cgen, json: make errors more informative (resolve empty panics) (related: #21184) (#22898)

Author: ttytm

vlib/json/tests/json_decode_test.v

@@ -90,15 +90,15 @@ struct DbConfig {
 
 fn test_decode_error_message_should_have_enough_context_empty() {
 	json.decode(DbConfig, '') or {
-		assert err.msg().len < 2
+		assert err.msg() == 'failed to decode JSON string'
 		return
 	}
 	assert false
 }
 
 fn test_decode_error_message_should_have_enough_context_just_brace() {
 	json.decode(DbConfig, '{') or {
-		assert err.msg() == '{'
+		assert err.msg() == 'failed to decode JSON string: {'
 		return
 	}
 	assert false
@@ -111,7 +111,7 @@ fn test_decode_error_message_should_have_enough_context_trailing_comma_at_end()
     "user": "alex",
 }'
 	json.decode(DbConfig, txt) or {
-		assert err.msg() == '    "user": "alex",\n}'
+		assert err.msg().contains('    "user": "alex",\n}')
 		return
 	}
 	assert false
@@ -120,7 +120,7 @@ fn test_decode_error_message_should_have_enough_context_trailing_comma_at_end()
 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":'
+		assert err.msg().contains('ost", "dbname": "alex" "user":')
 		return
 	}
 	assert false

vlib/v/gen/c/json.v

@@ -95,11 +95,11 @@ ${dec_fn_dec} {
 	${init_styp};
 	if (!root) {
 		const char *error_ptr = cJSON_GetErrorPtr();
-		if (error_ptr != NULL)	{
+		if (error_ptr != NULL) {
 			const int error_pos = (int)cJSON_GetErrorPos();
 			int maxcontext_chars = 30;
 			byte *buf = vcalloc_noscan(maxcontext_chars + 10);
-			if(error_pos > 0) {
+			if (error_pos > 0) {
 				int backlines = 1;
 				int backchars = error_pos < maxcontext_chars-7 ? (int)error_pos : maxcontext_chars-7 ;
 				char *prevline_ptr = (char*)error_ptr;
@@ -119,7 +119,12 @@ ${dec_fn_dec} {
 				int maxchars = vstrlen_char(prevline_ptr);
 				vmemcpy(buf, prevline_ptr, (maxchars < maxcontext_chars ? maxchars : maxcontext_chars));
 			}
-			return (${result_name}_${ret_styp}){.is_error = true,.err = _v_error(tos2(buf)),.data = {0}};
+			string msg;
+			msg = _SLIT("failed to decode JSON string");
+			if (buf[0] != \'\\0\') {
+				msg = string__plus(msg, _SLIT(": "));
+			}
+			return (${result_name}_${ret_styp}){.is_error = true,.err = _v_error(string__plus(msg, tos2(buf))),.data = {0}};
 		}
 	}
 ')