windows-rwlock: New module.

* lib/windows-rwlock.h: New file, extracted from lib/glthread/lock.h.
* lib/windows-rwlock.c: New file, extracted from lib/glthread/lock.c.
* lib/glthread/lock.h: Include windows-rwlock.h. Don't include
windows-initguard.h.
(gl_rwlock_t): Define using glwthread_rwlock_t.
(gl_rwlock_initializer): Define using GLWTHREAD_RWLOCK_INIT.
(glthread_rwlock_init): Define using glwthread_rwlock_init.
(glthread_rwlock_rdlock): Define using glwthread_rwlock_rdlock.
(glthread_rwlock_wrlock): Define using glwthread_rwlock_wrlock.
(glthread_rwlock_unlock): Define using glwthread_rwlock_unlock.
(glthread_rwlock_destroy): Define using glwthread_rwlock_destroy.
(glthread_rwlock_init_func, glthread_rwlock_rdlock_func,
glthread_rwlock_wrlock_func, glthread_rwlock_unlock_func,
glthread_rwlock_destroy_func): Remove declarations.
* lib/glthread/lock.c (gl_waitqueue_t): Remove type.
(gl_waitqueue_init, gl_waitqueue_add, gl_waitqueue_notify_first,
gl_waitqueue_notify_all, glthread_rwlock_init_func,
glthread_rwlock_rdlock_func, glthread_rwlock_wrlock_func,
glthread_rwlock_unlock_func, glthread_rwlock_destroy_func): Remove
functions.
* modules/windows-rwlock: New file.
* modules/lock (Depends-on): Add windows-rwlock.

7 files changed, 505 insertions, 314 deletions

diff --git a/ChangeLog b/ChangeLog
index 316ae110e5..3a0d6f1189 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,31 @@
 2019-07-14  Bruno Haible  <bruno@clisp.org>
 
+	windows-rwlock: New module.
+	* lib/windows-rwlock.h: New file, extracted from lib/glthread/lock.h.
+	* lib/windows-rwlock.c: New file, extracted from lib/glthread/lock.c.
+	* lib/glthread/lock.h: Include windows-rwlock.h. Don't include
+	windows-initguard.h.
+	(gl_rwlock_t): Define using glwthread_rwlock_t.
+	(gl_rwlock_initializer): Define using GLWTHREAD_RWLOCK_INIT.
+	(glthread_rwlock_init): Define using glwthread_rwlock_init.
+	(glthread_rwlock_rdlock): Define using glwthread_rwlock_rdlock.
+	(glthread_rwlock_wrlock): Define using glwthread_rwlock_wrlock.
+	(glthread_rwlock_unlock): Define using glwthread_rwlock_unlock.
+	(glthread_rwlock_destroy): Define using glwthread_rwlock_destroy.
+	(glthread_rwlock_init_func, glthread_rwlock_rdlock_func,
+	glthread_rwlock_wrlock_func, glthread_rwlock_unlock_func,
+	glthread_rwlock_destroy_func): Remove declarations.
+	* lib/glthread/lock.c (gl_waitqueue_t): Remove type.
+	(gl_waitqueue_init, gl_waitqueue_add, gl_waitqueue_notify_first,
+	gl_waitqueue_notify_all, glthread_rwlock_init_func,
+	glthread_rwlock_rdlock_func, glthread_rwlock_wrlock_func,
+	glthread_rwlock_unlock_func, glthread_rwlock_destroy_func): Remove
+	functions.
+	* modules/windows-rwlock: New file.
+	* modules/lock (Depends-on): Add windows-rwlock.
+
+2019-07-14  Bruno Haible  <bruno@clisp.org>
+
 	windows-thread: Add support for creating a thread in detached state.
 	* lib/windows-thread.h (GLWTHREAD_ATTR_DETACHED): New macro.
 	(glwthread_thread_create): Add attr argument.
diff --git a/lib/glthread/lock.c b/lib/glthread/lock.c
index 89f1f50ab1..852d84d3bb 100644
--- a/lib/glthread/lock.c
+++ b/lib/glthread/lock.c
@@ -15,7 +15,7 @@
    along with this program; if not, see <https://www.gnu.org/licenses/>.  */
 
