Assert the GVL is held when performing various string functions.

This is being done to debug issues in the `zlib` extension.

[Feature #20877]

common.mk

@@ -17839,6 +17839,7 @@ string.$(OBJEXT): $(top_srcdir)/internal/sanitizers.h
 string.$(OBJEXT): $(top_srcdir)/internal/serial.h
 string.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 string.$(OBJEXT): $(top_srcdir)/internal/string.h
+string.$(OBJEXT): $(top_srcdir)/internal/thread.h
 string.$(OBJEXT): $(top_srcdir)/internal/transcode.h
 string.$(OBJEXT): $(top_srcdir)/internal/variable.h
 string.$(OBJEXT): $(top_srcdir)/internal/vm.h

string.c

@@ -37,6 +37,7 @@
 #include "internal/re.h"
 #include "internal/sanitizers.h"
 #include "internal/string.h"
+#include "internal/thread.h"
 #include "internal/transcode.h"
 #include "probes.h"
 #include "ruby/encoding.h"
@@ -2660,6 +2661,8 @@ rb_str_modify(VALUE str)
 void
 rb_str_modify_expand(VALUE str, long expand)
 {
+    RUBY_ASSERT(ruby_thread_has_gvl_p());
+
     int termlen = TERM_LEN(str);
     long len = RSTRING_LEN(str);
 
@@ -3264,6 +3267,8 @@ rb_str_locktmp_ensure(VALUE str, VALUE (*func)(VALUE), VALUE arg)
 void
 rb_str_set_len(VALUE str, long len)
 {
+    RUBY_ASSERT(ruby_thread_has_gvl_p());
+
     long capa;
     const int termlen = TERM_LEN(str);