259 lines
9.2 KiB
Markdown
259 lines
9.2 KiB
Markdown
---
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author: Sanchayan Maity
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title: Automate debugging using GDB scripting
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tags: linux, gdb, gdb scripting, gstreamer
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---
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For a while, have had the pleasure of working on a GStreamer
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[plugin](https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/572)
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in Rust at work. The plugin basically rounds the corners of an incoming video,
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something akin to the `border-radius` property in CSS. Below is how it looks
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like when running on a video.
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The GStreamer pipeline for the same.
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```bash
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gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! roundedcorners border-radius-px=100 ! videoconvert ! gtksink
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```
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This was the first time working on a video plugin in GStreamer. Had a lot to
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learn on how to use the `BaseTransform` class from GStreamer, among other
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things. Without getting into the GStreamer specific details here, basically ran
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into a problem for which needed some debugging for figuring out what was going
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on in the internals of GStreamer.
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Now, while using GDB from the command line has never been a problem, but, the
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straight forward regular approach is time-consuming. Start the pipeline, then
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attach gdb to a running process, place breakpoints by manually typing out the
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whole thing and then start. For one off debugging sessions, where perhaps you
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just want to inspect the backtrace from a crash or may be look into a
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deadlock condition where your code hung, this could be fine. However, when you
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have to repeat this multiple times do a source code change compile and then
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select again it becomes frustrating.
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## GDB Dashboard
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Looking for a better way, [gdb-dashboard](https://github.com/cyrus-and/gdb-dashboard)
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is what first came up as an option. This is quite useful since it can give the needed
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information without having to type anything. Using gdb
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[hooks](https://git.sr.ht/~sanchayanmaity/dotfiles/tree/master/item/gdb/.gdbinit.d/hooks),
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the dashboard can be triggered when appropriate. See the rest of the [gdb
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configuration](https://git.sr.ht/~sanchayanmaity/dotfiles/tree/master/item/gdb/.gdbinit.d)
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to get an idea. This is useful in scenarios like where code is stuck due to a
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deadlock and one needs to look at the backtrace of a crash or any such one off
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simple investigation.
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## Construct breakpoint command in neovim & copy to clipboard
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The next small improvement is more specific to neovim. Navigating source code
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with neovim opened in one kitty tab and gdb running in terminal in next tab or
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a split is a preferred workflow personally. Being able to place a breakpoint
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without having to type anything out on the gdb prompt would be convenient. The
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vimscript code below generates the gdb command, considering the current line
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and file on which the cursor is at in the source when opened in neovim.
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```vimscript
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function! CopyBpLocToClipboard() abort
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let linenumber = line(".")
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let filepath = expand("%")
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let breakpoint = "break " . filepath . ":" . linenumber
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silent execute "!wl-copy " . breakpoint
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endfunction
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nnoremap <silent> <Leader>yb :<C-U>call CopyBpLocToClipboard()<CR>
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```
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By using the preceding key binding, a command like below gets copied to the
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clipboard which can be just pasted on gdb prompt.
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```bash
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break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:104
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```
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Nifty!!!
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## GDB scripting
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Now imagine a scenario where perhaps one wants to look at multiple places in the
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source code and when the program is running, inspect certain variables or just
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print out a back trace each time a specific code point is reached.
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The manual way to do this is to load the executable in gdb or attach to a
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running process, place a break point, run, inspect the local variables or print
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stack trace, place the next break point and repeat this whole process. Just
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time-consuming.
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GDB can completely automate the preceding process like below.
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Below is the `.gdbinit` file applicable for the problem facing encountered at
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work. This is what's called a command file by gdb.
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```bash
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set confirm off
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set breakpoint pending on
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set logging on
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set logging overwrite on
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set print pretty on
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set pagination off
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break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:104 if meta->n_planes == 4
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:228
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break subprojects/gstreamer/gst/gstbuffer.c:1410
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:231
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:237
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break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:136
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commands 1
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print i
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print *frame
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enable 2
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enable 3
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enable 4
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enable 5
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enable 6
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continue
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end
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commands 2
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print offset
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continue
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end
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commands 3
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print offset
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print size
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continue
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end
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commands 4
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print *(GstBufferImpl *)buffer
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print idx
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print length
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print skip
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continue
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end
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commands 5
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disable 2
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disable 3
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disable 4
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print *(GstBufferImpl *)buffer
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print info->data
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print skip
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print *data
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continue
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end
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commands 6
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print *frame
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quit
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end
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disable 2
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disable 3
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disable 4
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disable 5
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disable 6
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run
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```
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Below is the command to debug the GStreamer plugin in this pipeline with gdb.