 /* Written by Bruno Haible <bruno@clisp.org>, 2005.
-   Based on GCC's gthr-posix.h, gthr-posix95.h, gthr-win32.h.  */
+   Based on GCC's gthr-posix.h, gthr-posix95.h.  */
 
 #include <config.h>
 
@@ -501,286 +501,6 @@ glthread_once_singlethreaded (pthread_once_t *once_control)
 
 #if USE_WINDOWS_THREADS
 
-/* ------------------------- gl_rwlock_t datatype ------------------------- */
-
-/* In this file, the waitqueues are implemented as circular arrays.  */
-#define gl_waitqueue_t gl_carray_waitqueue_t
-
-static void
-gl_waitqueue_init (gl_waitqueue_t *wq)
-{
-  wq->array = NULL;
-  wq->count = 0;
-  wq->alloc = 0;
-  wq->offset = 0;
-}
-
-/* Enqueues the current thread, represented by an event, in a wait queue.
-   Returns INVALID_HANDLE_VALUE if an allocation failure occurs.  */
-static HANDLE
-gl_waitqueue_add (gl_waitqueue_t *wq)
-{
-  HANDLE event;
-  unsigned int index;
-
-  if (wq->count == wq->alloc)
-    {
-      unsigned int new_alloc = 2 * wq->alloc + 1;
-      HANDLE *new_array =
-        (HANDLE *) realloc (wq->array, new_alloc * sizeof (HANDLE));
-      if (new_array == NULL)
-        /* No more memory.  */
-        return INVALID_HANDLE_VALUE;
-      /* Now is a good opportunity to rotate the array so that its contents
-         starts at offset 0.  */
-      if (wq->offset > 0)
-        {
-          unsigned int old_count = wq->count;
-          unsigned int old_alloc = wq->alloc;
-          unsigned int old_offset = wq->offset;
-          unsigned int i;
-          if (old_offset + old_count > old_alloc)
-            {
-              unsigned int limit = old_offset + old_count - old_alloc;
-              for (i = 0; i < limit; i++)
-                new_array[old_alloc + i] = new_array[i];
-            }
-          for (i = 0; i < old_count; i++)
-            new_array[i] = new_array[old_offset + i];
-          wq->offset = 0;
-        }
-      wq->array = new_array;
-      wq->alloc = new_alloc;
-    }
-  /* Whether the created event is a manual-reset one or an auto-reset one,
-     does not matter, since we will wait on it only once.  */
-  event = CreateEvent (NULL, TRUE, FALSE, NULL);
-  if (event == INVALID_HANDLE_VALUE)
-    /* No way to allocate an event.  */
-    return INVALID_HANDLE_VALUE;
-  index = wq->offset + wq->count;
-  if (index >= wq->alloc)
-    index -= wq->alloc;
-  wq->array[index] = event;
-  wq->count++;
-  return event;
-}
-
-/* Notifies the first thread from a wait queue and dequeues it.  */
-static void
-gl_waitqueue_notify_first (gl_waitqueue_t *wq)
-{
-  SetEvent (wq->array[wq->offset + 0]);
-  wq->offset++;
-  wq->count--;
-  if (wq->count == 0 || wq->offset == wq->alloc)
-    wq->offset = 0;
-}
-
-/* Notifies all threads from a wait queue and dequeues them all.  */
-static void
-gl_waitqueue_notify_all (gl_waitqueue_t *wq)
-{
-  unsigned int i;
-
-  for (i = 0; i < wq->count; i++)
-    {
-      unsigned int index = wq->offset + i;
-      if (index >= wq->alloc)
-        index -= wq->alloc;
-      SetEvent (wq->array[index]);
-    }
-  wq->count = 0;
-  wq->offset = 0;
-}
-
-void
-glthread_rwlock_init_func (gl_rwlock_t *lock)
-{
-  InitializeCriticalSection (&lock->lock);
-  gl_waitqueue_init (&lock->waiting_readers);
-  gl_waitqueue_init (&lock->waiting_writers);
-  lock->runcount = 0;
-  lock->guard.done = 1;
-}
-
-int
-glthread_rwlock_rdlock_func (gl_rwlock_t *lock)
-{
-  if (!lock->guard.done)
-    {
-      if (InterlockedIncrement (&lock->guard.started) == 0)
-        /* This thread is the first one to need this lock.  Initialize it.  */
-        glthread_rwlock_init (lock);
-      else
-        {
-          /* Don't let lock->guard.started grow and wrap around.  */
-          InterlockedDecrement (&lock->guard.started);
-          /* Yield the CPU while waiting for another thread to finish
-             initializing this lock.  */
-          while (!lock->guard.done)
-            Sleep (0);
-        }
-    }
-  EnterCriticalSection (&lock->lock);
-  /* Test whether only readers are currently running, and whether the runcount
-     field will not overflow, and whether no writer is waiting.  The latter
-     condition is because POSIX recommends that "write locks shall take
-     precedence over read locks", to avoid "writer starvation".  */
-  if (!(lock->runcount + 1 > 0 && lock->waiting_writers.count == 0))
-    {
-      /* This thread has to wait for a while.  Enqueue it among the
-         waiting_readers.  */
-      HANDLE event = gl_waitqueue_add (&lock->waiting_readers);
