diff --git a/service/src/com/android/server/connectivity/NetworkRanker.java b/service/src/com/android/server/connectivity/NetworkRanker.java
index e839837..a4085cd 100644
--- a/service/src/com/android/server/connectivity/NetworkRanker.java
+++ b/service/src/com/android/server/connectivity/NetworkRanker.java
@@ -108,7 +108,58 @@
         }
     }
 
-    @Nullable private <T extends Scoreable> T getBestNetworkByPolicy(
+    private <T extends Scoreable> boolean isBadWiFi(@NonNull final T candidate) {
+        return candidate.getScore().hasPolicy(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD)
+                && candidate.getCapsNoCopy().hasTransport(TRANSPORT_WIFI);
+    }
+
+    /**
+     * Apply the "yield to bad WiFi" policy.
+     *
+     * This function must run immediately after the validation policy.
+     *
+     * If any of the accepted networks has the "yield to bad WiFi" policy AND there are some
+     * bad WiFis in the rejected list, then move the networks with the policy to the rejected
+     * list. If this leaves no accepted network, then move the bad WiFis back to the accepted list.
+     *
+     * This function returns nothing, but will have updated accepted and rejected in-place.
+     *
+     * @param accepted networks accepted by the validation policy
+     * @param rejected networks rejected by the validation policy
+     */
+    private <T extends Scoreable> void applyYieldToBadWifiPolicy(@NonNull ArrayList<T> accepted,
+            @NonNull ArrayList<T> rejected) {
+        if (!CollectionUtils.any(accepted, n -> n.getScore().hasPolicy(POLICY_YIELD_TO_BAD_WIFI))) {
+            // No network with the policy : do nothing.
+            return;
+        }
+        if (!CollectionUtils.any(rejected, n -> isBadWiFi(n))) {
+            // No bad WiFi : do nothing.
+            return;
+        }
+        if (CollectionUtils.all(accepted, n -> n.getScore().hasPolicy(POLICY_YIELD_TO_BAD_WIFI))) {
+            // All validated networks yield to bad WiFis : keep bad WiFis alongside with the
+            // yielders. This is important because the yielders need to be compared to the bad
+            // wifis by the following policies (e.g. exiting).
+            final ArrayList<T> acceptedYielders = new ArrayList<>(accepted);
+            final ArrayList<T> rejectedWithBadWiFis = new ArrayList<>(rejected);
+            partitionInto(rejectedWithBadWiFis, n -> isBadWiFi(n), accepted, rejected);
+            accepted.addAll(acceptedYielders);
+            return;
+        }
+        // Only some of the validated networks yield to bad WiFi : keep only the ones who don't.
+        final ArrayList<T> acceptedWithYielders = new ArrayList<>(accepted);
+        partitionInto(acceptedWithYielders, n -> !n.getScore().hasPolicy(POLICY_YIELD_TO_BAD_WIFI),
+                accepted, rejected);
+    }
+
+    /**
+     * Get the best network among a list of candidates according to policy.
+     * @param candidates the candidates
+     * @param currentSatisfier the current satisfier, or null if none
+     * @return the best network
+     */
+    @Nullable public <T extends Scoreable> T getBestNetworkByPolicy(
             @NonNull List<T> candidates,
             @Nullable final T currentSatisfier) {
         // Used as working areas.
@@ -148,24 +199,15 @@
         if (accepted.size() == 1) return accepted.get(0);
         if (accepted.size() > 0 && rejected.size() > 0) candidates = new ArrayList<>(accepted);
 
