os: Don't crash in AttendClient if the client is gone

If a client is in the process of being closed down, then its client->osPrivate
pointer will be set to NULL by CloseDownConnection. This can cause a crash if
freeing the client's resources results in a call to AttendClient. For example,
if the client has a pending sync fence:

 Thread 1 "X" received signal SIGSEGV, Segmentation fault.
 AttendClient (client=0x5571c4aed9a0) at ../os/connection.c:942
 (gdb) bt
 #0  AttendClient (client=0x5571c4aed9a0) at ../os/connection.c:942
 #1  0x00005571c3dbb865 in SyncAwaitTriggerFired (pTrigger=<optimized out>) at ../Xext/sync.c:694
 #2  0x00005571c3dd5749 in miSyncDestroyFence (pFence=0x5571c5063980) at ../miext/sync/misync.c:120
 #3  0x00005571c3dbbc69 in FreeFence (obj=<optimized out>, id=<optimized out>) at ../Xext/sync.c:1909
 #4  0x00005571c3d7a01d in doFreeResource (res=0x5571c506e3d0, skip=skip@entry=0) at ../dix/resource.c:880
 #5  0x00005571c3d7b1dc in FreeClientResources (client=0x5571c4aed9a0) at ../dix/resource.c:1146
 #6  FreeClientResources (client=0x5571c4aed9a0) at ../dix/resource.c:1109
 #7  0x00005571c3d5525f in CloseDownClient (client=0x5571c4aed9a0) at ../dix/dispatch.c:3473
 #8  0x00005571c3d55eeb in Dispatch () at ../dix/dispatch.c:492
 #9  0x00005571c3d59e96 in dix_main (argc=3, argv=0x7ffe7854bc28, envp=<optimized out>) at ../dix/main.c:276
 #10 0x00007fea4837cb6b in __libc_start_main (main=0x5571c3d1d060 <main>, argc=3, argv=0x7ffe7854bc28, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffe7854bc18) at ../csu/libc-start.c:308
 #11 0x00005571c3d1d09a in _start () at ../Xext/sync.c:2378
 (gdb) print client->osPrivate
 $1 = (void *) 0x0

Since the client is about to be freed, its ignore count doesn't matter and
AttendClient can simply be a no-op. Check for client->clientGone in AttendClient
and remove similar checks from two callers that had them.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
(cherry picked from commit 4308f5d3d1fbd0f5dce81e22c0c3f08c65a7c9d8)

4 files changed, 12 insertions, 12 deletions

diff --git a/Xext/sleepuntil.c b/Xext/sleepuntil.c
index 68a7a9bee..334c2784a 100644
--- a/Xext/sleepuntil.c
+++ b/Xext/sleepuntil.c
@@ -125,8 +125,7 @@ ClientSleepUntil(ClientPtr client,
 static void
 ClientAwaken(ClientPtr client, void *closure)
 {
-    if (!client->clientGone)
-        AttendClient(client);
+    AttendClient(client);
 }
 
 static int
diff --git a/dix/dixutils.c b/dix/dixutils.c
index 2983174a1..331ccfb96 100644
--- a/dix/dixutils.c
+++ b/dix/dixutils.c
@@ -662,14 +662,7 @@ ClientWakeup(ClientPtr client)
         if (q->client == client) {
             *prev = q->next;
             free(q);
-            if (client->clientGone)
-                /* Oops -- new zombie cleanup code ensures this only
-                 * happens from inside CloseDownClient; don't want to
-                 * recurse here...
-                 */
-                /* CloseDownClient(client) */ ;
-            else
-                AttendClient(client);
+            AttendClient(client);
             break;
         }
         prev = &q->next;
diff --git a/os/connection.c b/os/connection.c
index 229bbe745..32d2cda2a 100644
--- a/os/connection.c
+++ b/os/connection.c
@@ -935,6 +935,14 @@ AttendClient(ClientPtr client)
 {
     OsCommPtr oc = (OsCommPtr) client->osPrivate;
 
+    if (client->clientGone) {
+        /*
+         * client is gone, so any pending requests will be dropped and its
+         * ignore count doesn't matter.
+         */
+        return;
+    }
+
     client->ignoreCount--;
     if (client->ignoreCount)
         return;
diff --git a/record/record.c b/record/record.c
index fdcee7e00..f0b739b0c 100644
--- a/record/record.c
+++ b/record/record.c
@@ -2362,9 +2362,9 @@ RecordDisableContext(RecordContextPtr pContext)
     if (!pContext->pRecordingClient->clientGone) {
         RecordAProtocolElement(pContext, NULL, XRecordEndOfData, NULL, 0, 0, 0);
         RecordFlushReplyBuffer(pContext, NULL, 0, NULL, 0);
-        /* Re-enable request processing on this connection. */
-        AttendClient(pContext->pRecordingClient);
     }
+    /* Re-enable request processing on this connection. */
+    AttendClient(pContext->pRecordingClient);
 
     for (pRCAP = pContext->pListOfRCAP; pRCAP; pRCAP = pRCAP->pNextRCAP) {
         RecordUninstallHooks(pRCAP, 0);