Re: WAL format and API changes (9.5)

Looking at the backtrace, it seems that a page for gin is corrupted. The buffer capture patch does some sanity checks on the page format when performing masking and it is failing in one of them on a standby when kicking ginRedoUpdateMetapage:
    if (pd_lower > pd_upper || pd_special < pd_upper ||
        pd_lower < SizeOfPageHeaderData || pd_special > BLCKSZ)
    {
        elog(ERROR, "invalid page at %X/%08X\n",
                ((PageHeader) page)->pd_lsn.xlogid,
                ((PageHeader) page)->pd_lsn.xrecoff);
    }

    frame #4: 0x000000010437dec5 postgres`CheckForBufferLeaks + 165 at bufmgr.c:1778
    frame #5: 0x000000010437df1e postgres`AtProcExit_Buffers(code=1, arg=0) + 30 at bufmgr.c:1750
    frame #6: 0x000000010438fded postgres`shmem_exit(code=1) + 301 at ipc.c:263
    frame #7: 0x000000010438fc1c postgres`proc_exit_prepare(code=1) + 124 at ipc.c:187
    frame #8: 0x000000010438fb63 postgres`proc_exit(code=1) + 19 at ipc.c:102
    frame #9: 0x0000000104555b3c postgres`errfinish(dummy=0) + 1180 at elog.c:555
    frame #10: 0x00000001045590de postgres`elog_finish(elevel=20, fmt=0x0000000104633d4f) + 830 at elog.c:1362
    frame #11: 0x000000010437c1af postgres`mask_unused_space(page=0x00007fff5bc20a70) + 159 at bufcapt.c:78
    frame #12: 0x000000010437b53d postgres`mask_heap_page(page=0x00007fff5bc20a70) + 29 at bufcapt.c:95
    frame #13: 0x000000010437b1cd postgres`buffer_capture_write(newcontent=0x0000000104ab3980, blkno=0) + 205 at bufcapt.c:329
    frame #14: 0x000000010437bc7d postgres`buffer_capture_forget(buffer=82) + 349 at bufcapt.c:433
    frame #15: 0x00000001043801c9 postgres`LockBuffer(buffer=82, mode=0) + 233 at bufmgr.c:2773
    frame #16: 0x00000001043800c8 postgres`UnlockReleaseBuffer(buffer=82) + 24 at bufmgr.c:2554
    frame #17: 0x0000000103fefb03 postgres`ginRedoUpdateMetapage(lsn=335350144, record=0x00007fb1740382f0) + 1843 at ginxlog.c:580
    frame #18: 0x0000000103fede96 postgres`gin_redo(lsn=335350144, record=0x00007fb1740382f0) + 534 at ginxlog.c:724
    frame #19: 0x00000001040ad692 postgres`StartupXLOG + 8482 at xlog.c:6810
    frame #20: 0x0000000104330e0e postgres`StartupProcessMain + 430 at startup.c:224
    frame #21: 0x00000001040c64d9 postgres`AuxiliaryProcessMain(argc=2, argv=0x00007fff5bc231b0) + 1897 at bootstrap.c:416
    frame #22: 0x000000010432b1a8 postgres`StartChildProcess(type=StartupProcess) + 328 at postmaster.c:5090
    frame #23: 0x00000001043292b9 postgres`PostmasterMain(argc=3, argv=0x00007fb173c044e0) + 5401 at postmaster.c:1212
    frame #24: 0x000000010426f995 postgres`main(argc=3, argv=0x00007fb173c044e0) + 773 at main.c:219