From d2663582ca9305a36f9107cf95b951393a1563dc Mon Sep 17 00:00:00 2001
From: David Robillard <d@drobilla.net>
Date: Sun, 23 Jun 2024 16:36:12 -0400
Subject: Gracefully handle failure to terminate written anonymous nodes

This case shouldn't be reachable when driven directly from a reader, but can be
reached by invalid use of the writer in an application.
---
 src/writer.c       | 28 ++++++++++++++--------------
 test/test_writer.c | 30 ++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 14 deletions(-)

diff --git a/src/writer.c b/src/writer.c
index 96e6135d..58d7e64f 100644
--- a/src/writer.c
+++ b/src/writer.c
@@ -1041,24 +1041,24 @@ serd_writer_write_statement(SerdWriter*        writer,
 
   } else {
     // No abbreviation
-    if (serd_stack_is_empty(&writer->anon_stack)) {
-      if (writer->context.subject.type) {
-        TRY(st, write_sep(writer, SEP_END_S));
-      }
 
-      if (writer->last_sep == SEP_END_S || writer->last_sep == SEP_END_DIRECT) {
-        TRY(st, write_newline(writer));
-      }
+    if (!serd_stack_is_empty(&writer->anon_stack)) {
+      return SERD_ERR_BAD_ARG;
+    }
 
-      TRY(st, write_node(writer, subject, NULL, NULL, FIELD_SUBJECT, flags));
-      if ((flags & (SERD_ANON_S_BEGIN | SERD_LIST_S_BEGIN))) {
-        TRY(st, write_sep(writer, SEP_ANON_S_P));
-      } else {
-        TRY(st, write_sep(writer, SEP_S_P));
-      }
+    if (writer->context.subject.type) {
+      TRY(st, write_sep(writer, SEP_END_S));
+    }
 
-    } else {
+    if (writer->last_sep == SEP_END_S || writer->last_sep == SEP_END_DIRECT) {
+      TRY(st, write_newline(writer));
+    }
+
+    TRY(st, write_node(writer, subject, NULL, NULL, FIELD_SUBJECT, flags));
+    if ((flags & (SERD_ANON_S_BEGIN | SERD_LIST_S_BEGIN))) {
       TRY(st, write_sep(writer, SEP_ANON_S_P));
+    } else {
+      TRY(st, write_sep(writer, SEP_S_P));
     }
 
     reset_context(writer, 0U);
diff --git a/test/test_writer.c b/test/test_writer.c
index 75b311cf..b478f052 100644
--- a/test/test_writer.c
+++ b/test/test_writer.c
@@ -175,6 +175,35 @@ test_writer_cleanup(void)
   serd_env_free(env);
 }
 
+static void
+test_write_bad_anon_stack(void)
+{
+  SerdStatus  st  = SERD_SUCCESS;
+  SerdEnv*    env = serd_env_new(NULL);
+  SerdWriter* writer =
+    serd_writer_new(SERD_TURTLE, (SerdStyle)0U, env, NULL, null_sink, NULL);
+
+  SerdNode s  = serd_node_from_string(SERD_URI, USTR("http://example.org/s"));
+  SerdNode p  = serd_node_from_string(SERD_URI, USTR("http://example.org/p"));
+  SerdNode b0 = serd_node_from_string(SERD_BLANK, USTR("b0"));
+  SerdNode b1 = serd_node_from_string(SERD_BLANK, USTR("b1"));
+  SerdNode b2 = serd_node_from_string(SERD_BLANK, USTR("b2"));
+
+  assert(!(st = serd_writer_write_statement(
+             writer, SERD_ANON_O_BEGIN, NULL, &s, &p, &b0, NULL, NULL)));
+
+  // (missing call to end the anonymous node here)
+
+  st = serd_writer_write_statement(
+    writer, SERD_ANON_O_BEGIN, NULL, &b1, &p, &b2, NULL, NULL);
+
+  assert(st == SERD_ERR_BAD_ARG);
+
+  assert(!(st = serd_writer_finish(writer)));
+  serd_writer_free(writer);
+  serd_env_free(env);
+}
+
 static void
 test_strict_write(void)
 {
@@ -243,6 +272,7 @@ main(void)
   test_write_long_literal();
   test_write_nested_anon();
   test_writer_cleanup();
+  test_write_bad_anon_stack();
   test_strict_write();
   test_write_error();
 
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