Allowing to apply patches with build.sh

Currently the way the build script works, especially when compiling with Playwright support is that it tries to ensure that the Git worktree is clean, i.e., has no uncommitted changes. This aims to prevent that by accident one ends up in a situation where substantial changes have been made and the build script happily applies the Playwright patches. Reverting them requires a bit of understanding of the underlying build, so we try to safeguard against this.

However, this is at times a bit annoying. One could always set an environment variable to skip the check, but there are situation where that is cumbersome. Consider for example, you'd want to remove the `ChromeOnly` attribute from the [DOMParser.parseFromSafeString()](https://github.com/SAP/project-foxhound/blob/main/dom/webidl/DOMParser.webidl#L30). This is an easy change, but if your memory is as bad as mine remembering in which folder to locate the change when redoing it after an update is annoying. This can be worked around with Git stash trickery, but I feel like we can offer a nicer way to do this. This PR aims to implement the nicer approach.

The core idea is, that you can provide the build script with a sequence of patch files, which are applied before and reverted after a successful build. This ensures your working tree is clean, and allows you to apply changes rather easily.

The workflow now becomes as follows:
1. Remove the troublesome attribute and save the webidl file.
2. `git diff >> ../patches/expose_safe_parse_to_webpages.patch`
3. `FOXHOUND_PATCHES="$(pwd)/../expose_safe_parse_to_webpages.patch" bash build.sh -ap`

Several patches can be passed to `$FOXHOUND_PATCHES` separated by semicolons.

The script tries to ensure that all provided patches exist and errors out if that is not the case or they fail to apply, so it aims to make this as bulletproof as possible.

Author: leeN

build.sh

@@ -42,6 +42,8 @@ SKIP_BUILD=
 SKIP_PACKAGE=
 RESET_GIT_REPO=
 DRY_RUN=
+APPLY_PATCHES=
+PATCH_FILES=()
 
 _determine_obj_dir() {
   if [ ! -d "${FOXHOUND_OBJ_DIR}" ]; then
@@ -64,7 +66,47 @@ _determine_bin_name() {
     fi
     FOXHOUND_NAME="$BIN_NAME"
 }
+_apply_patches() {
+  pushd "${FOXHOUND_DIR}" > /dev/null || _die "Can't change into foxhound dir: ${FOXHOUND_DIR}"
+  for pf in ${PATCH_FILES[@]}; do
+    local SUCCESS
+    if [ -z "$DRY_RUN" ]; then
+      git apply "${pf}" || _die "Can not apply ${pf}.."
+    else
+      git apply --check "${pf}" || _die "Can not apply ${pf}.."
+    fi
+    _status "Successfully applied ${pf}"
+  done
+  popd > /dev/null || exit 1
+}
 
+_revert_patches() {
+  pushd "${FOXHOUND_DIR}" > /dev/null || _die "Can't change into foxhound dir: ${FOXHOUND_DIR}"
+  for pf in ${PATCH_FILES[@]}; do
+    local SUCCESS
+    if [ -z "$DRY_RUN" ]; then
+      git apply -R "${pf}" || _die "Can not revert ${pf}.."
+    fi
+    _status "Successfully reverted ${pf}"
+  done
+  popd > /dev/null || exit 1
+}
+
+_prepare_foxhound_patches() {
+  if [ -z "${FOXHOUND_PATCHES}" ]; then
+    _die "FOXHOUND_PATCHES env variable is empty depite -a flag"
+  fi
+  local PATCHES
+  PATCHES=(${FOXHOUND_PATCHES//;/ })
+  for p in ${PATCHES[@]}; do
+    if [ ! -f "$p" ]; then
+      _die "Patch file ${p} does not exist, please check your syntax"
+    else
+      _status "Adding $p to patch file queue to apply"
+      PATCH_FILES+=("$p")
+    fi
+  done
+}
 
 _make_git_commit() {
   pushd "${FOXHOUND_DIR}" > /dev/null || _die "Can't change into foxhound dir: ${FOXHOUND_DIR}"
@@ -213,7 +255,9 @@ main() {
   PLAYWRIGHT_DIR="${BASEDIR}/playwright"
 
   _status "Starting Foxhound build in ${FOXHOUND_DIR}"
-
+  if [ -n "$APPLY_PATCHES" ]; then
+    _prepare_foxhound_patches
+  fi
   # First get playwright / rust versions
   _get_playwright_version
 
@@ -231,13 +275,21 @@ main() {
     _prepare_playwright
     _patch_foxhound
   fi
+  if [ -n "$APPLY_PATCHES" ] && (( ${#PATCH_FILES[@]} )); then
+    _status "Applying ${#PATCH_FILES[@]} patches prior to building Foxhound"
+    _apply_patches
+  fi
   _determine_obj_dir
   _status "Determined MOZ_OBJDIR as: $FOXHOUND_OBJ_DIR"
   _determine_bin_name
   _status "Determined binary name as: $FOXHOUND_NAME"
   if [ -z "$SKIP_BUILD" ]; then
       _build_foxhound
   fi
+  if [ -n "$APPLY_PATCHES" ] && (( ${#PATCH_FILES[@]} )); then
+    _status "Reverting ${#PATCH_FILES[@]} patches after successfull build"
+    _revert_patches
+  fi
   if [ -z "$SKIP_PACKAGE" ]; then
      _package_foxhound
   fi
@@ -246,7 +298,7 @@ main() {
 _help() {
    echo "Builds project foxhound"
    echo
-   echo "Syntax: build.sh [-c|s|t|u|v|b|r|p|g|n|h]"
+   echo "Syntax: build.sh [-c|s|t|u|v|b|r|p|g|n|a|h]"
    echo
    echo "For example, to build from scratch with playwright:"
    echo "> bash build.sh -p"
@@ -262,19 +314,23 @@ _help() {
    echo "p     Builds with playwright integration."
    echo "g     Create a Git commit with the playwright patches."
    echo "n     Dry run. Only print the step the script would perform."
+   echo "a     Apply patches provided in env variable FOXHOUND_PATCHES: ${FOXHOUND_PATCHES} as a semicolon separated list"
    echo "h     Print this Help."
    echo
    echo
    echo "Environment variables:"
    echo
+   echo "FOXHOUND_PATCHES:"
+   echo "Combined with the -a flag, this splits the content into a list of semicolon separated patch files that we apply before the build and revert afterwards."
+   echo
    echo "These are meant to be used if you know what you are doing and can lead to states that are difficult to revert. Use with caution!"
    echo
    echo "SKIP_GIT_CHECK:"
    echo "If set to any value this skips the check whether the git repository is dirty."
    echo "Warning: When used together with Playwright support this can lead to a state where untangling the Playwright patches from your changes to commit them is very cumbersome!"
 }
 
-while getopts "hpcstuvrbgn" option; do
+while getopts "hpcstuvrbgna" option; do
   case "$option" in
     h)
         _help
@@ -299,6 +355,8 @@ while getopts "hpcstuvrbgn" option; do
         WITH_PLAYWRIGHT_INTEGRATION=1;;
     n)
         DRY_RUN=1;;
+    a)
+        APPLY_PATCHES=1;;
     \?)
         _die "Error: Invalid option: $option";;
   esac