epicsStdio.c allow stderr from inside epicsThreadInit()

Circular initialization problem that Murali discovered when you
try to fprintf(stderr) from the posix/osdThread.c once() routine.
This change lets the stderr stream through in that case.

src/libCom/osi/epicsStdio.c

@@ -27,7 +27,7 @@
 static epicsThreadOnceId onceId = EPICS_THREAD_ONCE_INIT;
 static epicsThreadPrivateId stdinThreadPrivateId;
 static epicsThreadPrivateId stdoutThreadPrivateId;
-static epicsThreadPrivateId stderrThreadPrivateId;
+static epicsThreadPrivateId stderrThreadPrivateId = 0;
 
 static void once(void *junk)
 {
@@ -39,21 +39,27 @@ static void once(void *junk)
 FILE * epicsShareAPI epicsGetStdin(void)
 {
     FILE *fp = epicsGetThreadStdin();
-    if(!fp) fp = stdin;
+
+    if (!fp)
+        fp = stdin;
     return fp;
 }
 
 FILE * epicsShareAPI epicsGetStdout(void)
 {
     FILE *fp = epicsGetThreadStdout();
-    if(!fp) fp = stdout;
+
+    if (!fp)
+        fp = stdout;
     return fp;
 }
 
 FILE * epicsShareAPI epicsGetStderr(void)
 {
     FILE *fp = epicsGetThreadStderr();
-    if(!fp) fp = stderr;
+
+    if (!fp)
+        fp = stderr;
     return fp;
 }
 
@@ -71,7 +77,14 @@ FILE * epicsShareAPI epicsGetThreadStdout(void)
 
 FILE * epicsShareAPI epicsGetThreadStderr(void)
 {
-    epicsThreadOnce(&onceId,once,0);
+    /* Deliberately don't do the epicsThreadOnce() here; epicsThreadInit()
+     * is allowed to use stderr inside its once() routine, in which case we
+     * must return the OS's stderr instead. There may be a tiny chance of a
+     * race happening here during initialization for some architectures, so
+     * we only use it for stderr to reduce the chance of that happening.
+     */
+    if (!stderrThreadPrivateId)
+        return NULL;
     return epicsThreadPrivateGet(stderrThreadPrivateId);
 }