-      if (event != INVALID_HANDLE_VALUE)
-        {
-          DWORD result;
-          LeaveCriticalSection (&lock->lock);
-          /* Wait until another thread signals this event.  */
-          result = WaitForSingleObject (event, INFINITE);
-          if (result == WAIT_FAILED || result == WAIT_TIMEOUT)
-            abort ();
-          CloseHandle (event);
-          /* The thread which signalled the event already did the bookkeeping:
-             removed us from the waiting_readers, incremented lock->runcount.  */
-          if (!(lock->runcount > 0))
-            abort ();
-          return 0;
-        }
-      else
-        {
-          /* Allocation failure.  Weird.  */
-          do
-            {
-              LeaveCriticalSection (&lock->lock);
-              Sleep (1);
-              EnterCriticalSection (&lock->lock);
-            }
-          while (!(lock->runcount + 1 > 0));
-        }
-    }
-  lock->runcount++;
-  LeaveCriticalSection (&lock->lock);
-  return 0;
-}
-
-int
-glthread_rwlock_wrlock_func (gl_rwlock_t *lock)
-{
-  if (!lock->guard.done)
-    {
-      if (InterlockedIncrement (&lock->guard.started) == 0)
-        /* This thread is the first one to need this lock.  Initialize it.  */
-        glthread_rwlock_init (lock);
-      else
-        {
-          /* Don't let lock->guard.started grow and wrap around.  */
-          InterlockedDecrement (&lock->guard.started);
-          /* Yield the CPU while waiting for another thread to finish
-             initializing this lock.  */
-          while (!lock->guard.done)
-            Sleep (0);
-        }
-    }
-  EnterCriticalSection (&lock->lock);
-  /* Test whether no readers or writers are currently running.  */
-  if (!(lock->runcount == 0))
-    {
-      /* This thread has to wait for a while.  Enqueue it among the
-         waiting_writers.  */
-      HANDLE event = gl_waitqueue_add (&lock->waiting_writers);
-      if (event != INVALID_HANDLE_VALUE)
-        {
-          DWORD result;
-          LeaveCriticalSection (&lock->lock);
-          /* Wait until another thread signals this event.  */
-          result = WaitForSingleObject (event, INFINITE);
-          if (result == WAIT_FAILED || result == WAIT_TIMEOUT)
-            abort ();
-          CloseHandle (event);
-          /* The thread which signalled the event already did the bookkeeping:
-             removed us from the waiting_writers, set lock->runcount = -1.  */
-          if (!(lock->runcount == -1))
-            abort ();
-          return 0;
-        }
-      else
-        {
-          /* Allocation failure.  Weird.  */
-          do
-            {
-              LeaveCriticalSection (&lock->lock);
-              Sleep (1);
-              EnterCriticalSection (&lock->lock);
-            }
-          while (!(lock->runcount == 0));
-        }
-    }
-  lock->runcount--; /* runcount becomes -1 */
-  LeaveCriticalSection (&lock->lock);
-  return 0;
-}
-
-int
-glthread_rwlock_unlock_func (gl_rwlock_t *lock)
-{
-  if (!lock->guard.done)
-    return EINVAL;
-  EnterCriticalSection (&lock->lock);
-  if (lock->runcount < 0)
-    {
-      /* Drop a writer lock.  */
-      if (!(lock->runcount == -1))
-        abort ();
-      lock->runcount = 0;
-    }
-  else
-    {
-      /* Drop a reader lock.  */
-      if (!(lock->runcount > 0))
-        {
-          LeaveCriticalSection (&lock->lock);
-          return EPERM;
-        }
-      lock->runcount--;
-    }
-  if (lock->runcount == 0)
-    {
-      /* POSIX recommends that "write locks shall take precedence over read
-         locks", to avoid "writer starvation".  */
-      if (lock->waiting_writers.count > 0)
-        {
-          /* Wake up one of the waiting writers.  */
-          lock->runcount--;
-          gl_waitqueue_notify_first (&lock->waiting_writers);
-        }
-      else
-        {
-          /* Wake up all waiting readers.  */
-          lock->runcount += lock->waiting_readers.count;
-          gl_waitqueue_notify_all (&lock->waiting_readers);
-        }
-    }
-  LeaveCriticalSection (&lock->lock);
-  return 0;
-}
-
-int
-glthread_rwlock_destroy_func (gl_rwlock_t *lock)
-{
-  if (!lock->guard.done)
-    return EINVAL;
-  if (lock->runcount != 0)
-    return EBUSY;
-  DeleteCriticalSection (&lock->lock);
-  if (lock->waiting_readers.array != NULL)
-    free (lock->waiting_readers.array);
-  if (lock->waiting_writers.array != NULL)
-    free (lock->waiting_writers.array);
-  lock->guard.done = 0;
-  return 0;
-}
-
 #endif
 
