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

@@ -1,4 +1,5 @@
 import groovy.transform.Memoized
+import groovy.xml.XmlParser
 
 apply plugin: 'jacoco'
 
@@ -167,3 +168,39 @@ def androidTestReport = tasks.register('jacocoAndroidTestReport', JacocoReport)
     ])
 }
 androidTestReport.configure { dependsOn('connectedPlayDebugAndroidTest') }
+
+// Issue 16640 - some emulators run, but register zero coverage
+tasks.register('assertNonzeroAndroidTestCoverage') {
+    doLast {
+        File jacocoReport = layout.buildDirectory.dir("reports/jacoco/jacocoAndroidTestReport/jacocoAndroidTestReport.xml").get().asFile
+
+        if (!jacocoReport.exists())
+            throw new FileNotFoundException("jacocoAndroidTestReport.xml was not found after running jacocoAndroidTestReport")
+
+        XmlParser xmlParser = new XmlParser()
+        xmlParser.setFeature("http://apache.org/xml/features/disallow-doctype-decl", false)
+        xmlParser.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false)
+
+        Node reportRoot = xmlParser.parse(jacocoReport)
+        boolean hasCovered = false
+
+        // https://github.com/jacoco/jacoco/blob/5aabb2eb60bbcd05df968005f1746ba19dcd5361/org.jacoco.report/src/org/jacoco/report/internal/xml/ReportElement.java#L190
+        for (Node child : reportRoot.children()) {
+            if (child.name() != "counter")
+                continue;
+
+            if (child.attribute("covered") == "0") {
+                logger.warn("jacoco registered zero code coverage for counter type %s", child.attribute("type"))
+            } else {
+                hasCovered = true
+            }
+        }
+
+        if (!hasCovered)
+            throw new GradleScriptException("androidTest registered zero code coverage in jacocoAndroidTestReport.xml. Probably some incompatibilities in the toolchain.", null)
+    }
+}
+afterEvaluate {
+    tasks.named('jacocoAndroidTestReport').configure { finalizedBy('assertNonzeroAndroidTestCoverage') }
+    tasks.named('jacocoTestReport').configure { finalizedBy('assertNonzeroAndroidTestCoverage') }
+}