test: ensure non-zero coverage for androidTest reports

there are many toolchain incompatibilities which can result in a
false-positive androidTest test run, but with zero actual coverage
tallied

this adds a finalization task that scans the coverage report looking
for an aggregate coverage count that is non-zero

AnkiDroid/jacoco.gradle

@@ -167,3 +167,28 @@ def androidTestReport = tasks.register('jacocoAndroidTestReport', JacocoReport)
     ])
 }
 androidTestReport.configure { dependsOn('connectedPlayDebugAndroidTest') }
+
+// Issue 16640 - some emulators run, but register zero coverage
+tasks.register('assertNonzeroAndroidTestCoverage') {
+    doLast {
+        // androidTestReport currently creates one .xml file results in this dir
+        File folder = file("./build/reports/jacoco/jacocoAndroidTestReport/")
+        File[] listOfFiles = folder.listFiles({ d, f -> f ==~ /.*.xml/ } as FilenameFilter)
+        for (File file : listOfFiles) {
+            // The aggregate results file currently ends with a final aggregate coverage amount
+            // it is non-zero in any sane execution of our tests
+            // the last characters in the report look like ' covered="393"/></report>'
+            String[] matches = file.readLines().findAll { it.contains('"/></report>') }
+            if (matches.length != 1) {
+                throw new GradleScriptException("Unable to determine coverage for tests executed for " + file.name + ". Regex pattern out of date?", null)
+            }
+            if (!(matches[0] =~ /covered="\d+"\/><\/report>/) || matches[0].contains('covered="0"/></report>')) {
+                throw new GradleScriptException("androidTest registered zero coverage for " + file.name + " - Probably some incompatibilities in the toolchain.", null)
+            }
+        }
+    }
+}
+afterEvaluate {
+    tasks.named('jacocoAndroidTestReport').configure { finalizedBy('assertNonzeroAndroidTestCoverage') }
+    tasks.named('jacocoTestReport').configure { finalizedBy('assertNonzeroAndroidTestCoverage') }
+}