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Dear XRootD developers,

In testing the OSG build of xrootd v4.7.1 on our StashCache server at Nebraska, we encountered a segfault in XrdXrootdMonFile::DoXFR(). @bbockelm suggested sending the backtrace upstream:

Core was generated by `/usr/bin/xrootd -l /var/log/xrootd/xrootd.log -c /etc/xrootd/xrootd-stashcache-'.
Program terminated with signal 11, Segmentation fault.
#0  XrdXrootdMonFile::DoXFR () at /usr/src/debug/xrootd-4.7.1/src/XrdXrootd/XrdXrootdMonFile.cc:271
271	                 {if (fsP->xfrXeq) DoXFR(fsP);
Missing separate debuginfos, ...
(gdb) bt
#0  XrdXrootdMonFile::DoXFR () at /usr/src/debug/xrootd-4.7.1/src/XrdXrootd/XrdXrootdMonFile.cc:271
#1  0x00007f09b61d03a5 in XrdXrootdMonFile::DoIt (this=0x1983fa0) at /usr/src/debug/xrootd-4.7.1/src/XrdXrootd/XrdXrootdMonFile.cc:230
#2  0x00007f09b5f5ecff in XrdScheduler::Run (this=0x610e98 <XrdMain::Config+440>) at /usr/src/debug/xrootd-4.7.1/src/Xrd/XrdScheduler.cc:357
#3  0x00007f09b5f5ee49 in XrdStartWorking (carg=<optimized out>) at /usr/src/debug/xrootd-4.7.1/src/Xrd/XrdScheduler.cc:87
#4  0x00007f09b5f1b4d7 in XrdSysThread_Xeq (myargs=0x7f0834513270) at /usr/src/debug/xrootd-4.7.1/src/XrdSys/XrdSysPthread.cc:86
#5  0x00007f09b5ad7e25 in start_thread (arg=0x7f0412ded700) at pthread_create.c:308
#6  0x00007f09b4ddd34d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113

And here's a bit more info on the loop it's running

(gdb) # fsP doesn't point to allocated memory
(gdb) print fsP
$1 = (XrdXrootdFileStats *) 0x7f036c700d98
(gdb) print *fsP
Cannot access memory at address 0x7f036c700d98

(gdb) # Loop variables
(gdb) print i
$2 = 124
(gdb) print n
$3 = 232

(gdb) # Bad fMap entry and adjacent good entries
(gdb) print XrdXrootdMonFile::fmMap[i].fMap[n-2]
$4 = {{cVal = 139672740028968, cPtr = 0x7f08180de228, vPtr = 0x7f08180de228}}
(gdb) print XrdXrootdMonFile::fmMap[i].fMap[n-1]
$5 = {{cVal = 139652680912280, cPtr = 0x7f036c700d98, vPtr = 0x7f036c700d98}}
(gdb) print XrdXrootdMonFile::fmMap[i].fMap[n]
$6 = {{cVal = 139675760573464, cPtr = 0x7f08cc17bc18, vPtr = 0x7f08cc17bc18}}

(gdb) # Bad fMap.vPtr entry and adjacent good entries
(gdb) print *(XrdXrootdMonFile::fmMap[i].fMap[n-2].vPtr)
$7 = {FileID = 1276444928, MonEnt = -1818, monLvl = 2 '\002', xfrXeq = 0 '\000', fSize = 180785925, xfr = {read = 12931845, readv = 0, write = 0}, ops = {read = 13, readv = 0, write = 0, rsMin = 32767, rsMax = 0, rsegs = 0, rdMin = 348933, rdMax = 1048576, 
    rvMin = 2147483647, rvMax = 0, wrMin = 2147483647, wrMax = 0}, ssq = {read = 0, readv = 0, rsegs = 0, write = 0}}
(gdb) print *(XrdXrootdMonFile::fmMap[i].fMap[n-1].vPtr)
Cannot access memory at address 0x7f036c700d98
(gdb) print *(XrdXrootdMonFile::fmMap[i].fMap[n].vPtr)
$8 = {FileID = 1309999360, MonEnt = -1816, monLvl = 2 '\002', xfrXeq = 0 '\000', fSize = 178914241, xfr = {read = 33570816, readv = 0, write = 0}, ops = {read = 33, readv = 0, write = 0, rsMin = 32767, rsMax = 0, rsegs = 0, rdMin = 16384, rdMax = 1048576, 
    rvMin = 2147483647, rvMax = 0, wrMin = 2147483647, wrMax = 0}, ssq = {read = 0, readv = 0, rsegs = 0, write = 0}}

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