automatically add a breakpoint that breaks on postgres errors (#7279)

When debugging postgres it is quite hard to get to the source for
`errfinish` in `elog.c`. Instead of relying on the developer to set a
breakpoint in the `elog.c` file for `errfinish` for `elevel == ERROR`,
this change adds the breakpoint to `.gdbinit`. This makes sure that
whenever a debugger is attached to a postgres backend it will break on
postgres errors.

When attaching the debugger a small banner is printed that explains how
to disable the breakpoint.

.devcontainer/.gdbinit

@@ -3,3 +3,31 @@
 # actually also works when debugging with vscode. Providing nice tools
 # to understand the internal datastructures we are working with.
 source /root/gdbpg.py
+
+# when debugging postgres it is convenient to _always_ have a breakpoint
+# trigger when an error is logged. Because .gdbinit is sourced before gdb
+# is fully attached and has the sources loaded. To make sure the breakpoint
+# is added when the library is loaded we temporary set the breakpoint pending
+# to on. After we have added out breakpoint we revert back to the default
+# configuration for breakpoint pending.
+# The breakpoint is hard to read, but at entry of the function we don't have
+# the level loaded in elevel. Instead we hardcode the location where the
+# level of the current error is stored. Also gdb doesn't understand the
+# ERROR symbol so we hardcode this to the value of ERROR. It is very unlikely
+# this value will ever change in postgres, but if it does we might need to
+# find a way to conditionally load the correct breakpoint.
+set breakpoint pending on
+break elog.c:errfinish if errordata[errordata_stack_depth].elevel == 21
+set breakpoint pending auto
+
+echo \n
+echo ----------------------------------------------------------------------------------\n
+echo when attaching to a postgres backend a breakpoint will be set on elog.c:errfinish \n
+echo it will only break on errors being raised in postgres \n
+echo \n
+echo to disable this breakpoint from vscode run `-exec disable 1` in the debug console \n
+echo this assumes it's the first breakpoint loaded as it is loaded from .gdbinit \n
+echo this can be verified with `-exec info break`, enabling can be done with \n
+echo `-exec enable 1` \n
+echo ----------------------------------------------------------------------------------\n
+echo \n