Improve the documentation for generating core dumps

Author: jschmidt-icinga

doc/21-development.md

@@ -267,73 +267,85 @@ $3 = std::vector of length 11, capacity 16 = {{static NPos = 1844674407370955161
 
 ### Core Dump <a id="development-debug-core-dump"></a>
 
-When the Icinga 2 daemon crashes with a `SIGSEGV` signal
-a core dump file should be written. This will help
-developers to analyze and fix the problem.
+When the Icinga 2 daemon crashes with a `SIGSEGV` signal a core dump file should be written. This will help developers to analyze and fix the problem.
 
-#### Core Dump File Size Limit <a id="development-debug-core-dump-limit"></a>
+#### Core Dump Kernel Pattern <a id="development-debug-core-dump-format"></a>
 
-This requires setting the core dump file size to `unlimited`.
+Core dumps are generated according to the format specified in `/proc/sys/kernel/core_pattern`.
+This can either be a path relative to the directory the program was started in, an absolute path or a pipe to a different program.
 
+For more information see the [core(5)](https://man7.org/linux/man-pages/man5/core.5.html) man page.
 
-##### Systemd
+#### Systemd Coredumpctl <a id="development-debug-core-dump-systemd"></a>
 
+Most distributions offer systemd's coredumpctl either by default or as a package. Distributions that offer it by default include RHEL and SLES, on others like Debian or Ubuntu it can be installed via the `systemd-coredump` package. When set up correctly, `core_pattern` will look something like this:
+```
+# cat /proc/sys/kernel/core_pattern
+|/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h`
 ```
-systemctl edit icinga2.service
 
-[Service]
-...
-LimitCORE=infinity
+You can look at the generated core dumps with the `coredumpctl list` command. You can show information, including a stack-trace using `coredumpctl show icinga2 -1` and retrieve the actual core-dump file with `coredumpctl dump icinga2 -1 --output <file>`.
 
-systemctl daemon-reload
+#### Ubuntu Apport <a id="development-debug-core-dump-apport"></a>
 
-systemctl restart icinga2
+Ubuntu uses their own application `apport` to record core dumps. When it is enabled your `core_pattern` will look like this:
+```
+# cat /proc/sys/kernel/core_pattern
+|/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E
 ```
 
-##### Init Script
+Apport is unsuitable for development work, because by default it only works with Ubuntu packages and it has a rather complicated interface for retrieving the core dump.
 
+To set it up to work with unpackaged programs, add the following (create the file if it doesn't exist) to `/etc/apport/settings`:
 ```
-vim /etc/init.d/icinga2
-...
-ulimit -c unlimited
-
-service icinga2 restart
+[main]
+unpackaged=true
 ```
-
-##### Verify
-
-Verify that the Icinga 2 process core file size limit is set to `unlimited`.
-
+and restart Apport:
 ```
-for pid in $(pidof icinga2); do cat /proc/$pid/limits; done
-
-...
-Max core file size        unlimited            unlimited            bytes
+systemctl restart apport.service
 ```
 
+When the program crashes you can then find an Apport crash report in `/var/crash/` that you can read with the interactive `apport-cli` command. To extract the core dump you run `apport-unpack /var/crash/<crash-file> <output-dir>` which then saves a `<outputdir>/CoreDump` file that contains the actual core dump.
+
+Unless you rely on Apport for some other workflow, systemd's coredumpctl is a much better option and is available on Ubuntu in the `systemd-coredump` package that can replace Apport on your system with no further setup required.
 
-#### Core Dump Kernel Format <a id="development-debug-core-dump-format"></a>
+#### Directly to a File <a id="development-debug-core-dump-direct"></a>
 
-The Icinga 2 daemon runs with the SUID bit set. Therefore you need
-to explicitly enable core dumps for SUID on Linux.
+If coredumpctl is not available, simply writing the core dump directly to a file is also sufficient. You can set up your `core_pattern` to write a file to a suitable path:
 
 ```bash
-sysctl -w fs.suid_dumpable=2
+sysctl -w kernel.core_pattern=/var/lib/cores/core.%e.%p.%h.%t
+install -m 1777 -d /var/lib/cores
 ```
 
-Adjust the coredump kernel format and file location on Linux:
+This will create core dump files in `/tmp/cores` where `%e` is the truncated name of the program, `%p` is the programs PID, `%h` is the hostname, and `%t` a timestamp.
+
+Note that unlike the other methods this requires the core size limit to be set for the process. When starting Icinga 2 via systemd you can set it to unlimited by adding the following to `/etc/systemd/system/icinga2.service.d/limits.conf`:
+```
+[Service]
+LimitCORE=infinity
+```
 
+When using an init script or starting manually you need to run `ulimit -c unlimited` before starting the program:
 ```bash
-sysctl -w kernel.core_pattern=/var/lib/cores/core.%e.%p
+ulimit -c unlimited
+./icinga2 daemon
+```
 
-install -m 1777 -d /var/lib/cores
+To verify that the limit has been set to `unlimited` run the following:
+```bash
+for pid in $(pidof icinga2); do cat /proc/$pid/limits; done
+```
+And look for the line:
+```
+Max core file size        unlimited            unlimited            bytes
 ```
 
-MacOS:
+#### MacOS <a id="development-debug-core-dump-macos"></a>
 
 ```bash
 sysctl -w kern.corefile=/cores/core.%P
-
 chmod 777 /cores
 ```