-        // Yield to bad wifi policy : if any wifi has ever been validated (even if it's now
-        // unvalidated), and unless it's been explicitly avoided when bad in UI, then keep only
-        // networks that don't yield to such a wifi network.
-        final boolean anyWiFiEverValidated = CollectionUtils.any(candidates,
-                nai -> nai.getScore().hasPolicy(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD)
-                        && nai.getCapsNoCopy().hasTransport(TRANSPORT_WIFI));
-        if (anyWiFiEverValidated) {
-            partitionInto(candidates, nai -> !nai.getScore().hasPolicy(POLICY_YIELD_TO_BAD_WIFI),
-                    accepted, rejected);
-            if (accepted.size() == 1) return accepted.get(0);
-            if (accepted.size() > 0 && rejected.size() > 0) candidates = new ArrayList<>(accepted);
-        }
-
         // If any network is validated (or should be accepted even if it's not validated), then
         // don't choose one that isn't.
         partitionInto(candidates, nai -> nai.getScore().hasPolicy(POLICY_IS_VALIDATED)
                         || nai.getScore().hasPolicy(POLICY_ACCEPT_UNVALIDATED),
                 accepted, rejected);
+        // Yield to bad wifi policy : if any network has the "yield to bad WiFi" policy and
+        // there are bad WiFis connected, then accept the bad WiFis and reject the networks with
+        // the policy.
+        applyYieldToBadWifiPolicy(accepted, rejected);
         if (accepted.size() == 1) return accepted.get(0);
         if (accepted.size() > 0 && rejected.size() > 0) candidates = new ArrayList<>(accepted);
 
@@ -194,16 +236,24 @@
         // subscription with the same transport.
         partitionInto(candidates, nai -> nai.getScore().hasPolicy(POLICY_TRANSPORT_PRIMARY),
                 accepted, rejected);
-        for (final Scoreable defaultSubNai : accepted) {
-            // Remove all networks without the DEFAULT_SUBSCRIPTION policy and the same transports
-            // as a network that has it.
-            final int[] transports = defaultSubNai.getCapsNoCopy().getTransportTypes();
-            candidates.removeIf(nai -> !nai.getScore().hasPolicy(POLICY_TRANSPORT_PRIMARY)
-                    && Arrays.equals(transports, nai.getCapsNoCopy().getTransportTypes()));
+        if (accepted.size() > 0) {
+            // Some networks are primary. For each transport, keep only the primary, but also
+            // keep all networks for which there isn't a primary (which are now in the |rejected|
+            // array).
+            // So for each primary network, remove from |rejected| all networks with the same
+            // transports as one of the primary networks. The remaining networks should be accepted.
+            for (final T defaultSubNai : accepted) {
+                final int[] transports = defaultSubNai.getCapsNoCopy().getTransportTypes();
+                rejected.removeIf(
+                        nai -> Arrays.equals(transports, nai.getCapsNoCopy().getTransportTypes()));
+            }
+            accepted.addAll(rejected);
+            candidates = new ArrayList<>(accepted);
         }
         if (1 == candidates.size()) return candidates.get(0);
-        // It's guaranteed candidates.size() > 0 because there is at least one with the
-        // TRANSPORT_PRIMARY policy and only those without it were removed.
+        // If there were no primary network, then candidates.size() > 0 because it didn't
+        // change from the previous result. If there were, it's guaranteed candidates.size() > 0
+        // because accepted.size() > 0 above.
 
         // If some of the networks have a better transport than others, keep only the ones with
         // the best transports.
diff --git a/tests/unit/java/com/android/server/connectivity/NetworkRankerTest.kt b/tests/unit/java/com/android/server/connectivity/NetworkRankerTest.kt
index 551b94c..4408958 100644
--- a/tests/unit/java/com/android/server/connectivity/NetworkRankerTest.kt
+++ b/tests/unit/java/com/android/server/connectivity/NetworkRankerTest.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2020 The Android Open Source Project
+ * Copyright (C) 2021 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,74 +17,156 @@
 package com.android.server.connectivity
 
