mdbx: provide SRWL stub for Windows XP/2000.

Change-Id: I438373a4952298efc88ca4c1a92c88e173b82f98

src/lck-windows.c

@@ -156,7 +156,7 @@ void mdbx_txn_unlock(MDBX_env *env) {
 #define LCK_UPPER LCK_UP_OFFSET, LCK_UP_LEN
 
 int mdbx_rdt_lock(MDBX_env *env) {
-  AcquireSRWLockShared(&env->me_remap_guard);
+  mdbx_srwlock_AcquireShared(&env->me_remap_guard);
   if (env->me_lfd == INVALID_HANDLE_VALUE)
     return MDBX_SUCCESS; /* readonly database in readonly filesystem */
 
@@ -165,7 +165,7 @@ int mdbx_rdt_lock(MDBX_env *env) {
     return MDBX_SUCCESS;
 
   int rc = GetLastError();
-  ReleaseSRWLockShared(&env->me_remap_guard);
+  mdbx_srwlock_ReleaseShared(&env->me_remap_guard);
   return rc;
 }
 
@@ -175,7 +175,7 @@ void mdbx_rdt_unlock(MDBX_env *env) {
     if (!funlock(env->me_lfd, LCK_UPPER))
       mdbx_panic("%s failed: errcode %u", mdbx_func_, GetLastError());
   }
-  ReleaseSRWLockShared(&env->me_remap_guard);
+  mdbx_srwlock_ReleaseShared(&env->me_remap_guard);
 }
 
 static int suspend_and_append(mdbx_handle_array_t **array,
@@ -573,3 +573,103 @@ int mdbx_rpid_check(MDBX_env *env, mdbx_pid_t pid) {
     return rc;
   }
 }
+
+//----------------------------------------------------------------------------
+// Stub for slim read-write lock
+// Copyright (C) 1995-2002 Brad Wilson
+
+static void WINAPI stub_srwlock_Init(MDBX_srwlock *srwl) {
+  srwl->readerCount = srwl->writerCount = 0;
+}
+
+static void WINAPI stub_srwlock_AcquireShared(MDBX_srwlock *srwl) {
+  while (true) {
+    assert(srwl->writerCount >= 0 && srwl->readerCount >= 0);
+
+    //  If there's a writer already, spin without unnecessarily
+    //  interlocking the CPUs
+    if (srwl->writerCount != 0) {
+      YieldProcessor();
+      continue;
+    }
+
+    //  Add to the readers list
+    _InterlockedIncrement(&srwl->readerCount);
+
+    // Check for writers again (we may have been pre-empted). If
+    // there are no writers writing or waiting, then we're done.
+    if (srwl->writerCount == 0)
+      break;
+
+    // Remove from the readers list, spin, try again
+    _InterlockedDecrement(&srwl->readerCount);
+    YieldProcessor();
+  }
+}
+
+static void WINAPI stub_srwlock_ReleaseShared(MDBX_srwlock *srwl) {
+  assert(srwl->readerCount > 0);
+  _InterlockedDecrement(&srwl->readerCount);
+}
+
+static void WINAPI stub_srwlock_AcquireExclusive(MDBX_srwlock *srwl) {
+  while (true) {
+    assert(srwl->writerCount >= 0 && srwl->readerCount >= 0);
+
+    //  If there's a writer already, spin without unnecessarily
+    //  interlocking the CPUs
+    if (srwl->writerCount != 0) {
+      YieldProcessor();
+      continue;
+    }
+
+    // See if we can become the writer (expensive, because it inter-
+    // locks the CPUs, so writing should be an infrequent process)
+    if (_InterlockedExchange(&srwl->writerCount, 1) == 0)
+      break;
+  }
+
+  // Now we're the writer, but there may be outstanding readers.
+  // Spin until there aren't any more; new readers will wait now
+  // that we're the writer.
+  while (srwl->readerCount != 0) {
+    assert(srwl->writerCount >= 0 && srwl->readerCount >= 0);
+    YieldProcessor();
+  }
+}
+
+static void WINAPI stub_srwlock_ReleaseExclusive(MDBX_srwlock *srwl) {
+  assert(srwl->writerCount == 1 && srwl->readerCount >= 0);
+  srwl->writerCount = 0;
+}
+
+static void WINAPI srwlock_thunk_init(MDBX_srwlock *srwl) {
+  HINSTANCE hInst = GetModuleHandleA("kernel32.dll");
+  MDBX_srwlock_function init =
+      (MDBX_srwlock_function)GetProcAddress(hInst, "InitializeSRWLock");
+  if (init != NULL) {
+    mdbx_srwlock_AcquireShared =
+        (MDBX_srwlock_function)GetProcAddress(hInst, "AcquireSRWLockShared");
+    mdbx_srwlock_ReleaseShared =
+        (MDBX_srwlock_function)GetProcAddress(hInst, "ReleaseSRWLockShared");
+    mdbx_srwlock_AcquireExclusive =
+        (MDBX_srwlock_function)GetProcAddress(hInst, "AcquireSRWLockExclusive");
+    mdbx_srwlock_ReleaseExclusive =
+        (MDBX_srwlock_function)GetProcAddress(hInst, "ReleaseSRWLockExclusive");
+  } else {
+    init = stub_srwlock_Init;
+    mdbx_srwlock_AcquireShared = stub_srwlock_AcquireShared;
+    mdbx_srwlock_ReleaseShared = stub_srwlock_ReleaseShared;
+    mdbx_srwlock_AcquireExclusive = stub_srwlock_AcquireExclusive;
+    mdbx_srwlock_ReleaseExclusive = stub_srwlock_ReleaseExclusive;
+  }
+  mdbx_compiler_barrier();
+  mdbx_srwlock_Init = init;
+  mdbx_srwlock_Init(srwl);
+}
+
+MDBX_srwlock_function mdbx_srwlock_Init = srwlock_thunk_init;
+MDBX_srwlock_function mdbx_srwlock_AcquireShared;
+MDBX_srwlock_function mdbx_srwlock_ReleaseShared;
+MDBX_srwlock_function mdbx_srwlock_AcquireExclusive;
+MDBX_srwlock_function mdbx_srwlock_ReleaseExclusive;