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In indented mode, YAML.stringify now emits a literal block scalar (| or |-, chosen by the trailing newline) for a multiline string when block form reproduces the string exactly. Strings that block form cannot represent keep the double-quoted output: lines that end with a space or tab, a first non-empty line that starts with a space, carriage returns and other control characters, YAML 1.1 line break characters, lone surrogates, and strings that end with two or more newlines. Keys and minified mode are unchanged. Fixes #41115
Walkthrough
ChangesYAML block scalar serialization
Suggested reviewers: Merge Risk: ⚪ Minimal · up to The PR changes multiline string serialization to improve readability while preserving round-trip behavior; no actionable merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@test/js/bun/yaml/yaml.test.ts`:
- Around line 3004-3010: Refactor the round-trip assertions around the cases
loop into a parameterized describe.each(cases) suite, with the existing
YAML.parse/YAML.stringify assertions inside a test for each case. Preserve all
current assertions while ensuring failures are reported separately per input.
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✅ @robobun, your commit 60c4086bc2566e6262f0bb24b005de588bc8472c passed in 🧪 To try this PR locally: bunx bun-pr 41116That installs a local version of the PR into your bun-41116 --bun |
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A compact nested sequence advances two columns per level while the indent advances one level. With a one-column unit the block scalar content lands at or before the column of its compact parent, so the parser reads an empty scalar. YAML.stringify([["x\ny"]], null, 1) did not roundtrip. A one-column indent now keeps the quoted output.
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Inline comments:
In `@test/js/bun/yaml/yaml.test.ts`:
- Line 2935: Refactor the compact-nesting test around the cases loop to use
describe.each(cases) and nest test.each([2, 3, 4]) for the independent indent
values. Preserve the existing assertions and test behavior while giving each
input shape and indentation combination a distinct test name.
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Thanks — commit 540d114 addresses the 1-column compact-nesting roundtrip break I flagged: indent_allows_block_scalar now rejects Number(1) and single-space Str, and the new tests cover [["x\ny"]] / [{a:"x\ny"}] at width 1 (quoted fallback) and widths 2–4 (block scalar roundtrips). I didn't find further bugs; given the number of YAML-spec edge cases the emitter now encodes, a human pass on the block-scalar eligibility rules would still be worthwhile.
What was reviewed:
- Re-checked compact nesting at width ≥ 2: content column
level × widthstays strictly past the innermost-column for the nesting depths tested, and the added roundtrip cases exercise 3-deep chains. literal_block_chompingindex arithmetic:len - 1 - trailing_newlinesand the surrogatei + 1lookahead are guarded (trailing_newlines < len,i + 1 >= body_len), no underflow or OOB.Space::Strpath:trunc(10)matches the emitter's existing clamp, and the new< 2guard also covers the empty-string case.
Extended reasoning...
Overview
The PR adds literal block scalar output (| / |-) to YAML.stringify for multiline string values in indented mode, with a conservative eligibility scan (literal_block_chomping) that falls back to the existing double-quoted path whenever exact roundtrip is not guaranteed. My earlier inline finding — that a 1-column indent unit inside compact same-line nesting (- - |-) produced content not strictly more indented than its parent, so YAML.parse read an empty scalar — was fixed in 540d114 by making indent_allows_block_scalar require at least a two-column unit for both Number and Str spaces, and the test file gained direct coverage of that case plus roundtrip checks for compact nesting at widths 2/3/4.
Security risks
None identified. This is output formatting of already-held JS string data into a text buffer; no parsing of untrusted input, no allocation sizing derived from external lengths, no filesystem/network/auth surface. The scan is a linear pass over UTF-16 code units with bounded index arithmetic.
Level of scrutiny
Moderate. It is a codec emit path where the invariant is exact parse(stringify(x)) == x, and the eligibility rules encode several YAML-1.1/1.2 subtleties (indentation auto-detection, chomping, trailing whitespace, NEL/LS/PS as line breaks, lone surrogates). The test coverage is strong — exact-output toBe assertions plus a test.each roundtrip sweep across positions and indent widths — but the correctness of each rejection/acceptance rule against the spec is the kind of thing a maintainer familiar with src/parsers/yaml.rs should eyeball once.
Other factors
The follow-up fix is minimal and well-scoped (two-line predicate change plus a comment explaining the compact-nesting rationale), keys still route through append_string so mapping-key output is unchanged, and minified/tab-indent modes are untouched. No outstanding CHANGES_REQUESTED from other reviewers; the coderabbit thread on the test file predates a commit that touched that area.
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Re-reviewed after 540d114 — gating block scalars on a ≥2-column all-space indent unit fixes the compact-nesting roundtrip break I flagged, and the new tests cover [["x\ny"]] / [{a:"x\ny"}] at widths 1–4. No further issues found; a human look is still worthwhile given this changes user-visible YAML.stringify output.
What was reviewed:
indent_allows_block_scalar:Number(1)and single-space/tabStrnow fall through to quoted output; verified against the compact-sequence column math from my earlier note.literal_block_chomping: traced trailing-newline counting, last-line trailing-space check, first-non-empty-line leading-space guard, and surrogate pairing — each fallback has a matching test and a roundtrip-sweep entry.append_block_scalar: root-level indent bump to 1 and unindented interior blank lines both roundtrip in the added tests.
