fdroid-publish: find_tool must test its result (ls|tail exits 0 on no match)

An empty SDK build-tools glob was reported as found, so the signer check ran
nothing and refused a correctly signed APK with an empty signer. An unreadable
signer now warns and defers to the host's pinned-cert check; only a real
mismatch fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

fdroid-publish/publish.sh

@@ -54,17 +54,26 @@ find_tool() { # newest build-tools copy of $1 under the Android SDK, or PATH
   command -v "$1" 2>/dev/null && return 0
   for sdk in "${ANDROID_HOME:-}" "${ANDROID_SDK_ROOT:-}" /opt/android-sdk /opt/android/sdk /usr/lib/android-sdk; do
     [ -n "$sdk" ] && [ -d "$sdk/build-tools" ] || continue
-    ls -d "$sdk"/build-tools/*/"$1" 2>/dev/null | sort -V | tail -n1 && return 0
+    # Test the RESULT, not the pipeline: `ls … | tail` exits 0 even when ls found
+    # nothing, which once reported an empty path as found (empty signer → refusal).
+    p=$(ls -d "$sdk"/build-tools/*/"$1" 2>/dev/null | sort -V | tail -n1)
+    [ -n "$p" ] && [ -x "$p" ] && { printf '%s
+' "$p"; return 0; }
   done
   return 1
 }
 
 # Signer check before anything leaves the runner, so a mis-signed build fails
 # here with a clear message instead of as CERT_MISMATCH on the host.
-if apksigner=$(find_tool apksigner); then
-  got=$("$apksigner" verify --print-certs "$apk" 2>/dev/null | sed -n 's/^Signer #1 certificate SHA-256 digest: //p' | head -n1 | tr 'A-F' 'a-f')
-  [ "$got" = "$cert" ] || fail "APK signer $got is not the pinned $app cert $cert — refusing to publish"
-  say "signer cert: OK ($cert)"
+if apksigner=$(find_tool apksigner) && [ -n "$apksigner" ]; then
+  got=$("$apksigner" verify --print-certs "$apk" 2>/dev/null | sed -n 's/^Signer #1 certificate SHA-256 digest: //p' | head -n1 | tr 'A-F' 'a-f' || true)
+  if [ -z "$got" ]; then
+    say "::warning::apksigner at $apksigner printed no signer; the repo host verifies the cert"
+  elif [ "$got" != "$cert" ]; then
+    fail "APK signer $got is not the pinned $app cert $cert — refusing to publish"
+  else
+    say "signer cert: OK ($cert)"
+  fi
 else
   say "signer cert: apksigner not found here; the repo host verifies it"
 fi
@@ -72,7 +81,7 @@ fi
 if [ -z "$version_code" ]; then
   # Needed for the no-downgrade guard below. A bare runner image (the promote
   # workflow's) has no Android SDK, so fetch Debian's aapt rather than guess.
-  if ! aapt=$(find_tool aapt2 || find_tool aapt); then
+  if ! aapt=$(find_tool aapt2 || find_tool aapt) || [ -z "$aapt" ]; then
     SUDO=; [ "$(id -u)" -eq 0 ] || SUDO=sudo
     { $SUDO apt-get update -qq && $SUDO apt-get install -y -qq aapt; } >/dev/null 2>&1 || true
     aapt=$(command -v aapt || true)