From f4528ff5e189e6e52054634b0ac396a5685b5c9d Mon Sep 17 00:00:00 2001
From: Andrew Johnson <anj@aps.anl.gov>
Date: Tue, 1 Mar 2011 15:03:33 -0600
Subject: [PATCH] libCom: Fix msgbufGetNode() in errlog.c

Systems that generate large numbers of errlog messages or have a slow
message listener could overwrite older messages in the message buffer
after the buffer wraps.  This also corrects and annotates the test
code to describe what's being checked.
---
 src/libCom/error/errlog.c         |  6 ++++--
 src/libCom/test/epicsErrlogTest.c | 20 +++++++++++---------
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/src/libCom/error/errlog.c b/src/libCom/error/errlog.c
index 36399a119..322eb9c4f 100644
--- a/src/libCom/error/errlog.c
+++ b/src/libCom/error/errlog.c
@@ -534,8 +534,10 @@ static msgNode *msgbufGetNode(void)
         char *plimit = pbuffer + pvtData.buffersize;
 
         pnextFree = plast->message + adjustToWorstCaseAlignment(plast->length);
-        if (pfirst <= plast &&
-            pnextFree + pvtData.msgNeeded > plimit) {
+        if (pfirst > plast) {
+            plimit = (char *)pfirst;
+        }
+        else if (pnextFree + pvtData.msgNeeded > plimit) {
             pnextFree = pbuffer;    /* Hit end, wrap to start */
             plimit = (char *)pfirst;
         }
diff --git a/src/libCom/test/epicsErrlogTest.c b/src/libCom/test/epicsErrlogTest.c
index c96013313..08389e438 100644
--- a/src/libCom/test/epicsErrlogTest.c
+++ b/src/libCom/test/epicsErrlogTest.c
@@ -260,29 +260,31 @@ MAIN(epicsErrlogTest)
     for (i = 0; i < N; i++) {
         errlogPrintfNoConsole(msg);
     }
+    epicsThreadSleep(0.1); /* should really be a second Event */
 
     testOk1(pvt.count == 0);
 
-    epicsThreadSleep(0.1); /* should really be a second Event */
-
-    pvt.jam = -2; /* Block before #th message */
+    /* Extract the first 2 messages, 2*(sizeof(msgNode) + 128) bytes */
+    pvt.jam = -2;
     epicsEventSignal(pvt.jammer);
     epicsThreadSleep(0.1);
 
-    testDiag("Drain %u messages", pvt.count);
+    testDiag("Drained %u messages", pvt.count);
     testOk1(pvt.count == 2);
 
-    testDiag("Add two more (%d total)", (int) N+2);
-    errlogPrintfNoConsole(msg);
+    /* The buffer has space for 1 more message: sizeof(msgNode) + 256 bytes */
+    errlogPrintfNoConsole(msg); /* Use up that space */
+
+    testDiag("Overflow the buffer");
     errlogPrintfNoConsole(msg);
 
     testOk1(pvt.count == 2);
 
-    epicsEventSignal(pvt.jammer);
+    epicsEventSignal(pvt.jammer); /* Empty */
     errlogFlush();
-    testDiag("Logged %u messages", pvt.count);
 
-    testOk1(pvt.count == N+2);
+    testDiag("Logged %u messages", pvt.count);
+    testOk1(pvt.count == N+1);
 
     /* Clean up */
     errlogRemoveListener(&logClient);