Extended reasoning...
Overview
This PR teaches Bun.YAML.stringify to emit multiline strings as literal block scalars (| / |-) in indented mode, falling back to the existing double-quoted path whenever exact roundtrip is not guaranteed. The Rust change adds append_value_string, indent_allows_block_scalar, append_block_scalar, and the literal_block_chomping classifier (~160 lines in src/runtime/api/YAMLObject.rs); the test file gains ~175 lines including a test.each roundtrip sweep over ~30 edge-case strings across several indent widths and nesting shapes.
Security risks
None identified. This is a pure output-formatting change to a serializer; input is JS strings already held in memory, no allocation sizing from untrusted headers, no filesystem/network/auth surface. The classifier only decides between two already-supported output styles and defaults to the pre-existing quoted path on anything ambiguous.
Level of scrutiny
Moderate. The change is self-contained and conservatively designed (fallback-to-quoted on every edge case), but it alters user-visible serialization output, and YAML block-scalar semantics (indentation auto-detection, chomping, compact nested sequences) are subtle enough that a maintainer sign-off is appropriate. My earlier 🔴 finding — 1-column indent units producing block content not strictly deeper than a compact - - parent — was addressed in 540d114 by requiring n >= 2 (and ≥2 all-space chars for a string space), which is exactly one of the two remedies I suggested; the added "a one-column indent unit keeps quoted output" and "compact nesting stays more indented than its parent" tests exercise both failing shapes from that report.
Other factors
Test coverage is thorough: chomping variants, interior/leading blanks, root scalars, tab/single-space space fallback, all documented quoting-fallback triggers, mid-line tabs/Unicode, and a parametrized roundtrip sweep. There are six unresolved github-actions[bot] inline comments on the Rust file posted between the last two commits whose content I cannot see; combined with this being a user-facing output change, I am deferring rather than approving.
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Problem
YAML.stringify(value, null, 2)emits every multiline string as a single-line double-quoted scalar with escaped\n. That makes the output hard to read and produces noisy git diffs. Reported in [Feature Request]: Support block scalar styles (|,|-,>,>-) and string wrapping inbun:yaml(YAML.stringify) #41115.append_stringinsrc/runtime/api/YAMLObject.rs) has only two styles: plain and double-quoted. It never uses YAML block scalars, even thoughYAML.parsesupports every block-scalar form.Fix
|-when the string has no trailing newline,|when it ends with exactly one newline.\rand other control characters,\^E/\u2028/\u2029(line breaks to YAML 1.1 parsers), lone surrogates, and two or more trailing newlines (that would need|+). Keys, minified mode, and a non-space stringspaceargument are unchanged, so no existing test output changes.- -) advances two columns per level, so with a one-column unit the content would not be more indented than its parent. Aspaceof 1 keeps the quoted output.YAML.parse(YAML.stringify(x, null, n))returns the identical string for every input. Verified by the edge-case sweep in the new tests and by a 200k-iteration random fuzz over newline-heavy strings and nested shapes (0 failures).test/js/bun/yaml/yaml.test.ts(47 new tests, most fail on the released build), plusyaml-test-suite.test.tsandyaml-block-scalar-matrix.test.ts(all pass).Background
|-strips it,|keeps exactly one.yaml, and Deno@std/yaml, all of which emit block scalars for multiline strings with a quoted fallback.Notes
>,>-), line wrapping, and an options argument (lineWidth,defaultStringType). Those are deferred: this change adds no API surface. If options land later, the npmyamlshape (widening the thirdspaceparameter to accept an object) fits better than a fourth argument.stringify_unwrapped, notappend_string, becauseappend_stringalso serves mapping keys and keys must stay quoted. This composes with the in-flight replacer rewrite in Implement the replacer argument of YAML, TOML and JSON5 stringify #39925, which keeps that seam. If Implement the replacer argument of YAML, TOML and JSON5 stringify #39925 lands first, the merge conflict is the one call site: route value-position strings throughappend_value_string.spaceargument participates only if it is all spaces and at least two columns: block scalar content cannot be indented with tabs, soYAML.stringify(v, null, "\t")keeps quoted multiline output.[["a", "b"]]) already fails to roundtrip on main at any indent other than 2, for every scalar kind: the between-item newline indent does not line up with the compact first item. For exampleYAML.parse(YAML.stringify([[1, 2]], null, 3))returns[["1 - 2"]]. That bug predates this PR and is out of its scope.\r,\^E,\u00a0,\u2028, surrogate halves, and control characters, across indents 1, 2, 3, 10," "," ", wrapped in flat and nested shapes. 0 roundtrip failures, about 27k block-scalar outputs.[human-review] gate passed · iteration 0 · 2 files touched
fails on main (without fix)
passes on PR (with fix)
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root cause · written by the author bot
The bug was that YAML.stringify always serialized multiline strings as double-quoted scalars with escaped \n sequences, making output hard to read and inconsistent with other YAML stringifiers. The fix changes the value-position serialization path in YAMLObject.rs so that eligible multiline strings are emitted as literal block scalars (| or |-) when using indented output, with an eligibility check covering indentation, control characters, line separators, trailing newlines, and surrogate pairs. Strings that cannot safely roundtrip through block scalar form, along with keys and minified outp…