 /* ========================================================================= */
diff --git a/lib/glthread/lock.h b/lib/glthread/lock.h
index 23d2d98506..78dd67f29f 100644
--- a/lib/glthread/lock.h
+++ b/lib/glthread/lock.h
@@ -410,8 +410,8 @@ extern int glthread_once_singlethreaded (pthread_once_t *once_control);
 # define WIN32_LEAN_AND_MEAN  /* avoid including junk */
 # include <windows.h>
 
-# include "windows-initguard.h"
 # include "windows-mutex.h"
+# include "windows-rwlock.h"
 # include "windows-recmutex.h"
 # include "windows-once.h"
 
@@ -450,48 +450,23 @@ typedef glwthread_mutex_t gl_lock_t;
 
 /* ------------------------- gl_rwlock_t datatype ------------------------- */
 
-/* It is impossible to implement read-write locks using plain locks, without
-   introducing an extra thread dedicated to managing read-write locks.
-   Therefore here we need to use the low-level Event type.  */
-
-typedef struct
-        {
-          HANDLE *array; /* array of waiting threads, each represented by an event */
-          unsigned int count; /* number of waiting threads */
-          unsigned int alloc; /* length of allocated array */
-          unsigned int offset; /* index of first waiting thread in array */
-        }
-        gl_carray_waitqueue_t;
-typedef struct
-        {
-          glwthread_initguard_t guard; /* protects the initialization */
-          CRITICAL_SECTION lock; /* protects the remaining fields */
-          gl_carray_waitqueue_t waiting_readers; /* waiting readers */
-          gl_carray_waitqueue_t waiting_writers; /* waiting writers */
-          int runcount; /* number of readers running, or -1 when a writer runs */
-        }
-        gl_rwlock_t;
+typedef glwthread_rwlock_t gl_rwlock_t;
 # define gl_rwlock_define(STORAGECLASS, NAME) \
     STORAGECLASS gl_rwlock_t NAME;
 # define gl_rwlock_define_initialized(STORAGECLASS, NAME) \
     STORAGECLASS gl_rwlock_t NAME = gl_rwlock_initializer;
 # define gl_rwlock_initializer \
-    { GLWTHREAD_INITGUARD_INIT }
+    GLWTHREAD_RWLOCK_INIT
 # define glthread_rwlock_init(LOCK) \
-    (glthread_rwlock_init_func (LOCK), 0)
+    (glwthread_rwlock_init (LOCK), 0)
 # define glthread_rwlock_rdlock(LOCK) \
-    glthread_rwlock_rdlock_func (LOCK)
+    glwthread_rwlock_rdlock (LOCK)
 # define glthread_rwlock_wrlock(LOCK) \
-    glthread_rwlock_wrlock_func (LOCK)
+    glwthread_rwlock_wrlock (LOCK)
 # define glthread_rwlock_unlock(LOCK) \
-    glthread_rwlock_unlock_func (LOCK)
+    glwthread_rwlock_unlock (LOCK)
 # define glthread_rwlock_destroy(LOCK) \
-    glthread_rwlock_destroy_func (LOCK)
-extern void glthread_rwlock_init_func (gl_rwlock_t *lock);
-extern int glthread_rwlock_rdlock_func (gl_rwlock_t *lock);
-extern int glthread_rwlock_wrlock_func (gl_rwlock_t *lock);
-extern int glthread_rwlock_unlock_func (gl_rwlock_t *lock);
-extern int glthread_rwlock_destroy_func (gl_rwlock_t *lock);
+    glwthread_rwlock_destroy (LOCK)
 
