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Steps Towards Debugging

KeyDB is developed with a great stress on stability: we do our best with every release to make sure you'll experience a very stable product and no crashes. However even with our best efforts it is impossible to avoid all the critical bugs with 100% success.

When KeyDB crashes it produces a detailed report of what happened, however sometimes looking at the crash report is not enough, nor is it possible for the KeyDB core team to reproduce the issue independently: in this scenario we need help from the user that is able to reproduce the issue.

This little guide shows how to use GDB to provide all the information the KeyDB developers will need to track the bug more easily.

What is GDB?#

GDB is the Gnu Debugger: a program that is able to inspect the internal state of another program. Usually tracking and fixing a bug is an exercise in gathering more information about the state of the program at the moment the bug happens, so GDB is an extremely useful tool.

GDB can be used in two ways:

  • It can attach to a running program and inspect the state of it at runtime.
  • It can inspect the state of a program that already terminated using what is called a core file, that is, the image of the memory at the time the program was running.

From the point of view of investigating KeyDB bugs we need to use both of these GDB modes: the user able to reproduce the bug attaches GDB to their running KeyDB instance, and when the crash happens, they create the core file that in turn the developer will use to inspect the KeyDB internals at the time of the crash.

This way the developer can perform all the inspections in his or her computer without the help of the user, and the user is free to restart KeyDB in the production environment.

Compiling KeyDB without optimizations#

By default KeyDB is compiled with the -O2 switch, this means that compiler optimizations are enabled. This makes the KeyDB executable faster, but at the same time it makes KeyDB (like any other program) harder to inspect using GDB.

It is better to attach GDB to KeyDB compiled without optimizations using the make noopt command to compile it (instead of just using the plain make command). However if you have an already running KeyDB in production there is no need to recompile and restart it if this is going to create problems on your side. Even if by a lesser extent GDB still works against executables compiled with optimizations.

It is great if you make sure to recompile KeyDB with make noopt after the first crash, so that the next time it will be simpler to track the issue.

You should not be concerned with the loss of performance compiling KeyDB without optimizations, it is very unlikely that this will cause problems in your environment since it is usually just a matter of a small percentage because KeyDB is not very CPU-bound (it does a lot of I/O to serve queries).

Attaching GDB to a running process#

If you have an already running KeyDB server, you can attach GDB to it, so that if KeyDB crashes it will be possible to both inspect the internals and generate a core dump file.

After you attach GDB to the KeyDB process it will continue running as usual without any loss of performance, so this is not a dangerous procedure.

In order to attach GDB the first thing you need is the process ID of the running KeyDB instance (the pid of the process). You can easily obtain it using keydb-cli:

$ keydb-cli info | grep process_id

In the above example the process ID is 58414.

  • Login into your KeyDB server.

  • (Optional but recommended) Start screen or tmux or any other program that will make sure that your GDB session will not be closed if your ssh connection will timeout. If you don't know what screen is do yourself a favor and Read this article

  • Attach GDB to the running KeyDB server typing:

    gdb <path-to-keydb-executable> <pid>

    For example: gdb /usr/local/bin/keydb-server 58414

GDB will start and will attach to the running server printing something like the following:

Reading symbols for shared libraries + done
0x00007fff8d4797e6 in epoll_wait ()
  • At this point GDB is attached but your KeyDB instance is blocked by GDB. In order to let the KeyDB instance continue the execution just type continue at the GDB prompt, and press enter.

    (gdb) continue
  • Done! Now your KeyDB instance has GDB attached. You can wait for... the next crash :)

  • Now it's time to detach your screen / tmux session, if you are running GDB using it, pressing the usual Ctrl-a a key combination.

After the crash#

KeyDB has a command to simulate a segmentation fault (in other words a bad crash) using the DEBUG SEGFAULT command (don't use it against a real production instance of course ;). So I'll use this command to crash my instance to show what happens in the GDB side:

(gdb) continue
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0xffffffffffffffff
debugCommand (c=0x7ffc32005000) at debug.c:220
220 *((char*)-1) = 'x';

As you can see GDB detected that KeyDB crashed, and was able to show me even the file name and line number causing the crash. This is already much better than the KeyDB crash report back trace (containing just function names and binary offsets).

Obtaining the stack trace#

The first thing to do is to obtain a full stack trace with GDB. This is as simple as using the bt command: (that is a short for backtrace):

(gdb) bt
#0 debugCommand (c=0x7ffc32005000) at debug.c:220
#1 0x000000010d246d63 in call (c=0x7ffc32005000) at KeyDB.c:1163
#2 0x000000010d247290 in processCommand (c=0x7ffc32005000) at KeyDB.c:1305
#3 0x000000010d251660 in processInputBuffer (c=0x7ffc32005000) at networking.c:959
#4 0x000000010d251872 in readQueryFromClient (el=0x0, fd=5, privdata=0x7fff76f1c0b0, mask=220924512) at networking.c:1021
#5 0x000000010d243523 in aeProcessEvents (eventLoop=0x7fff6ce408d0, flags=220829559) at ae.c:352
#6 0x000000010d24373b in aeMain (eventLoop=0x10d429ef0) at ae.c:397
#7 0x000000010d2494ff in main (argc=1, argv=0x10d2b2900) at KeyDB.c:2046

This shows the backtrace, but we also want to dump the processor registers using the info registers command:

(gdb) info registers
rax 0x0 0
rbx 0x7ffc32005000 140721147367424
rcx 0x10d2b0a60 4515891808
rdx 0x7fff76f1c0b0 140735188943024
rsi 0x10d299777 4515796855
rdi 0x0 0
rbp 0x7fff6ce40730 0x7fff6ce40730
rsp 0x7fff6ce40650 0x7fff6ce40650
r8 0x4f26b3f7 1327936503
r9 0x7fff6ce40718 140735020271384
r10 0x81 129
r11 0x10d430398 4517462936
r12 0x4b7c04f8babc0 1327936503000000
r13 0x10d3350a0 4516434080
r14 0x10d42d9f0 4517452272
r15 0x10d430398 4517462936
rip 0x10d26cfd4 0x10d26cfd4 <debugCommand+68>
eflags 0x10246 66118
cs 0x2b 43
ss 0x0 0
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0

Please make sure to include both this outputs in your bug report.

Obtaining the core file#

The next step is to generate the core dump, that is the image of the memory of the running KeyDB process. This is performed using the gcore command:

(gdb) gcore
Saved corefile core.58414

Now you have the core dump to send to the KeyDB developer, but it is important to understand that this happens to contain all the data that was inside the KeyDB instance at the time of the crash; KeyDB developers will make sure not to share the content with anyone else, and will delete the file as soon as it is no longer used for debugging purposes, but you are warned that by sending the core file you are sending your data.

What to send to developers#

Finally you can send everything to the KeyDB core team:

  • The KeyDB executable you are using.
  • The stack trace produced by the bt command, and the registers dump.
  • The core file you generated with gdb.
  • Information about the operating system and GCC version, and KeyDB version you are using.

Thank you#