Tolerate ENOSYS failure from sync_file_range().

One unintended consequence of commit 9ccdd7f6 was that Windows WSL
users started getting a panic whenever we tried to initiate data
flushing with sync_file_range(), because WSL does not implement that
system call.  Previously, they got a stream of periodic warnings,
which was also undesirable but at least ignorable.

Prevent the panic by handling ENOSYS specially and skipping the panic
promotion with data_sync_elevel().  Also suppress future attempts
after the first such failure so that the pre-existing problem of
noisy warnings is improved.

Back-patch to 9.6 (older branches were not affected in this way by
9ccdd7f6).

Author: Thomas Munro and James Sewell
Tested-by: James Sewell
Reported-by: Bruce Klein
Discussion: https://postgr.es/m/CA+mCpegfOUph2U4ZADtQT16dfbkjjYNJL1bSTWErsazaFjQW9A@mail.gmail.com

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 213de7698a4c6af0fba8d5c21c970aaeab247651..d46639d2edeeed9695a9a7efb39644bb503b0709 100644 (file)
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -420,6 +420,10 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 #if defined(HAVE_SYNC_FILE_RANGE)
    {
        int         rc;
+       static bool not_implemented_by_kernel = false;
+
+       if (not_implemented_by_kernel)
+           return;
 
        /*
         * sync_file_range(SYNC_FILE_RANGE_WRITE), currently linux specific,
@@ -434,7 +438,22 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
                             SYNC_FILE_RANGE_WRITE);
        if (rc != 0)
        {
-           ereport(data_sync_elevel(WARNING),
+           int         elevel;
+
+           /*
+            * For systems that don't have an implementation of
+            * sync_file_range() such as Windows WSL, generate only one
+            * warning and then suppress all further attempts by this process.
+            */
+           if (errno == ENOSYS)
+           {
+               elevel = WARNING;
+               not_implemented_by_kernel = true;
+           }
+           else
+               elevel = data_sync_elevel(WARNING);
+
+           ereport(elevel,
                    (errcode_for_file_access(),
                     errmsg("could not flush dirty data: %m")));
        }