From 54c6032a9a41ffcd4c3d185f27252fa4c5b15067 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Balv=C3=ADn?= <opensource@petrbalvin.org>
Date: Thu, 17 Sep 2026 23:06:59 +0200
Subject: [PATCH] fix(encode): propagate invalid UTF-8 key errors from headers
 and inline tables

Assisted-by: GLM 5.3
---
 CHANGELOG.md   |  4 ++++
 encode.go      | 48 +++++++++++++++++++++++++++++++-----------------
 encode_test.go | 11 +++++++++++
 3 files changed, 46 insertions(+), 17 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 49e0513..3ced0e6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -54,6 +54,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   cannot be carried verbatim by the literal form: a run of three single quotes,
   a control character, or a lone carriage return. Such values previously
   produced output that did not re-parse.
+- `Marshal` returns an error for a table header key or an inline-table key that
+  is not valid UTF-8, the way scalar keys already did, instead of silently
+  emitting corrupt TOML (a header that lost its key, an inline table with a
+  missing key).
 - A float with an exponent marker but no digits (`1e`, `0.0E`) is rejected;
   the exponent requires at least one digit.
 - A date-time offset outside 00:00 through 23:59 is rejected; such offsets
diff --git a/encode.go b/encode.go
index 8537add..f2318eb 100644
--- a/encode.go
+++ b/encode.go
@@ -579,7 +579,9 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 			path := append(append([]string{}, prefix...), t.key)
 			e.writeBlankLine()
 			e.buf.WriteByte('[')
-			writeKeyPath(&e.buf, path)
+			if err := e.writeKeyPath(path); err != nil {
+				return err
+			}
 			e.buf.WriteString("]\n")
 			if err := e.emitDoc(t.doc, path); err != nil {
 				return err
@@ -590,7 +592,9 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 			for _, sub := range a.docs {
 				e.writeBlankLine()
 				e.buf.WriteString("[[")
-				writeKeyPath(&e.buf, path)
+				if err := e.writeKeyPath(path); err != nil {
+					return err
+				}
 				e.buf.WriteString("]]\n")
 				if err := e.emitDoc(sub, path); err != nil {
 					return err
@@ -615,7 +619,9 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 			path := append(append([]string{}, prefix...), ent.key)
 			e.writeBlankLine()
 			e.buf.WriteByte('[')
-			writeKeyPath(&e.buf, path)
+			if err := e.writeKeyPath(path); err != nil {
+				return err
+			}
 			e.buf.WriteString("]\n")
 			if err := e.emitDoc(ent.doc, path); err != nil {
 				return err
@@ -625,7 +631,9 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 			for _, sub := range ent.docs {
 				e.writeBlankLine()
 				e.buf.WriteString("[[")
-				writeKeyPath(&e.buf, path)
+				if err := e.writeKeyPath(path); err != nil {
+					return err
+				}
 				e.buf.WriteString("]]\n")
 				if err := e.emitDoc(sub, path); err != nil {
 					return err
@@ -637,10 +645,9 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 }
 
 func (e *encoder) writeKV(key string, val any) error {
-	if !utf8.ValidString(key) {
-		return fmt.Errorf("interpres: key %q is not valid UTF-8", key)
+	if err := e.writeKey(key); err != nil {
+		return err
 	}
-	e.writeKey(key)
 	e.buf.WriteString(" = ")
 	if err := e.writeValue(val); err != nil {
 		return err
@@ -649,25 +656,30 @@ func (e *encoder) writeKV(key string, val any) error {
 	return nil
 }
 
-func writeKeyPath(buf *bytes.Buffer, path []string) {
+func (e *encoder) writeKeyPath(path []string) error {
 	for i, p := range path {
 		if i > 0 {
-			buf.WriteByte('.')
+			e.buf.WriteByte('.')
 		}
-		if isBareKey(p) {
-			buf.WriteString(p)
-			continue
+		if err := e.writeKey(p); err != nil {
+			return err
 		}
-		writeQuotedString(buf, p)
 	}
+	return nil
 }
 
-func (e *encoder) writeKey(key string) {
+// writeKey writes one key, bare when it qualifies and quoted otherwise. A key
+// that is not valid UTF-8 is an error; writing it anyway would emit corrupt
+// TOML, because the quoted form has no representation for it.
+func (e *encoder) writeKey(key string) error {
 	if isBareKey(key) {
 		e.buf.WriteString(key)
-		return
+		return nil
 	}
-	writeQuotedString(&e.buf, key)
+	if !utf8.ValidString(key) {
+		return fmt.Errorf("interpres: key %q is not valid UTF-8", key)
+	}
+	return writeQuotedString(&e.buf, key)
 }
 
 // writeQuotedString writes s as a TOML basic string (double-quoted) to buf.
@@ -787,7 +799,9 @@ func (e *encoder) writeInlineTable(m map[string]any) error {
 		if i > 0 {
 			e.buf.WriteString(", ")
 		}
-		e.writeKey(k)
+		if err := e.writeKey(k); err != nil {
+			return err
+		}
 		e.buf.WriteString(" = ")
 		if err := e.writeValue(m[k]); err != nil {
 			return err
diff --git a/encode_test.go b/encode_test.go
index adb4e18..90652f3 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -1128,6 +1128,17 @@ func TestMarshalKeyRequiresUTF8(t *testing.T) {
 	if _, err := Marshal(m); err == nil {
 		t.Errorf("expected error for invalid UTF-8 key")
 	}
+	// The check must reach the keys of table headers and of inline tables
+	// nested inside value arrays, not only scalar keys: both write keys
+	// through the same path.
+	nested := map[string]any{"\xff": map[string]any{"k": "v"}}
+	if _, err := Marshal(nested); err == nil {
+		t.Errorf("expected error for invalid UTF-8 table header key")
+	}
+	inline := map[string]any{"mix": []any{1, map[string]any{"\xff": 1}}}
+	if _, err := Marshal(inline); err == nil {
+		t.Errorf("expected error for invalid UTF-8 inline table key")
+	}
 }
 
 func TestMarshalStringRequiresUTF8(t *testing.T) {