From 05339176c3270beeee7cb43b3fbc99d0925926ca Mon Sep 17 00:00:00 2001
From: Constantin Hirsch <63846074+SiegfriedTobias1@users.noreply.github.com>
Date: Mon, 5 Oct 2020 16:52:01 +0200
Subject: [PATCH 1/3] Fix NPE in MDCContextMap

Accomodate for the fact that MDC.getCopyOfContextMap() may return null
---
 .../main/java/org/apache/logging/slf4j/MDCContextMap.java   | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/log4j-to-slf4j/src/main/java/org/apache/logging/slf4j/MDCContextMap.java b/log4j-to-slf4j/src/main/java/org/apache/logging/slf4j/MDCContextMap.java
index f03ee54797e..b04d3582362 100644
--- a/log4j-to-slf4j/src/main/java/org/apache/logging/slf4j/MDCContextMap.java
+++ b/log4j-to-slf4j/src/main/java/org/apache/logging/slf4j/MDCContextMap.java
@@ -71,7 +71,8 @@ public void clear() {
 
     @Override
     public boolean containsKey(final String key) {
-        return MDC.getCopyOfContextMap().containsKey(key);
+        Map<String, String> map = MDC.getCopyOfContextMap();
+        return map != null && map.containsKey(key);
     }
 
     @Override
@@ -88,7 +89,8 @@ public Map<String, String> getImmutableMapOrNull() {
 
     @Override
     public boolean isEmpty() {
-        return MDC.getCopyOfContextMap().isEmpty();
+        Map<String, String> map = MDC.getCopyOfContextMap();
+        return map == null || map.isEmpty();
     }
 
     @Override

From cd4f02c7e63183420eb3f9f6e582ecbcef676e51 Mon Sep 17 00:00:00 2001
From: Constantin Hirsch <63846074+SiegfriedTobias1@users.noreply.github.com>
Date: Mon, 5 Oct 2020 18:34:44 +0200
Subject: [PATCH 2/3] Add test for MDC NPE fix

LOG4J2-2939;
---
 .../test/java/org/apache/logging/slf4j/LoggerTest.java    | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java b/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
index e994d192c92..35f72c08033 100644
--- a/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
+++ b/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
@@ -32,6 +32,7 @@
 import org.junit.Before;
 import org.junit.ClassRule;
 import org.junit.Test;
+import org.slf4j.MDC;
 
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
@@ -165,5 +166,12 @@ public void mdc() {
         assertThat(list.strList, hasSize(2));
         assertTrue("Incorrect year", list.strList.get(0).startsWith("2010"));
     }
+
+    @Test
+    public void mdcNullBacked() {
+        assertNull("Setup wrong", MDC.getCopyOfContextMap());
+        assertTrue(ThreadContext.isEmpty());
+        assertFalse(ThreadContext.containsKey("something"));
+    }
 }
 

From 544e57c2730a91027610f8d330f0859f66c438ff Mon Sep 17 00:00:00 2001
From: Constantin Hirsch <63846074+SiegfriedTobias1@users.noreply.github.com>
Date: Tue, 6 Oct 2020 12:55:47 +0200
Subject: [PATCH 3/3] Add more tests for MDC NPE fix

LOG4J2-2939;
---
 .../org/apache/logging/slf4j/LoggerTest.java  | 35 +++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java b/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
index 35f72c08033..9cf7deb78d0 100644
--- a/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
+++ b/log4j-to-slf4j/src/test/java/org/apache/logging/slf4j/LoggerTest.java
@@ -168,10 +168,41 @@ public void mdc() {
     }
 
     @Test
-    public void mdcNullBacked() {
+    public void mdcNullBackedIsEmpty() {
         assertNull("Setup wrong", MDC.getCopyOfContextMap());
         assertTrue(ThreadContext.isEmpty());
+    }
+
+    @Test
+    public void mdcNullBackedContainsKey() {
+        assertNull("Setup wrong", MDC.getCopyOfContextMap());
         assertFalse(ThreadContext.containsKey("something"));
     }
-}
 
+    @Test
+    public void mdcNullBackedContainsNullKey() {
+        assertNull("Setup wrong", MDC.getCopyOfContextMap());
+        assertFalse(ThreadContext.containsKey(null));
+    }
+
+    @Test
+    public void mdcContainsNullKey() {
+        try {
+            ThreadContext.put("some", "thing");
+            assertNotNull("Setup wrong", MDC.getCopyOfContextMap());
+            assertFalse(ThreadContext.containsKey(null));
+        } finally {
+            ThreadContext.clearMap();
+        }
+    }
+
+    @Test
+    public void mdcCannotContainNullKey() {
+        try {
+            ThreadContext.put(null, "something");
+            fail("should throw");
+        } catch (IllegalArgumentException | NullPointerException e) {
+            // expected
+        }
+    }
+}