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--update-snapshots opened the .snap file with O_TRUNC and wrote back only the snapshots that the run took, in the order the tests ran. The run now reads the file. It replaces a changed value in place and appends a new key. It removes an entry that no test took only when the test that owns it ran to a pass.
…the start under --update-snapshots
A statement that is not an entry, for example a value with ${}, has no
place in the file. --update-snapshots writes such a file again from the
start, as it did before. Tests: a new file, --rerun-each, the three file
shapes that take this path. Docs: what --update-snapshots keeps.
- A statement that goes on after its value literal (the parser folds `(value)` and `value || other` to the literal) takes the full rewrite. - The tests that did not run to a pass add up over every run of a test file (the file listed twice, --rerun-each with a cached module). - A beforeAll/afterAll hook owns the entries with the name of its scope. They stay when the hook did not run to a pass. - One helper for the backtick escape. An invalid Loc is a parse error. - Tests: file layouts, the file run twice, hooks, more full-rewrite shapes. - Docs: say when the file is written again from the start.
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WalkthroughChangesSnapshot update mode now preserves existing entry order, replaces changed values, appends new entries, retains unfinished tests, removes obsolete entries when names are known, and resets malformed files in update mode. Tests and documentation cover these behaviors. Snapshot update flow
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Linked Issues check |  |
Issue #42969 requires --update-snapshots to preserve existing entry order and limit changes to changed values. The reviewed implementation indexes existing entries, replaces values in their existing… |
| Out of Scope Changes check | |
The changes stay within Issue #42969. Runtime changes implement in-place snapshot updates and unfinished-test tracking. Documentation describes the resulting behavior. Tests cover the required behavio… |
| Title check | |
The title clearly describes the main change: updating existing .snap files in place with --update-snapshots. |
| Description check | |
The description is complete and relevant. It explains the problem, implementation, behavior, edge cases, linked issues, and verification results, including extensive test coverage. Although it does no… |
Full details: Linked Issues check
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Explanation
Issue
#42969requires--update-snapshotsto preserve existing entry order and limit changes to changed values. The reviewed implementation indexes existing entries, replaces values in their existing ranges, and appends new entries. It preserves entries for filtered, skipped, todo, failing, thrown, or unfinished tests, and removes entries only after successful completion. The incremental change also keeps same-line comments with the associated entry. The added tests cover ordering, replacement, appending, preservation, removal, bail behavior, and comment handling.
Full details: Out of Scope Changes check
-
Explanation
The changes stay within Issue
#42969. Runtime changes implement in-place snapshot updates and unfinished-test tracking. Documentation describes the resulting behavior. Tests cover the required behavior and related snapshot-update cases. No unrelated product change is demonstrated.
Full details: Description check
-
Explanation
The description is complete and relevant. It explains the problem, implementation, behavior, edge cases, linked issues, and verification results, including extensive test coverage. Although it does not use the template headings exactly, it provides the required information in equivalent sections.
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Updated 1:56 AM PT - Oct 23rd, 2026
✅ @robobun, your commit 3913729da5c37fa44e76075ad2e247f407490bde passed in 🧪 To try this PR locally: bunx bun-pr 43043That installs a local version of the PR into your bun-43043 --bun |
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Status: the fix is in this PR. The review threads are resolved. It needs a maintainer review. How to reproduce on 1.4.3 (each case starts from a // filter.test.ts
import { describe, expect, test } from "bun:test";
test("alpha", () => { expect("a").toMatchSnapshot(); });
test("beta", () => { expect("b").toMatchSnapshot(); });
describe("grp", () => { test("gamma", () => { expect("g").toMatchSnapshot(); }); });
With this branch each of these runs keeps the entries and the order. |
`bun test --bail` exits with a bare exit(1) and skips the VM teardown, so under the ASAN lane's detect_leaks=1 the child aborts with 134 (#32183).
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In `@src/runtime/test_runner/snapshot.rs`:
- Line 240: Update the byte iteration in the snapshot test runner’s loop to use
a byte string literal instead of an array of single-byte literals, preserving
iteration over semicolon, carriage-return, and newline.
