diff --git a/.travis.yml b/.travis.yml
index cbb568a..3835300 100644
--- a/.travis.yml
+++ b/.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
diff --git a/helper/wait-for-network-idle.rb b/helper/wait-for-network-idle.rb
new file mode 100755
index 0000000..18b5a9a
--- /dev/null
+++ b/helper/wait-for-network-idle.rb
@@ -0,0 +1,29 @@
+#!/usr/bin/env ruby
+require 'open3'
+
+# Wait until the given PID had no network activity for `Timeout` seconds, then exit.
+
+pid = ARGV.first
+Timeout = 2
+
+stdin, out, err, wait_thread = Open3.popen3("strace -f -e trace=network -s 1 -q -p #{pid}")
+while IO.select([err], nil, nil, Timeout)
+  begin
+    out = err.read_nonblock(1 << 10)
+  rescue EOFError
+    status = wait_thread.value
+    if status.success?
+      puts "Monitored process #{pid} exited"
+      exit 0
+    else
+      puts "Strace failed with exit code #{status.to_i}. Last output:\n#{out}"
+      # strace often fails with code 256 which looks like success to shells. fail with 1 instead.
+      exit 1
+    end
+  end
+end
+
+# If we exit without an explicit kill,
+# ptrace can fail on reattachment:  ptrace(PTRACE_SEIZE, $PID): Operation not permitted
+# Only relevant for testing.
+Process.kill("TERM", wait_thread.pid)