Fix roundtrip of top-level string scalars that look like YAML structure

When UnwrapScalar is enabled (the default for yaml output), the yaml
encoder writes node.Value verbatim as a bare line. Any string whose
content is itself a valid YAML mapping, sequence, or alias then round
trips as that container instead of as a string. For example, the input
document `"this: should really work"` previously re-emitted as the bare
line `this: should really work`, which the next reader parses as a one
key map, destroying the original scalar. The same problem surfaces
whenever a multiline string literal happens to contain `key: value`
lines, which is the form the bug report uses for its second reproducer.

Guard the fast-path by re-parsing node.Value with yaml.v4: if the bare
form decodes to a non-scalar, fall through to the regular encoder so it
can apply the quoting style required by the YAML spec. The check is
limited to `!!str` nodes and to structural reinterpretations, so tag
expressions such as `!!int` and plain strings that re-read as integers,
booleans, or nulls are unaffected. An unparseable value (e.g. one
containing NUL) stays on the fast-path so downstream NUL-aware writers
still see the raw bytes.

Updates the base64 "decode yaml document" scenario whose expected
output was `a: apple\n` bare; it is now emitted as a block literal,
which round-trips back to the same string.

Reproducer:

```
printf '"this: should really work"\n' | yq -p yaml -o yaml
```

Before this fix the second run of yq parses the output as a map;
after, it remains the original string.

Fixes #2608

pkg/yqlib/base64_test.go

@@ -81,10 +81,13 @@ var base64Scenarios = []formatScenario{
 		scenarioType: "decode",
 	},
 	{
-		skipDoc:      true,
-		description:  "decode yaml document",
-		input:        base64EncodedYaml,
-		expected:     base64DecodedYaml + "\n",
+		skipDoc:     true,
+		description: "decode yaml document",
+		input:       base64EncodedYaml,
+		// The decoded payload ("a: apple\n") would re-parse as a map if
+		// emitted bare, so the yaml encoder keeps it as a block literal to
+		// preserve roundtrip safety. See issue #2608.
+		expected:     "|\n    a: apple\n",
 		scenarioType: "decode",
 	},
 	{

pkg/yqlib/encoder_yaml.go

@@ -35,7 +35,7 @@ func (ye *yamlEncoder) Encode(writer io.Writer, node *CandidateNode) error {
 	if strings.Contains(node.LeadingContent, "\r\n") {
 		lineEnding = "\r\n"
 	}
-	if node.Kind == ScalarNode && ye.prefs.UnwrapScalar {
+	if node.Kind == ScalarNode && ye.prefs.UnwrapScalar && !bareStringNeedsQuoting(node) {
 		valueToPrint := node.Value
 		if node.LeadingContent == "" || valueToPrint != "" {
 			valueToPrint = valueToPrint + lineEnding
@@ -78,3 +78,28 @@ func (ye *yamlEncoder) Encode(writer io.Writer, node *CandidateNode) error {
 	}
 	return nil
 }
+
+// bareStringNeedsQuoting reports whether a top-level string scalar would be
+// structurally reinterpreted if emitted as an unquoted bare value. The
+// unwrap-scalar fast-path writes node.Value verbatim, which silently turns a
+// string like "this: should really work" into a mapping on the next read, or
+// "- item" into a sequence. When this returns true the caller falls through
+// to the full yaml encoder, which applies the quoting style required by the
+// YAML spec. Scalar-to-scalar reinterpretations (e.g. "123" parsing as an int
+// tag) are not covered here: they preserve the node shape and are handled by
+// callers that care about explicit tag preservation.
+func bareStringNeedsQuoting(node *CandidateNode) bool {
+	if node.Tag != "!!str" {
+		return false
+	}
+	var parsed yaml.Node
+	if err := yaml.Unmarshal([]byte(node.Value), &parsed); err != nil {
+		// Unparseable bare form (e.g. control characters): leave it on the
+		// fast-path so callers that check for those characters still see them.
+		return false
+	}
+	if parsed.Kind != yaml.DocumentNode || len(parsed.Content) != 1 {
+		return false
+	}
+	return parsed.Content[0].Kind != yaml.ScalarNode
+}

pkg/yqlib/encoder_yaml_test.go

@@ -0,0 +1,84 @@
+package yqlib
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+// TestYamlEncoderUnwrapScalarRoundtripSafety verifies that a top-level string
+// scalar whose unquoted form would re-parse as a non-scalar node (map or
+// sequence) is emitted quoted even when UnwrapScalar is enabled. Safe plain
+// strings continue to round-trip through the existing fast-path. See #2608.
+func TestYamlEncoderUnwrapScalarRoundtripSafety(t *testing.T) {
+	cases := []struct {
+		name     string
+		value    string
+		wantBare bool // true: output equals value+"\n"; false: output must differ
+	}{
+		{name: "colon_parses_as_map", value: "this: should really work"},
+		{name: "dash_parses_as_seq", value: "- item"},
+		{name: "multiline_maplike", value: "a: a\nb: b"},
+		{name: "safe_plain_string", value: "hello world", wantBare: true},
+		{name: "safe_identifier", value: "cat", wantBare: true},
+		{name: "safe_digits_preserved", value: "123", wantBare: true},
+		{name: "safe_null_word_preserved", value: "null", wantBare: true},
+		{name: "safe_tag_shorthand_preserved", value: "!!int", wantBare: true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			prefs := NewDefaultYamlPreferences()
+			prefs.UnwrapScalar = true
+
+			var buf bytes.Buffer
+			err := NewYamlEncoder(prefs).Encode(&buf, &CandidateNode{
+				Kind:  ScalarNode,
+				Tag:   "!!str",
+				Value: tc.value,
+			})
+			if err != nil {
+				t.Fatalf("encode failed: %v", err)
+			}
+			got := buf.String()
+
+			if tc.wantBare {
+				if got != tc.value+"\n" {
+					t.Fatalf("expected bare %q, got %q", tc.value+"\n", got)
+				}
+				return
+			}
+
+			// Ambiguous input: must not be emitted as the bare value.
+			if got == tc.value+"\n" {
+				t.Fatalf("value %q was emitted bare; expected quoted form", tc.value)
+			}
+
+			// The output must round-trip back to a string scalar with the
+			// same value, proving structural roundtrip safety.
+			decoder := NewYamlDecoder(NewDefaultYamlPreferences())
+			nodes, err := readDocuments(strings.NewReader(got), "test.yaml", 0, decoder)
+			if err != nil {
+				t.Fatalf("decode of %q failed: %v", got, err)
+			}
+			if nodes.Len() != 1 {
+				t.Fatalf("expected one document, got %d", nodes.Len())
+			}
+			candidate := nodes.Front().Value.(*CandidateNode)
+			// readDocuments wraps the document; descend to the scalar.
+			scalar := candidate
+			for scalar.Kind != ScalarNode && len(scalar.Content) == 1 {
+				scalar = scalar.Content[0]
+			}
+			if scalar.Kind != ScalarNode {
+				t.Fatalf("round-tripped node is not a scalar: kind=%v value=%q", scalar.Kind, scalar.Value)
+			}
+			if scalar.Tag != "!!str" {
+				t.Fatalf("round-tripped tag is %q, want !!str", scalar.Tag)
+			}
+			if scalar.Value != tc.value {
+				t.Fatalf("round-tripped value is %q, want %q", scalar.Value, tc.value)
+			}
+		})
+	}
+}