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📒 Files selected for processing (6)
docs/test/snapshots.mdxsrc/runtime/cli/test_command.rssrc/runtime/test_runner/bun_test.rssrc/runtime/test_runner/expect.rssrc/runtime/test_runner/snapshot.rstest/js/bun/test/snapshot-tests/new-snapshot.test.ts
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clippy::byte_char_slices rejects the array of byte literals.
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…the entry A removed entry left its same-line comment behind, under the entry above. The rest of the line now goes with the entry when it is blank or a comment and the next statement starts on a later line.
…te-snapshots-keep-file
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Merged main (91e7e3b) into this branch and ran the tests again.
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…te-snapshots-keep-file # Conflicts: # src/runtime/test_runner/snapshot.rs
…te-snapshots-keep-file
`unfinished_test_names` called itself once for each nested `describe()`. With `--update-snapshots` and a file that nests `describe()` 10000 levels deep, the walk overflowed a 1 MiB native stack after the tests ran. Walk the describe tree with an explicit stack, as `Order.rs` does.
The `--bail` exit path tears the VM down on main now, so the child needs no LeakSanitizer option. The test `--update-snapshots keeps the old file until the new contents are written` from main holds the bail rule: the file stays as it was.
String.prototype.repeat is slow in debug JavaScriptCore builds.
…s sibling `describe.test.ts` already has the test for a deeply nested `describe()` and its stack limit setup. The new case uses the same setup.
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Merged main (5c6d4e2) into this branch. The PR merges clean again.
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Fixes #42969
Supersedes #42980.
Problem
bun test --update-snapshotsloses.snapentries: those of tests that-t,.only,.skipor.todoleave out, and of a test that throws before itstoMatchSnapshot(). The entries that stay move to run order (bun test --update-snapshots rewrites the .snap file in execution order, reordering unchanged entries #42969).-u,get_snapshot_file(src/runtime/test_runner/snapshot.rs) does not read the file. The file is built from the snapshots of this run only.Fix
-uthe run reads the file.parse_filerecords the byte range of each entry and its value.write_snapshot_fileputs a changed value in place and appends new keys. An unchanged entry keeps its bytes.BunTest::unfinished_test_nameslists the tests that did not. Jest has the same rule.test/js/bun/test/snapshot-tests/new-snapshot.test.tsand one intest/js/bun/test/describe.test.ts. 21 of the 28 fail on 1.4.3. The notes list the other suites.Background
.snapfile is a list ofexports[`name N`] = `value`;statements.nameis the describe and test names, then: hint.Nis a counter. Bun parses the file as JavaScript and does not run it.Snapshotsholds one.snapfile infile_buf. It writes it when the next test file opens its.snap, or at the end of the run..claude/hooks/pre-bash-guard.jsforbid-uwith-tbecause of this bug. They stay whileUSE_SYSTEM_BUN=1runs a released build.Notes
Other open PRs in the same functions. This PR should land first. It does not change when the file is written.
writes a new file in the order in which the tests run,writes the file again from the start when it has a syntax error).O_TRUNCout of the open and truncates after the write. A run that stops early (--bail,process.exit()) now leaves the file as it was. Its test--update-snapshots keeps the old file until the new contents are writtenholds that rule and passes with this PR. Before Hardening follow-up: fix regressions from #46021 and more crashes #46270 such a run left a 0-byte file..snapwhen a test file is done and on--bail. On this PR's data a bail write loses nothing, because nothing is removed before the file is done.afterAll(On --update-snapshots, snapshot file remains empty in afterAll() #12114). This PR fixes a part of On --update-snapshots, snapshot file remains empty in afterAll() #12114: under-uan existing file is not empty while the tests run. The new contents are still written afterafterAll..snapstatement that is not an entry, and Make concurrent bun test and bun install processes safe on shared files #39689 writes the file through a rename. Both need-uas the way out of a rejected file. Here-uwrites such a file again from the start, so that way stays.beforeEach/afterEachsnapshot after the running test. See the hook note below.Reproductions on 1.4.3-canary.1 (c6b7fcb), each with a
