Re: kern/56713 - tail -f doesn't follow (fwd)

[Paul Goyette <paul%whooppee.com@localhost>]

The following reply was made to PR kern/56713; it has been noted by GNATS.

From: Paul Goyette <paul%whooppee.com@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/56713 - tail -f doesn't follow (fwd)
Date: Sun, 6 Mar 2022 09:46:46 -0800 (PST)

 Logging to PR
 
 ---------- Forwarded message ----------
 Date: Sun, 6 Mar 2022 09:44:19 -0800 (PST)
 From: Paul Goyette <paul%whooppee.com@localhost>
 To: thorpej%netbsd.org@localhost
 Cc: martin%netbsd.org@localhost
 Subject: Re: kern/56713 - tail -f doesn't follow
 
 I still cannot get my test program to fail, but I have looked a bit
 closer into what's going on.
 
 I modified tail's forward.c to do a few printf()s before calls to
 kevent().  I'm using a pkg_build for gnucash to generate the target
 log file, but anything can substitute.
 
 xtail on log file provides the last 10 lines of the file.  The following (11th) 
 line would start with "===> Skipping vulnerability checks."
 
 ...
 => Full dependency swig3>=3.0.12: found swig3-3.0.12nb4
 => Full dependency hicolor-icon-theme>=0.9nb1: found hicolor-icon-theme-0.17nb1
 => Full dependency gdk-pixbuf2>=2.42.6: found gdk-pixbuf2-2.42.6nb1
 => Full dependency guile22>=2.2.7nb3: found guile22-2.2.7nb4
 => Full dependency desktop-file-utils>=0.10nb1: found 
 desktop-file-utils-0.26nb1
 => Full dependency icu>=70.1: found icu-70.1
 => Full dependency libxml2>=2.9.12nb1: found libxml2-2.9.12nb2
 => Full dependency libxslt>=1.1.34nb6: found libxslt-1.1.34nb7
 => Full dependency webkit-gtk>=2.34.2: found webkit-gtk-2.34.6
 => Full dependency gtk3+>=3.24.30nb1: found gtk3+-3.24.31
 
 At this point, xtail's ktrace shows one of my test printf's,
 followed by the set-watchlist kevent call, followed by an attempt
 to read any new data that might exist (there is none), followed
 by the kevent() call to wait-for-event.  The latter shows that
 it never returns, until I press ^C
 
 ...
   20519  20519 xtail    1646584937.737532152 CALL  write(1,0x764d4e699000,0x23)
   20519  20519 xtail    1646584937.737534380 GIO   fd 1 wrote 35 bytes
         "forward: ADD_EVENTS: kevent( n=1 )\n"
   20519  20519 xtail    1646584937.737534576 RET   write 35/0x23
   20519  20519 xtail    1646584937.737535715 CALL 
 __kevent50(4,0x7f7fff1f5ab0,1,0,0,0)
   20519  20519 xtail    1646584937.737537464 RET   __kevent50 0
   20519  20519 xtail    1646584937.737537905 CALL  read(3,0x764d4e69a040,0x8000)
   20519  20519 xtail    1646584937.737538452 GIO   fd 3 read 0 bytes
         ""
   20519  20519 xtail    1646584937.737538685 RET   read 0
   20519  20519 xtail    1646584937.737539221 CALL 
 __kevent50(4,0,0,0x7f7fff1f5ab0,1,0)
   20519  20519 xtail    1646585057.253611154 RET   __kevent50 -1 errno 4 
 Interrupted system call
 
 Note that the call to kevent() to establish the event watch-list is at
 timestamp 37.737535715, and the never-ending wait starts at timestamp
 37.737539221
 
 From the ktrace of the process writing to the log file we see the
 point where it writes the next line:
 
   29343  29343 sh       1646584941.086235727 CALL  write(1,0x74bbbc55b000,0x24)
   29343  29343 sh       1646584941.086240495 GIO   fd 1 wrote 36 bytes
         "===> Skipping vulnerability checks.\n"
   29343  29343 sh       1646584941.086240792 RET   write 36/0x24
 
 The write to the log file for the "===> Skipping ..." text occurred
 at 41.086240495 which is after the kevent() starts waiting, so it
 should have triggered the kevent.
 
 I tried to update my test program to add the read-new-data but it
 still doesn't fail.  :(
 
 I'm quickly reaching my limit of understanding here...
 
 
 
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