 import android.net.NetworkCapabilities
-import android.net.NetworkRequest
+import android.net.NetworkCapabilities.TRANSPORT_CELLULAR
+import android.net.NetworkCapabilities.TRANSPORT_WIFI
 import android.net.NetworkScore.KEEP_CONNECTED_NONE
+import android.net.NetworkScore.POLICY_EXITING
+import android.net.NetworkScore.POLICY_TRANSPORT_PRIMARY
+import android.net.NetworkScore.POLICY_YIELD_TO_BAD_WIFI
+import android.os.Build
 import androidx.test.filters.SmallTest
-import androidx.test.runner.AndroidJUnit4
+import com.android.server.connectivity.FullScore.POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD
+import com.android.server.connectivity.FullScore.POLICY_IS_VALIDATED
+import com.android.testutils.DevSdkIgnoreRule
+import com.android.testutils.DevSdkIgnoreRunner
 import org.junit.Test
 import org.junit.runner.RunWith
-import org.mockito.ArgumentMatchers.any
-import org.mockito.Mockito.doReturn
-import org.mockito.Mockito.mock
 import kotlin.test.assertEquals
-import kotlin.test.assertNull
 
-@RunWith(AndroidJUnit4::class)
+private fun score(vararg policies: Int) = FullScore(0,
+        policies.fold(0L) { acc, e -> acc or (1L shl e) }, KEEP_CONNECTED_NONE)
+private fun caps(transport: Int) = NetworkCapabilities.Builder().addTransportType(transport).build()
+
 @SmallTest
+@RunWith(DevSdkIgnoreRunner::class)
+@DevSdkIgnoreRule.IgnoreUpTo(Build.VERSION_CODES.R)
 class NetworkRankerTest {
-    private val ranker = NetworkRanker()
+    private val mRanker = NetworkRanker()
 
-    private fun makeNai(satisfy: Boolean, legacyScore: Int) =
-            mock(NetworkAgentInfo::class.java).also {
-                doReturn(satisfy).`when`(it).satisfies(any())
-                val fs = FullScore(legacyScore, 0 /* policies */, KEEP_CONNECTED_NONE)
-                doReturn(fs).`when`(it).getScore()
-                val nc = NetworkCapabilities.Builder().build()
-                doReturn(nc).`when`(it).getCapsNoCopy()
-            }
-
-    @Test
-    fun testGetBestNetwork() {
-        val scores = listOf(20, 50, 90, 60, 23, 68)
-        val nais = scores.map { makeNai(true, it) }
-        val bestNetwork = nais[2] // The one with the top score
-        val someRequest = mock(NetworkRequest::class.java)
-        assertEquals(bestNetwork, ranker.getBestNetwork(someRequest, nais, bestNetwork))
+    private class TestScore(private val sc: FullScore, private val nc: NetworkCapabilities)
+            : NetworkRanker.Scoreable {
+        override fun getScore() = sc
+        override fun getCapsNoCopy(): NetworkCapabilities = nc
     }
 
     @Test
-    fun testIgnoreNonSatisfying() {
-        val nais = listOf(makeNai(true, 20), makeNai(true, 50), makeNai(false, 90),
-                makeNai(false, 60), makeNai(true, 23), makeNai(false, 68))
-        val bestNetwork = nais[1] // Top score that's satisfying
-        val someRequest = mock(NetworkRequest::class.java)
-        assertEquals(bestNetwork, ranker.getBestNetwork(someRequest, nais, nais[1]))
+    fun testYieldToBadWiFiOneCell() {
+        // Only cell, it wins
+        val winner = TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                caps(TRANSPORT_CELLULAR))
+        val scores = listOf(winner)
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
     }
 
     @Test
-    fun testNoMatch() {
-        val nais = listOf(makeNai(false, 20), makeNai(false, 50), makeNai(false, 90))
-        val someRequest = mock(NetworkRequest::class.java)
-        assertNull(ranker.getBestNetwork(someRequest, nais, null))
+    fun testYieldToBadWiFiOneCellOneBadWiFi() {
+        // Bad wifi wins against yielding validated cell
+        val winner = TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD),
+                caps(TRANSPORT_WIFI))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
     }
 