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```bash
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gdb --nx -x .gdbinit --args env RUST_BACKTRACE=1 GST_DEBUG=3,basetransform:6 GST_PLUGIN_PATH=$GST_PLUGIN_PATH:~/GitSources/gst-plugins-rs/target/debug gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! video/x-raw,format=I420 ! roundedcorners border-radius-px=100 ! video/x-raw,format=A420 ! videoconvert ! gtksink
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```
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In the preceding command, the `-x` parameter tells gdb to use the command file.
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The `--nx` flag tells gdb to not read any `.gdbinit` files in any directory, as
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`gdb-dashboard` isn't intended to be used for this. `--args` is how one tells
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gdb what to run, which is the GStreamer pipeline in this case. See `gdb --help`
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for details on the flags.
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Now, consider what the command file does. The ones below are just some
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settings for gdb to use. Note that logging and pretty printing are enabled.
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```bash
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set confirm off
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set breakpoint pending on
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set logging on
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set logging overwrite on
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set print pretty on
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set pagination off
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```
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Next, specify the breakpoints. There are six breakpoints. These are the source
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code locations of interest.
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```bash
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break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:104 if meta->n_planes == 4
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:228
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break subprojects/gstreamer/gst/gstbuffer.c:1410
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:231
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break subprojects/gst-plugins-base/gst-libs/gst/video/gstvideometa.c:237
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break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:136
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```
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Breakpoints can be enabled conditionally. The `if meta->n_planes == 4` implies
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to consider this breakpoint only when a video frame with 4 planes is received.
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Next gdb has to be told what should be done when each of the preceding
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breakpoints is hit.
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```bash
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commands 1
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print i
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print *frame
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enable 2
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enable 3
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enable 4
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enable 5
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enable 6
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continue
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end
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```
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`commands 1` implies these are the commands for gdb to execute when breakpoint
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1 is hit. When breakpoint 1 is hit, the value of `i` and `frame` gets printed.
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The other breakpoints get enabled only after the first one is hit. This is
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because at the end of command file, the following commands
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```bash
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disable 2
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disable 3
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disable 4
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disable 5
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disable 6
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```
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instruct gdb to start with these breakpoints off. These get enabled only
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when breakpoint 1 is hit. The `continue` just tells gdb to continue, as gdb
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shouldn't stop on hitting a breakpoint and logs can be inspected in the
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end using gdb log.
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Other breakpoints are specified similarly.
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The `run` at the end tells gdb to start executing immediately. In normal usage
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one would have to explicitly type `run` on the gdb prompt to make gdb start
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debugging.
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If it's not clear so far, basically whatever gdb commands would have been used
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for debugging at the gdb prompt, is what gets specified in the command file as
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well.
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After running the below on the terminal
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```bash
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gdb --nx -x .gdbinit --args env RUST_BACKTRACE=1 GST_DEBUG=3,basetransform:6 GST_PLUGIN_PATH=$GST_PLUGIN_PATH:~/GitSources/gst-plugins-rs/target/debug gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! video/x-raw,format=I420 ! roundedcorners border-radius-px=100 ! video/x-raw,format=A420 ! videoconvert ! gtksink
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```
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The pipeline gets executed by gdb, considering the command file it was passed
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and log whatever it was asked to log when each breakpoint is encountered. Since
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logging and pretty printing were enabled earlier, gdb logs everything in
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default `gdb.txt` file. The exact log text file can be seen
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[here](https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/572#note_1107146),
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with `gdbinit` and the other two log files attached.
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Now, one can comfortably look at this log and see what's going on. Once the
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command file is written, the whole debugging process is completely automated.
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Run, sit back and then look at the logs.
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Using gdb is now a breeze and hassle-free experience. Being able to automate
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and log the debugging process like this, also means you could share your
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command file and someone else can replicate this.
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