Tentatively fix amd64 build failures (https://bugzilla.mozilla.org/1800857).

This is a temporary workaround, we really need to get to the bottom of the underlying issue.

Author: Olivier Tilloy

snapcraft.yaml

@@ -332,6 +332,22 @@ parts:
       MACH="/usr/bin/python3 ./mach"
       $MACH configure --prefix=$SNAPCRAFT_PART_INSTALL/usr
       $MACH uniffi generate # workaround for https://bugzilla.mozilla.org/1797714
+      # Unset SNAP_INSTANCE_NAME because when snapcraft runs as a snap it is set
+      # (and its value is "snapcraft"), and this interferes with firefox's
+      # detection of whether it's running sandboxed (see
+      # https://searchfox.org/mozilla-central/search?q=IsRunningUnderSnap()).
+      # That codes is compiled in the xpcshell binary that's used to generate
+      # files at build time, and the environment it runs in prevents it from
+      # locating services it needs, causing a crash (see
+      # https://searchfox.org/mozilla-central/rev/aa329cf7506ddd966542e642ec00223fd7461599/xpcom/io/SpecialSystemDirectory.cpp#196).
+      # This is a temporary workaround to get builds going again, we really need
+      # to get to the bottom of this and understand why xpcshell cannot find the
+      # appinfo service when run in the build environment.
+      # Unsetting SNAP_INSTANCE_NAME has the undesirable side-effect that when
+      # generating a profile for PGO-enabled builds, a potentially different
+      # code path will be exercised (not confined as a snap), so that's not a
+      # good solution.
+      unset SNAP_INSTANCE_NAME
       if [ $SNAPCRAFT_TARGET_ARCH = "amd64" ]; then
         # xvfb is only needed when doing a PGO-enabled build
         xvfb-run '--server-args=-screen 0 1920x1080x24' $MACH build -j$SNAPCRAFT_PARALLEL_BUILD_COUNT