Work around snap's old cabal + new snapcraft proxy.

.snapsquid.conf

@@ -0,0 +1,14 @@
+# In 2015, cabal-install had a http bug triggered when proxies didn't keep
+# the connection open. This version made it into Ubuntu Xenial as used by
+# Snapcraft. In June 2018, Snapcraft's proxy started triggering this bug.
+#
+#     https://bugs.launchpad.net/launchpad-buildd/+bug/1797809
+#
+# Workaround: add more proxy
+
+visible_hostname localhost
+http_port 8888
+cache_peer 10.10.10.1 parent 8222 0 no-query default
+cache_peer_domain localhost !.internal
+http_access allow all
+

snap/snapcraft.yaml

@@ -37,9 +37,16 @@ parts:
     source: ./
     build-packages:
       - cabal-install
+      - squid3
     build: |
+      # See comments in .snapsquid.conf
+      [ "$http_proxy" ] && {
+        squid3 -f .snapsquid.conf
+        export http_proxy="http://localhost:8888"
+        sleep 3
+      }
       cabal sandbox init
-      cabal update
+      cabal update || cat /var/log/squid/*
       cabal install -j
     install: |
       install -d $SNAPCRAFT_PART_INSTALL/usr/bin