     @Test
-    fun testEmpty() {
-        val someRequest = mock(NetworkRequest::class.java)
-        assertNull(ranker.getBestNetwork(someRequest, emptyList(), null))
+    fun testYieldToBadWiFiOneCellTwoBadWiFi() {
+        // Bad wifi wins against yielding validated cell. Prefer the one that's primary.
+        val winner = TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                POLICY_TRANSPORT_PRIMARY), caps(TRANSPORT_WIFI))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD),
+                        caps(TRANSPORT_WIFI)),
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
     }
 
-    // Make sure the ranker is "stable" (as in stable sort), that is, it always returns the FIRST
-    // network satisfying the request if multiple of them have the same score.
     @Test
-    fun testStable() {
-        val nais1 = listOf(makeNai(true, 30), makeNai(true, 30), makeNai(true, 30),
-                makeNai(true, 30), makeNai(true, 30), makeNai(true, 30))
-        val someRequest = mock(NetworkRequest::class.java)
-        assertEquals(nais1[0], ranker.getBestNetwork(someRequest, nais1, nais1[0]))
+    fun testYieldToBadWiFiOneCellTwoBadWiFiOneNotAvoided() {
+        // Bad wifi ever validated wins against bad wifi that never was validated (or was
+        // avoided when bad).
+        val winner = TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD),
+                caps(TRANSPORT_WIFI))
+        val scores = listOf(
+                winner,
+                TestScore(score(), caps(TRANSPORT_WIFI)),
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
+    }
 
-        val nais2 = listOf(makeNai(true, 30), makeNai(true, 50), makeNai(true, 20),
-                makeNai(true, 50), makeNai(true, 50), makeNai(true, 40))
-        assertEquals(nais2[1], ranker.getBestNetwork(someRequest, nais2, nais2[1]))
+    @Test
+    fun testYieldToBadWiFiOneCellOneBadWiFiOneGoodWiFi() {
+        // Good wifi wins
+        val winner = TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                POLICY_IS_VALIDATED), caps(TRANSPORT_WIFI))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                        POLICY_TRANSPORT_PRIMARY), caps(TRANSPORT_WIFI)),
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
+    }
+
+    @Test
+    fun testYieldToBadWiFiTwoCellsOneBadWiFi() {
+        // Cell that doesn't yield wins over cell that yields and bad wifi
+        val winner = TestScore(score(POLICY_IS_VALIDATED), caps(TRANSPORT_CELLULAR))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                        POLICY_TRANSPORT_PRIMARY), caps(TRANSPORT_WIFI)),
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
+    }
+
+    @Test
+    fun testYieldToBadWiFiTwoCellsOneBadWiFiOneGoodWiFi() {
+        // Good wifi wins over cell that doesn't yield and cell that yields
+        val winner = TestScore(score(POLICY_IS_VALIDATED), caps(TRANSPORT_WIFI))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                        POLICY_TRANSPORT_PRIMARY), caps(TRANSPORT_WIFI)),
+                TestScore(score(POLICY_IS_VALIDATED), caps(TRANSPORT_CELLULAR)),
+                TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                        caps(TRANSPORT_CELLULAR))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
+    }
+
+    @Test
+    fun testYieldToBadWiFiOneExitingGoodWiFi() {
+        // Yielding cell wins over good exiting wifi
+        val winner = TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                caps(TRANSPORT_CELLULAR))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_IS_VALIDATED, POLICY_EXITING), caps(TRANSPORT_WIFI))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
+    }
+
+    @Test
+    fun testYieldToBadWiFiOneExitingBadWiFi() {
+        // Yielding cell wins over bad exiting wifi
+        val winner = TestScore(score(POLICY_YIELD_TO_BAD_WIFI, POLICY_IS_VALIDATED),
+                caps(TRANSPORT_CELLULAR))
+        val scores = listOf(
+                winner,
+                TestScore(score(POLICY_EVER_VALIDATED_NOT_AVOIDED_WHEN_BAD,
+                        POLICY_EXITING), caps(TRANSPORT_WIFI))
+        )
+        assertEquals(winner, mRanker.getBestNetworkByPolicy(scores, null))
     }
 }