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Merge branch 'docs-python-gdb-fixes' into 'main'

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Cleanup and add content to glib debugging using gdb scripts

See merge request GNOME/glib!3638
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Philip Withnall 2023-10-16 08:50:40 +00:00
commit 54c9d9c0f2
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@ -145,25 +145,131 @@ functions like `strftime()`.
## Debugging with GDB
GLib ships with a set of Python macros for the GDB debugger. These includes
pretty printers for lists, hashtables and GObject types. It also has a
backtrace filter that makes backtraces with signal emissions easier to read.
GLib ships with a set of Python macros for the GDB debugger. These
macros make it easier to debug applications written using GLib.
To use this you need a version of GDB that supports Python scripting;
anything from 7.0 should be fine. You then need to install GLib in the same
prefix as GDB so that the Python GDB autoloaded files get installed in the
right place for GDB to pick up.
To use this you need to install GLib in the same prefix as GDB so that
the Python GDB autoloaded files get installed in the right place for
GDB to pick up.
General pretty printing should just happen without having to do anything
special. To get the signal emission filtered backtrace you must use the
"new-backtrace" command instead of the standard one.
You can check if gdb has picked up the GLib gdb scripts correctly by
running the following command in your gdb session.
There is also a new command called gforeach that can be used to apply a
command on each item in a list. E.g. you can do
```
(gdb) info auto-load python-scripts
Loaded Script
Yes /usr/share/gdb/auto-load/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7800.0-gdb.py
Yes /usr/share/gdb/auto-load/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.7800.0-gdb.py
```
`gforeach i in some_list_variable: print *(GtkWidget *)l`
The version numbers and paths might differ based on the OS
distribution, but if you see the entries listed for libglib and
libgobject, then the gdb scripts should work.
Which would print the contents of each widget in a list of widgets.
GLib Python macros provide the following benefits while debugging
applications written using GLib.
1. Pretty printing GLib types
2. Iterating lists using `gforeach` command
3. Backtrace decorations
### 1. Pretty printing GLib types:
General pretty printing GLib types should work without having to do
anything special.
For example, printing a hash table with string keys will display the hash
contents in the following format.
```
(gdb) print my_string_hash_variable
$1 = 0x5555556d9660 = {
[0x7ffff76f6592 "GdkWaylandPopup"] = 0x555555768a50,
[0x7ffff76e67e8 "GtkStyleProvider"] = 0x5555557a19c0,
[0x7ffff7db7151 "GHttpProxy"] = 0x5555556f5bc0
}
```
Printing a hash table with pointer keys will display the hash contents
in the following format.
```
(gdb) print my_pointer_hash_variable
$2 = 0x5555556dcb50 = {
[0x555555b59ec0] = 0x555555b59770,
[0x555555c79f80] = 0x555555c86b00,
[0x555555c68f40] = 0x555555c69630
}
```
Printing a list will display the list contents in the following
format.
```
(gdb) p my_appname_list
$3 = 0x55c4582deef0 = {0x7f16ab85dc90, 0x7f16abb02700, 0x7f16a80066e0}
```
It's not possible for us to decode and print string entries even if
the list has string entries, as GLib stores list entries as
`gpointer`s internally. But, that can be achieved using the `gforeach`
command.
### 2. Iterating lists using "gforeach" command:
The `gforeach` command can be used to apply a command on each item in
a list (`GList`/`GSList`). We can also type cast the `gpointer`s in the
list to desired type.
For example, to print all strings in a string list named `my_appname_list`,
you can do the following.
```
(gdb) gforeach appname in my_appname_list: print (gchar*) $appname
$4 = (gchar *) 0x7f16ab85dc90 "Boxes"
$5 = (gchar *) 0x7f16abb02700 "GNOME Application Platform version 45"
$6 = (gchar *) 0x7f16a80066e0 "Firefox ESR"
```
You can also call functions on each entry in the list, as below:
```
(gdb) gforeach appname in my_appname_list: call (int) strlen($appname)
$7 = 5
$8 = 37
$9 = 11
```
### 3. Backtrace decorations:
Backtraces are decorated with GLib type names and signal names.
E.g. From the below backtrace, we can say that
`gs_updates_page_button_refresh_cb ()` callback function was called
when a button (via the `GtkButton` type decoration) was clicked (via
the `clicked` signal decoration)
```
#0 gs_updates_page_button_refresh_cb (widget=0x5602f986b670 [GtkButton], self=0x5602f93aeb80 [GsUpdatesPage]) at ../src/gs-updates-page.c:826
#1 0x00007f09da7651de in <emit signal 'clicked' on instance 0x5602f986b670 [GtkButton]> (instance=0x5602f986b670, signal_id=158, detail=0) at ../gobject/gsignal.c:3675
#2 0x00007f09da747bad in g_cclosure_marshal_VOID__VOIDv (closure=0x5602f986c6a0, return_value=0x0, instance=0x5602f986b670, ...)
at ../gobject/gmarshal.c:165
…
#20 0x00007f09da52c250 in g_application_run (application=0x5602f9068ae0 [GsApplication], argc=1, argv=0x7ffcfb3bb468) at ../gio/gapplication.c:2577
#21 0x00005602f8b53ccd in main (argc=1, argv=0x7ffcfb3bb468) at ../src/gs-main.c:49
```
Following is the same backtrace without any GLib decorations, which is
not very useful.
```
#0 gs_updates_page_button_refresh_cb (widget=0x5602f986b670, self=0x5602f93aeb80) at ../src/gs-updates-page.c:826
#1 0x00007f09da747bad in g_cclosure_marshal_VOID__VOIDv (closure=0x5602f986c6a0, return_value=0x0, instance=0x5602f986b670, ...)
at ../gobject/gmarshal.c:165
…
#20 0x00007f09da52c250 in g_application_run (application=0x5602f9068ae0, argc=1, argv=0x7ffcfb3bb468) at ../gio/gapplication.c:2577
#21 0x00005602f8b53ccd in main (argc=1, argv=0x7ffcfb3bb468) at ../src/gs-main.c:49
```
## SystemTap