Fix round-trip of block scalars whose content starts with a space

src/stringify/foldFlowLines.ts

@@ -68,6 +68,14 @@ export function foldFlowLines(
   let escStart = -1
   let escEnd = -1
   if (mode === FOLD_BLOCK) {
+    if (text[0] === ' ' || text[0] === '\t') {
+      // Unlike the following ones, the first line does not include the block
+      // indent, which the caller adds. Any white space here is therefore
+      // content of a more-indented line, which must not be folded.
+      const nl = text.indexOf('\n')
+      if (nl === -1) return text
+      i = nl
+    }
     i = consumeMoreIndentedLines(text, i, indent.length)
     if (i !== -1) end = i + endStep
   }

src/stringify/stringifyString.ts

@@ -190,9 +190,14 @@ function blockString(
     return quotedString(value, ctx)
   }
 
+  // Content starting with a space requires an explicit indentation indicator,
+  // which is only unambiguous if the block itself is indented as well.
+  const startsWithSpace = /^[\n ]*[ ]/.test(value)
   const indent =
     ctx.indent ||
-    (ctx.forceBlockIndent || containsDocumentMarker(value) ? '  ' : '')
+    (ctx.forceBlockIndent || startsWithSpace || containsDocumentMarker(value)
+      ? '  '
+      : '')
   const literal =
     blockQuote === 'literal'
       ? true
@@ -227,14 +232,12 @@ function blockString(
   }
 
   // determine indent indicator from whitespace at value start
-  let startWithSpace = false
   let startEnd: number
   let startNlPos = -1
   for (startEnd = 0; startEnd < value.length; ++startEnd) {
     const ch = value[startEnd]
-    if (ch === ' ') startWithSpace = true
-    else if (ch === '\n') startNlPos = startEnd
-    else break
+    if (ch === '\n') startNlPos = startEnd
+    else if (ch !== ' ') break
   }
   let start = value.substring(
     0,
@@ -245,9 +248,8 @@ function blockString(
     start = start.replace(/\n+/g, `$&${indent}`)
   }
 
-  const indentSize = indent ? '2' : '1' // root is at -1
   // Leading | or > is added later
-  let header = (startWithSpace ? indentSize : '') + chomp
+  let header = (startsWithSpace ? '2' : '') + chomp
   if (comment) {
     header += ' ' + commentString(comment.replace(/ ?[\r\n]+/g, ' '))
     if (onComment) onComment()

tests/doc/foldFlowLines.ts

@@ -321,7 +321,8 @@ describe('end-to-end', () => {
   test('More-indented first line (#55)', () => {
     const str = ' first more-indented line\nnext line\n'
     const ys = YAML.stringify(str, foldOptions)
-    expect(ys).toBe('>1\n first more-indented line\nnext line\n')
+    expect(ys).toBe('>2\n   first more-indented line\n  next line\n')
+    expect(YAML.parse(ys)).toBe(str)
   })
 
   test('plain string', () => {

tests/doc/stringify.ts

@@ -1407,6 +1407,38 @@ describe('Document markers in top-level scalars', () => {
   })
 })
 
+describe('Block scalars with leading spaces (#692)', () => {
+  for (const [value, exp] of [
+    ['  a\n  b', '|2-\n    a\n    b\n'],
+    ['  indented\nlines', '|2-\n    indented\n  lines\n'],
+    [' a\nb', '|2-\n   a\n  b\n'],
+    ['   x\n y\nz', '|2-\n     x\n   y\n  z\n'],
+    ['    deep\nshallow', '|2-\n      deep\n  shallow\n'],
+    ['\n  a', '|2-\n  \n    a\n'],
+    ['  \nabc', '|2-\n    \n  abc\n'],
+    [' a\n', '|2\n   a\n'],
+    ['  a\n\n', '|2+\n    a\n\n']
+  ]) {
+    test(JSON.stringify(value), () => {
+      const str = YAML.stringify(value)
+      expect(str).toBe(exp)
+      expect(YAML.parse(str)).toBe(value)
+    })
+  }
+
+  test('as a map value', () => {
+    const str = YAML.stringify({ key: '  a\n  b' })
+    expect(str).toBe('key: |2-\n    a\n    b\n')
+    expect(YAML.parse(str)).toEqual({ key: '  a\n  b' })
+  })
+
+  test('as a seq item', () => {
+    const str = YAML.stringify(['  a\n  b'])
+    expect(str).toBe('- |2-\n    a\n    b\n')
+    expect(YAML.parse(str)).toEqual(['  a\n  b'])
+  })
+})
+
 describe('Document markers in top-level map keys (#431)', () => {
   test('---', () => {
     const str = YAML.stringify({ '--- x': 42 })