 /* --------------------- gl_recursive_lock_t datatype --------------------- */
 
diff --git a/lib/windows-rwlock.c b/lib/windows-rwlock.c
new file mode 100644
index 0000000000..d777307b09
--- /dev/null
+++ b/lib/windows-rwlock.c
@@ -0,0 +1,373 @@
+/* Read-write locks (native Windows implementation).
+   Copyright (C) 2005-2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2005.
+   Based on GCC's gthr-win32.h.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "windows-rwlock.h"
+
+#include <errno.h>
+#include <stdlib.h>
+
+/* In this file, the waitqueues are implemented as circular arrays.  */
+#define glwthread_waitqueue_t glwthread_carray_waitqueue_t
+
+static void
+glwthread_waitqueue_init (glwthread_waitqueue_t *wq)
+{
+  wq->array = NULL;
+  wq->count = 0;
+  wq->alloc = 0;
+  wq->offset = 0;
+}
+
+/* Enqueues the current thread, represented by an event, in a wait queue.
+   Returns INVALID_HANDLE_VALUE if an allocation failure occurs.  */
+static HANDLE
+glwthread_waitqueue_add (glwthread_waitqueue_t *wq)
+{
+  HANDLE event;
+  unsigned int index;
+
+  if (wq->count == wq->alloc)
+    {
+      unsigned int new_alloc = 2 * wq->alloc + 1;
+      HANDLE *new_array =
+        (HANDLE *) realloc (wq->array, new_alloc * sizeof (HANDLE));
+      if (new_array == NULL)
+        /* No more memory.  */
+        return INVALID_HANDLE_VALUE;
+      /* Now is a good opportunity to rotate the array so that its contents
+         starts at offset 0.  */
+      if (wq->offset > 0)
+        {
+          unsigned int old_count = wq->count;
+          unsigned int old_alloc = wq->alloc;
+          unsigned int old_offset = wq->offset;
+          unsigned int i;
+          if (old_offset + old_count > old_alloc)
+            {
+              unsigned int limit = old_offset + old_count - old_alloc;
+              for (i = 0; i < limit; i++)
+                new_array[old_alloc + i] = new_array[i];
+            }
+          for (i = 0; i < old_count; i++)
+            new_array[i] = new_array[old_offset + i];
+          wq->offset = 0;
+        }
+      wq->array = new_array;
+      wq->alloc = new_alloc;
+    }
+  /* Whether the created event is a manual-reset one or an auto-reset one,
+     does not matter, since we will wait on it only once.  */
+  event = CreateEvent (NULL, TRUE, FALSE, NULL);
+  if (event == INVALID_HANDLE_VALUE)
+    /* No way to allocate an event.  */
+    return INVALID_HANDLE_VALUE;
+  index = wq->offset + wq->count;
+  if (index >= wq->alloc)
+    index -= wq->alloc;
+  wq->array[index] = event;
+  wq->count++;
+  return event;
+}
+
+/* Notifies the first thread from a wait queue and dequeues it.  */
+static void
+glwthread_waitqueue_notify_first (glwthread_waitqueue_t *wq)
+{
+  SetEvent (wq->array[wq->offset + 0]);
+  wq->offset++;
+  wq->count--;
+  if (wq->count == 0 || wq->offset == wq->alloc)
+    wq->offset = 0;
+}
+
+/* Notifies all threads from a wait queue and dequeues them all.  */
+static void
+glwthread_waitqueue_notify_all (glwthread_waitqueue_t *wq)
+{
+  unsigned int i;
+
+  for (i = 0; i < wq->count; i++)
+    {
+      unsigned int index = wq->offset + i;
+      if (index >= wq->alloc)
+        index -= wq->alloc;
+      SetEvent (wq->array[index]);
+    }
+  wq->count = 0;
+  wq->offset = 0;
+}
+
+void
+glwthread_rwlock_init (glwthread_rwlock_t *lock)
+{
+  InitializeCriticalSection (&lock->lock);
+  glwthread_waitqueue_init (&lock->waiting_readers);
+  glwthread_waitqueue_init (&lock->waiting_writers);
+  lock->runcount = 0;
+  lock->guard.done = 1;
+}
+
+int
+glwthread_rwlock_rdlock (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    {
+      if (InterlockedIncrement (&lock->guard.started) == 0)
