fix(parsing): require flow continuation lines to be indented

YAML requires multi-line flow content to be indented past the enclosing
block context (s-flow-line-prefix). libyaml does not enforce this, so
'flow: [a,' / 'b]' with 'b' in column 0, and the equivalent multi-line
flow mapping, wrongly parsed.

Track whether reaching a token crossed a line break, and in flow context
reject a token at or below the parent block indent. A closing ']' / '}'
or a leading ',' may sit at any indentation, so those are exempt (a flow
mapping's closing brace in column 0 stays valid).

Surpasses libyaml on 9C9N, VJP3/00; whole-suite conformance 394/402,
canonical events 308/308.

CHANGELOG.md

@@ -22,6 +22,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Reject a flow-collection continuation line that is not indented past the parent
+  block. YAML requires multi-line flow content to be more indented than the
+  enclosing block context, so `flow: [a,` / `b]` (with `b` in column 0) and the
+  equivalent multi-line flow mapping are malformed; libyaml does not enforce it. A
+  closing `]`/`}` or a leading `,` may still sit at any indentation. Surpasses
+  libyaml on 9C9N and VJP3/00.
 - Reject content after a `...` document-end marker on the same line. Only
   whitespace and a comment may follow `...`; unlike `--- x`, a `... x` cannot
   carry inline content. Surpasses libyaml on 3HFZ.

Sources/Parsing/LibYAML.Scanner.Fetch.swift

@@ -25,6 +25,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
         }
 
         try scanToNextToken()
+        try checkFlowContinuationIndent()
         mark = reader.mark
         try staleSimpleKeys()
         indentEngine.unrollIndent(column: reader.column, mark: reader.mark)
@@ -106,6 +107,28 @@ extension PureYAML.Parsing.LibYAML.Scanner {
         }
     }
 
+    /// Reject a flow-collection continuation line that is not indented past the
+    /// enclosing block context. YAML requires `s-flow-line-prefix` to be more
+    /// indented than the parent block; libyaml does not enforce it, so multi-line
+    /// flow content in column 0 (`[a,\nb]` under a key) wrongly parses.
+    private mutating func checkFlowContinuationIndent() throws {
+        guard flowLevel > 0, crossedLineBreak, let next = reader.peek() else {
+            return
+        }
+        // A closing `]`/`}` (or a `,` separator) may sit at any indentation; only
+        // flow CONTENT must be indented past the parent block.
+        if next == "]" || next == "}" || next == "," {
+            return
+        }
+        if reader.column <= indentEngine.indent {
+            throw ScannerError(
+                context: "while parsing a flow node",
+                problem: "found insufficiently indented line in a flow collection",
+                mark: reader.mark,
+            )
+        }
+    }
+
     /// The error raised when a tab is used where indentation is required (before a
     /// block indicator or an implicit mapping key). A tab is valid separation but
     /// never indentation.

Sources/Parsing/LibYAML.Scanner.SkipToToken.swift

@@ -8,6 +8,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
     /// here; `tabbedSeparation` records when one preceded the token so a block
     /// indicator or implicit key (where the tab would be indentation) can reject.
     mutating func scanToNextToken() throws {
+        crossedLineBreak = false
         while true {
             tabbedSeparation = false
             if reader.column == 0, reader.peek() == "\u{FEFF}" {
@@ -33,6 +34,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
             }
             if Classify.isBreak(reader.peek()) {
                 reader.skipLine()
+                crossedLineBreak = true
                 if flowLevel == 0 {
                     simpleKeyAllowed = true
                 }

Sources/Parsing/LibYAML.Scanner.swift

@@ -18,6 +18,10 @@ extension PureYAML.Parsing.LibYAML {
         /// tab is valid in-line separation but never indentation, so a block
         /// indicator or implicit mapping key reached through one is an error.
         var tabbedSeparation = false
+        /// Whether reaching the pending token crossed at least one line break. In
+        /// flow context a continuation line must be indented past the parent block,
+        /// so a token on a new line at or below that indent is an error.
+        var crossedLineBreak = false
         /// `parser->mark`, kept in sync with the reader at each fetch step.
         var mark: PureYAML.Parsing.Mark = .start
         /// `parser->stream_start_produced`.

Tests/Fixtures/yaml-test-suite-manifest.yaml

@@ -10,8 +10,8 @@ source:
 # oracle:   json if in.json present, else none
 # status:   passing (PureYAML conformant today) | gap (not yet conformant)
 total: 402
-passing: 392
-gap: 10
+passing: 394
+gap: 8
 cases:
   - id: "229Q"
     name: "Spec Example 2.4. Sequence of Mappings"
@@ -986,7 +986,7 @@ cases:
     localPath: yaml-test-suite/9C9N/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 20
     lineCount: 4
   - id: "9CWY"
@@ -2938,7 +2938,7 @@ cases:
     localPath: yaml-test-suite/VJP3/00/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 13
     lineCount: 5
   - id: "VJP3/01"