From 49fddaa13e10b3c524fa1d294f6d31392aa288af Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Tue, 15 Nov 2022 07:59:22 -0800
Subject: [PATCH] errlogRemoveListeners() handle self-removal

Handle errlogRemoveListeners() during a callback.
---
 modules/libcom/src/error/errlog.c | 27 +++++++++++++++++++++++----
 modules/libcom/src/error/errlog.h |  3 +++
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/modules/libcom/src/error/errlog.c b/modules/libcom/src/error/errlog.c
index 79361e42a..6ae267323 100644
--- a/modules/libcom/src/error/errlog.c
+++ b/modules/libcom/src/error/errlog.c
@@ -66,6 +66,8 @@ typedef struct listenerNode{
     ELLNODE node;
     errlogListener listener;
     void *pPrivate;
+    unsigned active:1;
+    unsigned removed:1;
 } listenerNode;
 
 typedef struct {
@@ -442,9 +444,15 @@ int errlogRemoveListeners(errlogListener listener, void *pPrivate)
         listenerNode *pnext = (listenerNode *)ellNext(&plistenerNode->node);
 
         if (plistenerNode->listener == listener &&
-            plistenerNode->pPrivate == pPrivate) {
-            ellDelete(&pvt.listenerList, &plistenerNode->node);
-            free(plistenerNode);
+            plistenerNode->pPrivate == pPrivate)
+        {
+            if(plistenerNode->active) { /* callback removing itself */
+                plistenerNode->removed = 1;
+
+            } else {
+                ellDelete(&pvt.listenerList, &plistenerNode->node);
+                free(plistenerNode);
+            }
             ++count;
         }
         plistenerNode = pnext;
@@ -674,8 +682,19 @@ static void errlogThread(void)
                 epicsMutexMustLock(pvt.listenerLock);
                 plistenerNode = (listenerNode *)ellFirst(&pvt.listenerList);
                 while (plistenerNode) {
+                    listenerNode *next;
+
+                    plistenerNode->active = 1;
                     (*plistenerNode->listener)(plistenerNode->pPrivate, base+1u);
-                    plistenerNode = (listenerNode *)ellNext(&plistenerNode->node);
+                    plistenerNode->active = 0;
+
+                    next = (listenerNode *)ellNext(&plistenerNode->node);
+                    if(plistenerNode->removed) {
+                        /* listener() called errlogRemoveListeners() */
+                        ellDelete(&pvt.listenerList, &plistenerNode->node);
+                        free(plistenerNode);
+                    }
+                    plistenerNode = next;
                 }
                 epicsMutexUnlock(pvt.listenerLock);
 
diff --git a/modules/libcom/src/error/errlog.h b/modules/libcom/src/error/errlog.h
index 133fe8be7..7cec39653 100644
--- a/modules/libcom/src/error/errlog.h
+++ b/modules/libcom/src/error/errlog.h
@@ -178,6 +178,9 @@ LIBCOM_API void errlogAddListener(errlogListener listener, void *pPrivate);
  *
  * \param listener Function pointer of type ::errlogListener
  * \param pPrivate This will be passed as the first argument of listener()
+ *
+ * \since UNRELEASED Safe to call from a listener callback.
+ * \until UNRELEASED Self-removal from a listener callback caused corruption.
  */
 LIBCOM_API int errlogRemoveListeners(errlogListener listener,
     void *pPrivate);