+        /* This thread is the first one to need this lock.  Initialize it.  */
+        glwthread_rwlock_init (lock);
+      else
+        {
+          /* Don't let lock->guard.started grow and wrap around.  */
+          InterlockedDecrement (&lock->guard.started);
+          /* Yield the CPU while waiting for another thread to finish
+             initializing this lock.  */
+          while (!lock->guard.done)
+            Sleep (0);
+        }
+    }
+  EnterCriticalSection (&lock->lock);
+  /* Test whether only readers are currently running, and whether the runcount
+     field will not overflow, and whether no writer is waiting.  The latter
+     condition is because POSIX recommends that "write locks shall take
+     precedence over read locks", to avoid "writer starvation".  */
+  if (!(lock->runcount + 1 > 0 && lock->waiting_writers.count == 0))
+    {
+      /* This thread has to wait for a while.  Enqueue it among the
+         waiting_readers.  */
+      HANDLE event = glwthread_waitqueue_add (&lock->waiting_readers);
+      if (event != INVALID_HANDLE_VALUE)
+        {
+          DWORD result;
+          LeaveCriticalSection (&lock->lock);
+          /* Wait until another thread signals this event.  */
+          result = WaitForSingleObject (event, INFINITE);
+          if (result == WAIT_FAILED || result == WAIT_TIMEOUT)
+            abort ();
+          CloseHandle (event);
+          /* The thread which signalled the event already did the bookkeeping:
+             removed us from the waiting_readers, incremented lock->runcount.  */
+          if (!(lock->runcount > 0))
+            abort ();
+          return 0;
+        }
+      else
+        {
+          /* Allocation failure.  Weird.  */
+          do
+            {
+              LeaveCriticalSection (&lock->lock);
+              Sleep (1);
+              EnterCriticalSection (&lock->lock);
+            }
+          while (!(lock->runcount + 1 > 0));
+        }
+    }
+  lock->runcount++;
+  LeaveCriticalSection (&lock->lock);
+  return 0;
+}
+
+int
+glwthread_rwlock_wrlock (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    {
+      if (InterlockedIncrement (&lock->guard.started) == 0)
+        /* This thread is the first one to need this lock.  Initialize it.  */
+        glwthread_rwlock_init (lock);
+      else
+        {
+          /* Don't let lock->guard.started grow and wrap around.  */
+          InterlockedDecrement (&lock->guard.started);
+          /* Yield the CPU while waiting for another thread to finish
+             initializing this lock.  */
+          while (!lock->guard.done)
+            Sleep (0);
+        }
+    }
+  EnterCriticalSection (&lock->lock);
+  /* Test whether no readers or writers are currently running.  */
+  if (!(lock->runcount == 0))
+    {
+      /* This thread has to wait for a while.  Enqueue it among the
+         waiting_writers.  */
+      HANDLE event = glwthread_waitqueue_add (&lock->waiting_writers);
+      if (event != INVALID_HANDLE_VALUE)
+        {
+          DWORD result;
+          LeaveCriticalSection (&lock->lock);
+          /* Wait until another thread signals this event.  */
+          result = WaitForSingleObject (event, INFINITE);
+          if (result == WAIT_FAILED || result == WAIT_TIMEOUT)
+            abort ();
+          CloseHandle (event);
+          /* The thread which signalled the event already did the bookkeeping:
+             removed us from the waiting_writers, set lock->runcount = -1.  */
+          if (!(lock->runcount == -1))
+            abort ();
+          return 0;
+        }
+      else
+        {
+          /* Allocation failure.  Weird.  */
+          do
+            {
