travis: cache all build outputs with cachix

This further speeds up builds, in particular the modules test in the
next commit. By checking if the expected final build output has already
been cached, we can even skip the download of cached builds.

.travis.yml

@@ -14,6 +14,9 @@ install:
   - cachix use nix-bitcoin
   - VER="$(nix eval nixpkgs.lib.version)"
 env:
+  global:
+    # CACHIX_SIGNING_KEY
+  - secure: "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"
   jobs:
   - PKG=nodeinfo STABLE=1
   - PKG=hwi STABLE=1
@@ -30,4 +33,25 @@ env:
   - PKG=liquid-swap STABLE=1
 script:
   - printf '%s (%s)\n' "$NIX_PATH" "$VER"
-  - nix-build -A $PKG
+  - time nix-instantiate -A $PKG --add-root ./drv --indirect
+  - outPath=$(nix-store --query ./drv)
+  - |
+    if nix path-info --store https://nix-bitcoin.cachix.org $outPath &>/dev/null; then
+      echo "$outPath" has already been built successfully.
+      travis_terminate 0
+    fi
+    # Travis doesn't expose secrets to pull-request builds,
+    # so skip cache uploading in this case
+  - |
+    if [[ $CACHIX_SIGNING_KEY ]]; then
+      cachix push nix-bitcoin --watch-store &
+      cachixPid=$!
+    fi
+  - nix-build ./drv
+  - |
+    if [[ $CACHIX_SIGNING_KEY ]]; then
+      # Wait until cachix has finished uploading
+      # Run as root because yama/ptrace_scope != 0
+      ruby=$(nix-build '<nixpkgs>' -A ruby)/bin/ruby
+      time sudo $ruby helper/wait-for-network-idle.rb $cachixPid
+    fi