2021-09-17  Bruno Haible  <bruno@clisp.org>

	threadlib: Avoid crashes in thread-related functions on Cygwin 3.2.0.
	Reported by Brian Inglis via Akim Demaille in
	<https://lists.gnu.org/archive/html/bug-gnulib/2021-09/msg00063.html>.
	* m4/threadlib.m4 (gl_WEAK_SYMBOLS): Force a "guessing no" result on
	Cygwin.

diff -w --git origsrc/m4/threadlib.m4 src/m4/threadlib.m4
index 37b797c18..6b43bbdfa 100644
--- origsrc/m4/threadlib.m4	2021-08-02 23:51:08.000000000 -0600
+++ src/m4/threadlib.m4	2021-09-17 15:17:37.103525500 -0600
@@ -1,4 +1,4 @@
-# threadlib.m4 serial 31
+# threadlib.m4 serial 32
 dnl Copyright (C) 2005-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -84,38 +84,48 @@ AC_DEFUN([gl_WEAK_SYMBOLS],
   AC_REQUIRE([AC_CANONICAL_HOST])
   AC_CACHE_CHECK([whether imported symbols can be declared weak],
     [gl_cv_have_weak],
-    [gl_cv_have_weak=no
-     dnl First, test whether the compiler accepts it syntactically.
-     AC_LINK_IFELSE(
-       [AC_LANG_PROGRAM(
-          [[extern void xyzzy ();
+    [case "$host_os" in
+       cygwin*)
+         dnl On Cygwin 3.2.0 with gcc 10.2, the test below would succeed, but
+         dnl programs that use pthread_in_use() with weak symbol references
+         dnl crash miserably at runtime.
+         gl_cv_have_weak="guessing no"
+         ;;
+       *)
+         gl_cv_have_weak=no
+	 dnl First, test whether the compiler accepts it syntactically.
+	 AC_LINK_IFELSE(
+	   [AC_LANG_PROGRAM(
+	      [[extern void xyzzy ();
 #pragma weak xyzzy]],
-          [[xyzzy();]])],
-       [gl_cv_have_weak=maybe])
-     if test $gl_cv_have_weak = maybe; then
-       dnl Second, test whether it actually works. On Cygwin 1.7.2, with
-       dnl gcc 4.3, symbols declared weak always evaluate to the address 0.
-       AC_RUN_IFELSE(
-         [AC_LANG_SOURCE([[
+	      [[xyzzy();]])],
+	   [gl_cv_have_weak=maybe])
+	 if test $gl_cv_have_weak = maybe; then
+	   dnl Second, test whether it actually works. On Cygwin 1.7.2, with
+	   dnl gcc 4.3, symbols declared weak always evaluate to the address 0.
+	   AC_RUN_IFELSE(
+	     [AC_LANG_SOURCE([[
 #include <stdio.h>
 #pragma weak fputs
 int main ()
 {
   return (fputs == NULL);
 }]])],
-         [gl_cv_have_weak=yes],
-         [gl_cv_have_weak=no],
-         [dnl When cross-compiling, assume that only ELF platforms support
-          dnl weak symbols.
-          AC_EGREP_CPP([Extensible Linking Format],
-            [#ifdef __ELF__
-             Extensible Linking Format
-             #endif
-            ],
-            [gl_cv_have_weak="guessing yes"],
-            [gl_cv_have_weak="guessing no"])
-         ])
-     fi
+	     [gl_cv_have_weak=yes],
+	     [gl_cv_have_weak=no],
+	     [dnl When cross-compiling, assume that only ELF platforms support
+	      dnl weak symbols.
+	      AC_EGREP_CPP([Extensible Linking Format],
+		[#ifdef __ELF__
+		 Extensible Linking Format
+		 #endif
+		],
+		[gl_cv_have_weak="guessing yes"],
+		[gl_cv_have_weak="guessing no"])
+	     ])
+	 fi
+	 ;;
+     esac
      dnl But when linking statically, weak symbols don't work.
      case " $LDFLAGS " in
        *" -static "*) gl_cv_have_weak=no ;;