.snapfile from a first plain run:bun test -u -t betawith testsalpha,beta,grp > gamma: the file keeps onlybeta 1.bun test -uwith a test that throws beforetoMatchSnapshot(): its entry is gone.bun test -uon Linux with atest.skipIf(process.platform !== "win32")test: its entry is gone. No filter is needed.bun test -u --bailwith one failing test: the file has 0 bytes, because the bail path exits before the write. Main fixed this case in Hardening follow-up: fix regressions from #46021 and more crashes #46270.bun test -uover a hand-sorted file writeszeta, alphaagain (bun test --update-snapshots rewrites the .snap file in execution order, reordering unchanged entries #42969).When nothing is removed. A describe callback threw, so the list of tests is not complete. Or the run stopped before the test file was done (
--bail,process.exit()).How an entry maps to its owner. The key without its counter is the snapshot name. The name is the test name, or the test name,
:, and a hint.ExistingEntry::is_of_one_oftries the name, then each prefix that ends before a:. A wrong match can only keep an entry. The names add up over every run of a test file in one process (the file listed twice,--rerun-each), because a later run can register fewer tests: its imported modules are in the module cache.Hooks. A snapshot that a hook takes has the name of the hook's scope plus
(unnamed). An entry of abeforeAll/afterAllhook stays when that hook did not run to a pass: it threw, or no test of its scope ran. The entries ofbeforeEach/afterEach/onTestFinishedhooks and of--preloadhooks are not kept when the hook runs fewer times, as before this PR. Their counter depends on how many tests ran, so-uwith a filter also writes them to the wrong keys, before and after this PR. With #38820 a per-test hook snapshot has the name of the running test, and the rule for tests covers it.Which files take the in-place path. A file where each statement is one entry, in any order, with CRLF or LF, with comments, with a BOM, with
'or"literals. The bytes between the value literal and the next statement must be whitespace,;and comments. When an entry is removed, a comment after it on its line goes with it. The parser folds(value),value || otherandtrue ? value : otherto the literal, so this check is what finds a statement that goes on after it. All 61.snapfiles undertest/take the in-place path: a-urun that removes nothing leaves each of them byte for byte, plus the new entry. A Jest-written file keeps its bytes for unchanged entries. Bun escapes U+2028 and Jest does not, so a writer that prints every entry again changes such a line with no change in the value.Source::range_of_stringfinds the end of a'or"literal only, and error ranges in the parser and the lockfile code depend on that, sostring_literal_endis local.Not changed. The
-usummary still counts each snapshot that the run takes asadded(the docs show that output). A removed entry is not reported.--rerun-eachunder-u: the first run writes the value and a later run compares with it, as before. The write is still not atomic and still happens when the next file opens its.snapor when the run ends. A.snapis opened by the first snapshot matcher of its test file, so a test file that takes no snapshot leaves its.snapas it is. In a CRLF file a replaced value with line breaks and an appended entry use LF, as appended entries already do without-u.Tests. 21 of the 28 new tests fail on 1.4.3. The other 7 hold behavior that must not change: the order of a new file, one layout whose result is the same as a full rewrite, and the five file shapes that take the full rewrite. No test is gated by platform. The test for a deeply nested
describe()under-uis indescribe.test.ts, next to the test it mirrors. Like that test, it lowers the stack to 1 MiB withulimit -son Linux. On macOS and Windows it runs with the default stack. Local runs: Linux x64 only, with a debug build (ASAN). macOS and Windows run the files in CI.Suites run with the debug build:
test/js/bun/test/snapshot-tests/(theerror snapshotstest fails with and without this change, it expects colors),test/js/bun/test/describe.test.ts,test/js/bun/test/ci-restrictions.test.ts,test/js/bun/test/bun_test.test.ts,test/cli/test/rerun-each.test.ts,test/cli/test/parallel.test.ts -t snapshot, and the source lintsbyte-search,port-era-markers,pre-port-identifiers,fn-long-mut-reborrow,uninit-assumed-init. Random.snaplayouts (about 5000 files: readable layouts,-tand--rerun-eachvariants, valid JavaScript that Bun does not write, byte mutations): no file that fails to parse, no lost entry of a skipped test, no change by a plain run after-u.Self-reviewed. The review asked for these changes, all made: the full rewrite for a statement that is not an entry or that goes on after its literal, the hook rule narrowed to
beforeAll/afterAll, names that add up over the runs of a file,--rerun-eachtests, the docs sentence about the full rewrite. A later review found that the list of unfinished tests used native recursion over nesteddescribe()scopes. It now uses an explicit stack, with a test at 10000 levels.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/bun/test/snapshot-tests/new-snapshot.test.ts