+              LeaveCriticalSection (&lock->lock);
+              Sleep (1);
+              EnterCriticalSection (&lock->lock);
+            }
+          while (!(lock->runcount == 0));
+        }
+    }
+  lock->runcount--; /* runcount becomes -1 */
+  LeaveCriticalSection (&lock->lock);
+  return 0;
+}
+
+int
+glwthread_rwlock_tryrdlock (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    {
+      if (InterlockedIncrement (&lock->guard.started) == 0)
+        /* This thread is the first one to need this lock.  Initialize it.  */
+        glwthread_rwlock_init (lock);
+      else
+        {
+          /* Don't let lock->guard.started grow and wrap around.  */
+          InterlockedDecrement (&lock->guard.started);
+          /* Yield the CPU while waiting for another thread to finish
+             initializing this lock.  */
+          while (!lock->guard.done)
+            Sleep (0);
+        }
+    }
+  /* It's OK to wait for this critical section, because it is never taken for a
+     long time.  */
+  EnterCriticalSection (&lock->lock);
+  /* Test whether only readers are currently running, and whether the runcount
+     field will not overflow, and whether no writer is waiting.  The latter
+     condition is because POSIX recommends that "write locks shall take
+     precedence over read locks", to avoid "writer starvation".  */
+  if (!(lock->runcount + 1 > 0 && lock->waiting_writers.count == 0))
+    {
+      /* This thread would have to wait for a while.  Return instead.  */
+      LeaveCriticalSection (&lock->lock);
+      return EBUSY;
+    }
+  lock->runcount++;
+  LeaveCriticalSection (&lock->lock);
+  return 0;
+}
+
+int
+glwthread_rwlock_trywrlock (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    {
+      if (InterlockedIncrement (&lock->guard.started) == 0)
+        /* This thread is the first one to need this lock.  Initialize it.  */
+        glwthread_rwlock_init (lock);
+      else
+        {
+          /* Don't let lock->guard.started grow and wrap around.  */
+          InterlockedDecrement (&lock->guard.started);
+          /* Yield the CPU while waiting for another thread to finish
+             initializing this lock.  */
+          while (!lock->guard.done)
+            Sleep (0);
+        }
+    }
+  /* It's OK to wait for this critical section, because it is never taken for a
+     long time.  */
+  EnterCriticalSection (&lock->lock);
+  /* Test whether no readers or writers are currently running.  */
+  if (!(lock->runcount == 0))
+    {
+      /* This thread would have to wait for a while.  Return instead.  */
+      LeaveCriticalSection (&lock->lock);
+      return EBUSY;
+    }
+  lock->runcount--; /* runcount becomes -1 */
+  LeaveCriticalSection (&lock->lock);
+  return 0;
+}
+
+int
+glwthread_rwlock_unlock (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    return EINVAL;
+  EnterCriticalSection (&lock->lock);
+  if (lock->runcount < 0)
+    {
+      /* Drop a writer lock.  */
+      if (!(lock->runcount == -1))
+        abort ();
+      lock->runcount = 0;
+    }
+  else
+    {
+      /* Drop a reader lock.  */
+      if (!(lock->runcount > 0))
+        {
+          LeaveCriticalSection (&lock->lock);
+          return EPERM;
+        }
+      lock->runcount--;
+    }
+  if (lock->runcount == 0)
+    {
+      /* POSIX recommends that "write locks shall take precedence over read
+         locks", to avoid "writer starvation".  */
+      if (lock->waiting_writers.count > 0)
+        {
+          /* Wake up one of the waiting writers.  */
+          lock->runcount--;
+          glwthread_waitqueue_notify_first (&lock->waiting_writers);
+        }
+      else
+        {
+          /* Wake up all waiting readers.  */
