Add small GDB plugin

Author: TheTumultuousUnicornOfDarkness

gdb/README.md

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+# linuxdeploy-plugin-gdb
+
+This plugin allows to debug the application within AppImage through GDB.  
+To allow GDB to find source files, `LINUXDEPLOY_PLUGIN_GDB_SRC` environment variable must be set: this plugin copies source files inside AppDir during AppImage build process.
+
+**:warning: This plugin is experimental and has not been tried with several applications. :warning:**
+
+## Usage
+
+```bash
+# get linuxdeploy and linuxdeploy-plugin-gdb
+> wget -c "https://github.com/linuxdeploy/linuxdeploy/releases/download/continuous/linuxdeploy-x86_64.AppImage"
+> wget -c "https://raw.githubusercontent.com/linuxdeploy/misc-plugins/master/gdb/linuxdeploy-plugin-gdb.sh"
+
+# first option: install your app into your AppDir via `make install` etc.
+# second option: bundle your app's main executables manually
+# see https://docs.appimage.org/packaging-guide/from-source/native-binaries.html for more information
+> [...]
+
+# call through linuxdeploy
+> export LINUXDEPLOY_PLUGIN_GDB_SRC="/some/path/to/app/source"
+> ./linuxdeploy-x86_64.AppImage --appdir AppDir --plugin gdb --output appimage --icon-file mypackage.png --desktop-file mypackage.desktop
+```
+
+## Start GDB
+
+By default, the AppImage works normally. To start GDB, set the `APPIMAGE_USE_GDB` environment variable, like this:
+```bash
+# start the AmmImage with GDB
+> APPIMAGE_USE_GDB=1 ./mypackage-x86_64.AppImage
+```

gdb/linuxdeploy-plugin-gdb.sh

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+#! /bin/bash
+
+# exit whenever a command called in this script fails
+set -e
+
+if [ "$DEBUG" != "" ]; then
+    set -x
+    verbose="--verbose"
+fi
+
+appdir=""
+
+show_usage() {
+    echo "Usage: bash $0 --appdir <AppDir>"
+    echo
+    echo "Bundles source files for debugging"
+    echo
+    echo "Required variables:"
+    echo "  LINUXDEPLOY_PLUGIN_GDB_SRC: path to source directory"
+    echo
+}
+
+while [ "$1" != "" ]; do
+    case "$1" in
+        --plugin-api-version)
+            echo "0"
+            exit 0
+            ;;
+        --appdir)
+            appdir="$2"
+            shift
+            shift
+            ;;
+        --help)
+            show_usage
+            exit 0
+            ;;
+        *)
+            echo "Invalid argument: $1"
+            echo
+            show_usage
+            exit 2
+    esac
+done
+
+if [[ "$appdir" == "" ]]; then
+    show_usage
+    exit 2
+fi
+
+if [ -z "$LINUXDEPLOY_PLUGIN_GDB_SRC" ]; then
+    echo "$0: LINUXDEPLOY_PLUGIN_GDB_SRC environment variable is not set"
+    show_usage
+    exit 2
+fi
+
+echo "Copying source files"
+cp --recursive $verbose "$LINUXDEPLOY_PLUGIN_GDB_SRC" "$appdir/usr/"
+
+echo "Installing new AppRun wrapper"
+mv $verbose "$appdir/AppRun.wrapped" "$appdir/AppRun.wrapped.real"
+cat > "$appdir/AppRun.wrapped" <<\EOF
+#! /bin/bash
+
+if [[ -n "$APPIMAGE_USE_GDB" ]]; then
+	gdb --cd "$APPDIR/usr/@SRCDIR@" --args "$APPDIR/AppRun.wrapped.real" "$@"
+else
+	exec "$APPDIR/AppRun.wrapped.real" "$@"
+fi
+EOF
+sed -i "s|@SRCDIR@|$(basename "$LINUXDEPLOY_PLUGIN_GDB_SRC")|" "$appdir/AppRun.wrapped"
+chmod $verbose 755 "$appdir/AppRun.wrapped"