lok: add SAL_DISABLE_FSYNC environment to disable fsync.

For state-less containers there is no benefit in fsync'ing,
file data is safe when it is up-loaded back to storage - but
profiling shows latency from stray fsyncs.

Change-Id: I9f03d5866dec05e5507deb56b0dca93b6876225e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167490
Tested-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
(cherry picked from commit 90bd45d7a9)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/172947
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins

external/nss/UnpackedTarball_nss.mk

@@ -27,6 +27,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,nss,\
     external/nss/nss-restore-manual-pre-dependencies.patch.1 \
     external/nss/Wincompatible-function-pointer-types.patch.0 \
     $(if $(filter LINUX,$(OS)), \
+        external/nss/nss.disablefsync.patch \
         external/nss/nss.getrandom.patch) \
     $(if $(filter iOS,$(OS)), \
         external/nss/nss-ios.patch) \

external/nss/nss.disablefsync.patch

@@ -0,0 +1,18 @@
+--- a/nss/nss/lib/sqlite/sqlite3.c
++++ b/nss/nss/lib/sqlite/sqlite3.c
+@@ -36136,6 +36136,15 @@
+ static int full_fsync(int fd, int fullSync, int dataOnly){
+   int rc;
+ 
++  static int disabledKnown = 0, disabled = 0;
++  if (!disabledKnown)
++  {
++    disabled = getenv("SAL_DISABLE_FSYNC") != NULL;
++    disabledKnown = 1;
++  }
++  if (disabled)
++    return 0;
++
+   /* The following "ifdef/elif/else/" block has the same structure as
+   ** the one below. It is replicated here solely to avoid cluttering 
+   ** up the real code with the UNUSED_PARAMETER() macros.

sal/osl/unx/file.cxx

@@ -1236,7 +1236,11 @@ oslFileError SAL_CALL osl_syncFile(oslFileHandle Handle)
     if (result != osl_File_E_None)
         return result;
 
-    if (fsync(pImpl->m_fd) == -1)
+    static bool disabled = getenv("SAL_DISABLE_FSYNC") != nullptr;
+
+    if (disabled)
+        SAL_INFO("sal.file", "fsync(" << pImpl->m_fd << "): Disabled");
+    else if (fsync(pImpl->m_fd) == -1)
     {
         int e = errno;
         SAL_INFO("sal.file", "fsync(" << pImpl->m_fd << "): " << UnixErrnoString(e));