+          lock->runcount += lock->waiting_readers.count;
+          glwthread_waitqueue_notify_all (&lock->waiting_readers);
+        }
+    }
+  LeaveCriticalSection (&lock->lock);
+  return 0;
+}
+
+int
+glwthread_rwlock_destroy (glwthread_rwlock_t *lock)
+{
+  if (!lock->guard.done)
+    return EINVAL;
+  if (lock->runcount != 0)
+    return EBUSY;
+  DeleteCriticalSection (&lock->lock);
+  if (lock->waiting_readers.array != NULL)
+    free (lock->waiting_readers.array);
+  if (lock->waiting_writers.array != NULL)
+    free (lock->waiting_writers.array);
+  lock->guard.done = 0;
+  return 0;
+}
diff --git a/lib/windows-rwlock.h b/lib/windows-rwlock.h
new file mode 100644
index 0000000000..ff4272a589
--- /dev/null
+++ b/lib/windows-rwlock.h
@@ -0,0 +1,68 @@
+/* Read-write locks (native Windows implementation).
+   Copyright (C) 2005-2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2005.
+   Based on GCC's gthr-win32.h.  */
+
+#ifndef _WINDOWS_RWLOCK_H
+#define _WINDOWS_RWLOCK_H
+
+#define WIN32_LEAN_AND_MEAN  /* avoid including junk */
+#include <windows.h>
+
+#include "windows-initguard.h"
+
+/* It is impossible to implement read-write locks using plain locks, without
+   introducing an extra thread dedicated to managing read-write locks.
+   Therefore here we need to use the low-level Event type.  */
+
+typedef struct
+        {
+          HANDLE *array; /* array of waiting threads, each represented by an event */
+          unsigned int count; /* number of waiting threads */
+          unsigned int alloc; /* length of allocated array */
+          unsigned int offset; /* index of first waiting thread in array */
+        }
+        glwthread_carray_waitqueue_t;
+typedef struct
+        {
+          glwthread_initguard_t guard; /* protects the initialization */
+          CRITICAL_SECTION lock; /* protects the remaining fields */
+          glwthread_carray_waitqueue_t waiting_readers; /* waiting readers */
+          glwthread_carray_waitqueue_t waiting_writers; /* waiting writers */
+          int runcount; /* number of readers running, or -1 when a writer runs */
+        }
+        glwthread_rwlock_t;
+
+#define GLWTHREAD_RWLOCK_INIT { GLWTHREAD_INITGUARD_INIT }
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern void glwthread_rwlock_init (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_rdlock (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_wrlock (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_tryrdlock (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_trywrlock (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_unlock (glwthread_rwlock_t *lock);
+extern int glwthread_rwlock_destroy (glwthread_rwlock_t *lock);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _WINDOWS_RWLOCK_H */
diff --git a/modules/lock b/modules/lock
index 0be823c76b..4b390ba765 100644
--- a/modules/lock
+++ b/modules/lock
@@ -11,6 +11,7 @@ Depends-on:
 extensions
 threadlib
 windows-mutex    [test $gl_threads_api = windows]
+windows-rwlock   [test $gl_threads_api = windows]
 windows-recmutex [test $gl_threads_api = windows]
 windows-once     [test $gl_threads_api = windows]
 
diff --git a/modules/windows-rwlock b/modules/windows-rwlock
new file mode 100644
index 0000000000..665d60245e
--- /dev/null
+++ b/modules/windows-rwlock
@@ -0,0 +1,28 @@
+Description:
+Read-write locks (native Windows implementation).
+
+Files:
+lib/windows-rwlock.h
+lib/windows-rwlock.c
+lib/windows-initguard.h
+
+Depends-on:
+
+configure.ac:
+AC_REQUIRE([AC_CANONICAL_HOST])
+case "$host_os" in
+  mingw*)
+    AC_LIBOBJ([windows-rwlock])
+    ;;
+esac
+
+Makefile.am:
+
+Include:
+"windows-rwlock.h"
+
+License:
+LGPLv2+
+
+Maintainer:
+all