Fix QA findings: FRE GC safety, WAIT mask=0, ENVIRON leak, sentinel collision

- FRE(): remove strpool_gc() call during expression evaluation — temporaries
  on the C stack would become dangling pointers after compaction
- WAIT: reject mask=0 (would infinite-loop since AND 0 is always 0)
- ENVIRON: use setenv()+free() instead of putenv() to avoid memory leak
  and dangling pointer if pool is compacted
- portio: add bounds check to com1_inp() (com1_out already had one)
- kernel: use CHR$(1) sentinel prefix to avoid collision with user output
- kernel: fix INPUT prompt rfind() edge case when no newline in buffer
- eval.c: enlarge DATE$ format buffer to silence -Wformat-truncation

gwbasickernel/kernel.py

@@ -34,8 +34,8 @@ from ipykernel.kernelbase import Kernel
 
 from . import __version__
 
-_SENTINEL = '<<<GWDONE>>>'
-_SENTINEL_CMD = f'PRINT "{_SENTINEL}"\n'
+_SENTINEL = '\x01GWDONE\x01'
+_SENTINEL_CMD = 'PRINT CHR$(1)+"GWDONE"+CHR$(1)\n'
 
 _ERROR_RE = re.compile(
     r'(?:Syntax error|Illegal function call|Type mismatch|Out of |'
@@ -378,13 +378,14 @@ class GWBasicKernel(Kernel):
                 return before, 'done'
 
             # Check for INPUT prompt: "? " at end of buffer (no newline after)
-            if allow_stdin and buf.endswith(b'? ') and b'\n' not in buf[buf.rfind(b'\n') + 1:]:
-                # Extract text before the "? " to show as prompt
+            if allow_stdin and buf.endswith(b'? '):
                 last_nl = buf.rfind(b'\n')
-                prompt_line = buf[last_nl + 1:].decode('utf-8', errors='replace')
-                reply = self.raw_input(prompt_line)
-                self._proc.stdin.write((reply + '\n').encode())
-                self._proc.stdin.flush()
+                tail = buf[last_nl + 1:] if last_nl >= 0 else buf
+                if b'\n' not in tail[:-2]:  # no embedded newline before "? "
+                    prompt_line = tail.decode('utf-8', errors='replace')
+                    reply = self.raw_input(prompt_line)
+                    self._proc.stdin.write((reply + '\n').encode())
+                    self._proc.stdin.flush()
 
     def _split_sixel(self, raw):
         """Split raw output into (text_parts, sixel_blobs) interleaved."""

src/eval.c

@@ -641,10 +641,9 @@ static gw_value_t eval_function(uint8_t prefix, uint8_t func_tok)
         gw_expect('(');
         arg = gw_eval();  /* can be string or numeric */
         gw_expect_rparen();
-        if (arg.type == VT_STR) {
-            gw_str_free(&arg.sval);
-            strpool_gc();
-        }
+        if (arg.type == VT_STR) gw_str_free(&arg.sval);
+        /* No GC here — unsafe during expression eval (temps on C stack).
+         * The run loop compacts at statement boundaries instead. */
         v.type = VT_SNG;
         v.fval = (float)strpool_free();
         return v;
@@ -996,7 +995,7 @@ static gw_value_t eval_atom(void)
             gw_chrget();
             gw_value_t v;
             v.type = VT_STR;
-            char tbuf[16];
+            char tbuf[40];
             time_t now = time(NULL);
             struct tm *tm = localtime(&now);
             if (xtok == XSTMT_DATE) {

src/interp.c

@@ -1377,7 +1377,12 @@ void gw_exec_stmt(void)
             gw_value_t val = gw_eval_str();
             char *s = gw_str_to_cstr(&val.sval);
             gw_str_free(&val.sval);
-            putenv(s);  /* s is intentionally not freed — putenv requires it */
+            char *eq = strchr(s, '=');
+            if (eq) {
+                *eq = '\0';
+                setenv(s, eq + 1, 1);
+            }
+            free(s);
             return;
         }
 
@@ -2617,7 +2622,7 @@ void gw_exec_stmt(void)
         gw_skip_spaces();
         gw_expect(',');
         int mask = gw_eval_int();
-        if (mask < 0 || mask > 255) gw_error(ERR_FC);
+        if (mask < 0 || mask > 255 || mask == 0) gw_error(ERR_FC);
         int xor_mask = 0;
         gw_skip_spaces();
         if (gw_chrgot() == ',') {

src/portio.c

@@ -122,7 +122,9 @@ static uint8_t com1_inp(uint16_t port)
     int reg = port - 0x3F8;
     if (reg == 5)  /* LSR: transmitter ready */
         return 0x60;
-    return com1_regs[reg];
+    if (reg >= 0 && reg < 7)
+        return com1_regs[reg];
+    return 0xFF;
 }
 
 static void com1_out(uint16_t port, uint8_t value)