json: return errors with more context, on failed json.decode() calls.

vlib/json/json_decode_test.v

@@ -69,3 +69,49 @@ fn test_skip_fields_should_be_initialised_by_json_decode() ? {
 	assert task.total_comments == 55
 	assert task.comments == []
 }
+
+//
+
+struct DbConfig {
+	host   string
+	dbname string
+	user   string
+}
+
+fn test_decode_error_message_should_have_enough_context_empty() {
+	json.decode(DbConfig, '') or {
+		assert err.msg().len < 2
+		return
+	}
+	assert false
+}
+
+fn test_decode_error_message_should_have_enough_context_just_brace() {
+	json.decode(DbConfig, '{') or {
+		assert err.msg() == '{'
+		return
+	}
+	assert false
+}
+
+fn test_decode_error_message_should_have_enough_context_trailing_comma_at_end() {
+	txt := '{
+    "host": "localhost",
+    "dbname": "alex",
+    "user": "alex",
+}'
+	json.decode(DbConfig, txt) or {
+		assert err.msg() == '    "user": "alex",\n}'
+		return
+	}
+	assert false
+}
+
+fn test_decode_error_message_should_have_enough_context_in_the_middle() {
+	txt := '{"host": "localhost", "dbname": "alex" "user": "alex", "port": "1234"}'
+	json.decode(DbConfig, txt) or {
+		assert err.msg() == 'ost", "dbname": "alex" "user":'
+		return
+	}
+	assert false
+}

vlib/v/gen/c/json.v

@@ -73,9 +73,28 @@ $dec_fn_dec {
 	if (!root) {
 		const char *error_ptr = cJSON_GetErrorPtr();
 		if (error_ptr != NULL)	{
-			// fprintf(stderr, "Error in decode() for $styp error_ptr=: %s\\n", error_ptr);
-			// printf("\\nbad js=%%s\\n", js.str);
-			return (Option_$styp){.state = 2,.err = _v_error(tos2((byteptr)error_ptr)),.data = {0}};
+			char *prevline_ptr = (char*)error_ptr;
+			int maxcontext_chars = 30;
+			int backlines = 1;
+			int backchars = maxcontext_chars-7;
+			while(backchars--){
+				 char prevc = *(prevline_ptr - 1);
+				 if(0==prevc){
+					 break;
+				 }
+				 if(10==prevc && !backlines--){
+					 break;
+				 }
+				 prevline_ptr--;
+				 if(123==prevc) {
+					 break; // stop at `{` too
+				 }
+			}
+			byte *buf = _v_malloc(maxcontext_chars + 10);
+			vmemset(buf, 0, maxcontext_chars+10);
+			vmemcpy(buf, prevline_ptr, maxcontext_chars);
+			// for(int x=-10;x<10;x++){ char *xx = prevline_ptr+x; fprintf(stderr, "2 prevline_ptr + %d: %p | %c | %d \\n", x, xx, (int)(*(xx)), (int)(*(xx))); } fprintf(stderr, "--------\\n");
+			return (Option_$styp){.state = 2,.err = _v_error(tos2(buf)),.data = {0}};
 		}
 	}
 ')