so put_by_id no longer takes the replace path that carries the assert, and the first two doors exit 0.

The first door needs a structure change, not just re-entry. Same build, N=49:

• process.env.K = { toString() { process.env.K = "inner"; return "outer" } } aborts. toString() creates K, so the structure changes under the store.
• The same statement after process.env.K = "seed" exits 0. The nested store replaces an existing property, so the structure does not change.
• process.env.K = "seed" followed by a toString() that sets a different key aborts again.

With this branch's diff, all of the shapes above exit 0 at N=10, 49, 200 and 400, and the spawned script prints the expected four lines.

Other results from the same build:

• test/js/node/process/process.test.js: 173 pass, 4 skip, 1 fail. The failure is the process test, which requires process.env.USER. My container does not set it. It fails the same way with src/ reset to main.
• CI build 113787 has one failure that is not marked flaky: serve-pending-promise-abort-leak.test.ts on debian 13 x64-asan, expect(alive).toBe(0) received 1 after 20 Bun.gc(true) rounds. That is a WeakRef liveness assertion in the HTTP tests, not an env property path. It does not appear in the last 8 main builds, so it looks like an ASAN timing flake rather than a break on main.

[robobun]

A put-only alternative, measured. It shows that this fix does not have to cost the read cache.

The read cost of this PR comes from ProhibitsPropertyCaching, which turns off every inline cache for process.env, reads included. The put-only shape leaves reads alone:

 // JSEnvironmentVariableMap.h
-    static constexpr unsigned StructureFlags = Base::StructureFlags | JSC::OverridesPut;
+    static constexpr unsigned StructureFlags = Base::StructureFlags
+        | JSC::OverridesPut
+        | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero;

 // JSEnvironmentVariableMap.cpp, put(), after the inherited-receiver branch
+    slot.disableCaching();

slot.disableCaching() makes slot.isCacheablePut() false. The LLInt cache and the JIT put ICs then never cache the site, and neither RELEASE_ASSERT can be reached. JSC's own put() overrides that can change the structure before they store do the same (JSFunction::put, ErrorInstance::put, ProxyObject::put), and the comment above the assert asks for it. It cannot reach the DFG's structure-only fold, PutByStatus::computeFor(JSGlobalObject*, const StructureSet&, ...), because that path has no slot. oven-sh/WebKit#640 closes that path: it returns LikelyTakesSlowPath for a structure with OverridesPut.

Measured on main bf80d21 (WebKit 564ac2a6), debug ASAN build, one machine. The ns/op rows only separate a cached read (JIT code, about 1 ns) from an uncached read (C++ slow path). They are not release numbers.

	main	this PR	put-only	put-only + #640 stand-in
abort, 4 forms	exit 134 x4	ok	ok	ok
third execution of one site	number	string	string	string
integer-like keys	number	string	string	string
hot write, 200000 calls per site	raw at call 2	ok	raw at call 848 to 1198	ok
env.TZ from a hot function, 20000 calls	stale at call 4499	live	stale at call 4502	live
process.env.NODE_ENV !== "production"	1.0 ns	2217 ns	0.9 ns	1.0 ns
missing key, in, Bun.env.K	1.0 to 1.5 ns	1681 to 2919 ns	1.0 ns	0.9 to 1.0 ns
string write	0.5 ns (the bug: put() skipped)	7371 ns	7331 ns	7343 ns
the 5 new tests in this PR	0 of 5	5 of 5	3 of 5	5 of 5

About the stand-in. I could not build the real #640 here: its preview build is 63 WebKit commits behind main, and its bun base needs LLVM 21 while the container has LLVM 23. The stand-in adds OverridesGetOwnPropertySlot to the class flags. That makes PutByStatus.cpp:379 take the same early return that #640 adds for OverridesPut. That line is the only store-side check of the flag in bytecode/, dfg/, ftl/, jit/ and llint/, so for stores the stand-in and #640 take the same path. #640 does not touch the read path, so the put-only read numbers apply to it unchanged. The stand-in is a measuring device, not a proposal.

What follows for this PR:

• Put-only on today's WebKit removes the abort, the third-execution coercion bug and the integer-like key bug, with no read cost. The two JIT-tier tests fail at call 99 (call 99 stored number 99, call 99: stored UTC, offset -540), because the DFG still folds the store.
• With the Quoting Code does not work for bun install in the "Not implemented yet" section in README.md #640 early return, all 5 new tests pass, the whole -t "process.env" set of process.test.js passes (15 pass, 1 skip), and reads stay cached.
• The write cost is the same in every fixed variant. Every store has to reach put().

The PR stays as it is until a maintainer picks the shape. If the put-only shape is chosen, the two JIT-tier tests can only land together with a WebKit bump that contains #640.

[robobun]

Correction to my note of Aug 27: #44356 changes what this PR and #40571 should carry.

#44356 keeps reads of process.env inline-cached, and changes put() to pass its own PutPropertySlot to the base put. The caller's slot then never becomes cacheable, so the LLInt and JIT put caches do not skip put(). That is the put-only shape from the Sep 22 comment above. ProhibitsPropertyCaching from this PR makes Structure::propertyAccessesAreCacheable() false, which turns those read caches off again, so it should not land together with #44356.

What #44356 does not cover, from this PR:

• Integer-like keys. It does not change putByIndex() or the structure flags, so InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero and the integer-like key test are still needed.
• The DFG store fold. PutByStatus::computeFor(StructureSet) has no OverridesPut check. process: let inline caches work on process.env and process.argv #44356 names that as its gap and has an it.todo for it. The two JIT-tier tests here can land only with a WebKit change for that path.

#40571 is now a draft. After #44356 merges it drops ProhibitsPropertyCaching and keeps the integer-key flag, so it no longer takes over this whole PR.