From 809676a85690e6a5b265e038af260db0a4c1667c Mon Sep 17 00:00:00 2001
From: Delyan Angelov <delian66@gmail.com>
Date: Sat, 9 May 2020 15:21:40 +0300
Subject: [PATCH] cgen: fix a sporadic segfault when giving -width in a string
 interpolation

---
 vlib/v/gen/str.v                         |  6 +++++-
 vlib/v/tests/string_interpolation_test.v | 20 ++++++++++++++++++--
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/vlib/v/gen/str.v b/vlib/v/gen/str.v
index 5da8c560eb..e66b05e8be 100644
--- a/vlib/v/gen/str.v
+++ b/vlib/v/gen/str.v
@@ -9,6 +9,10 @@ fn (mut g Gen) write_str_fn_definitions() {
 void _STR_PRINT_ARG(const char *fmt, char** refbufp, int *nbytes, int *memsize, int guess, ...) {
 	va_list args;
 	va_start(args, guess);
+	// NB: (*memsize - *nbytes) === how much free space is left at the end of the current buffer refbufp
+	// *memsize === total length of the buffer refbufp
+	// *nbytes === already occupied bytes of buffer refbufp
+	// guess === how many bytes were taken during the current vsnprintf run
 	for(;;) {
 		if (guess < *memsize - *nbytes) {
 			guess = vsnprintf(*refbufp + *nbytes, *memsize - *nbytes, fmt, args);
@@ -62,7 +66,7 @@ string _STR(const char *fmt, int nfmts, ...) {
 						fwidth -= (s.len - utf8_str_visible_length(s));
 					else
 						fwidth += (s.len - utf8_str_visible_length(s));
-					_STR_PRINT_ARG(fmt, &buf, &nbytes, &memsize, k+fwidth-4, fwidth, s.len, s.str);
+					_STR_PRINT_ARG(fmt, &buf, &nbytes, &memsize, k+s.len-4, fwidth, s.len, s.str);
 				} else { // %.*s
 					_STR_PRINT_ARG(fmt, &buf, &nbytes, &memsize, k+s.len-4, s.len, s.str);
 				}
diff --git a/vlib/v/tests/string_interpolation_test.v b/vlib/v/tests/string_interpolation_test.v
index 78dff09c3d..b57bb517d8 100644
--- a/vlib/v/tests/string_interpolation_test.v
+++ b/vlib/v/tests/string_interpolation_test.v
@@ -56,7 +56,7 @@ fn test_implicit_str() {
 	assert text == '4242'
 }
 
-fn test_string_interpolation_percent_escaping(){
+fn test_string_interpolation_percent_escaping() {
 	test := 'hello'
 	hello := 'world'
 	x := '%.*s$hello$test |${hello:-30s}|'
@@ -111,7 +111,11 @@ fn test_utf8_string_interpolation() {
 	st := 'Sträßle'
 	m := '10€'
 	assert '$a $st $m' == 'à-côté Sträßle 10€'
-	assert '>${a:10}< >${st:-8}< >${m:5}<-' == '>    à-côté< >Sträßle < >  10€<-'
+	zz := '>${a:10}< >${st:-8}< >${m:5}<-'
+	zz_expected := '>    à-côté< >Sträßle < >  10€<-'
+	eprintln('         zz: $zz')
+	eprintln('zz_expected: $zz_expected')
+	assert zz == zz_expected
 	// e := '\u20AC' // Eurosign doesn' work with MSVC and tcc
 	e := '€'
 	assert '100.00 $e' == '100.00 €'
@@ -135,3 +139,15 @@ fn test_string_interpolation_str_evaluation() {
 	mut x := S{17, 13.455893}
 	assert '$x' == '[17, 13.456]'
 }
+
+
+fn test_string_interpolation_with_negative_format_width_should_compile_and_run_without_segfaulting() {
+	// discovered during debugging VLS
+	i := 3
+	input := '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}'
+	eprintln('---------------------------------------------------------------------------------------------')
+	eprintln('+60 ${i:10} | input.len: ${input.len:10} | ${input.bytes().hex():60} | $input')
+	eprintln('-60 ${i:10} | input.len: ${input.len:10} | ${input.bytes().hex():-60} | $input')
+	eprintln('---------------------------------------------------------------------------------------------')
+	assert true
+}