Fix nasa#1429, adjust pthread stack to account for TCB+TLS

The glibc pthread implementation puts additional data structures at the
base of the stack.  The size of these is not known, but the only thing
to go by is PTHREAD_MIN_STACK -- which should always be enough to hold
the required objects.

Instead of simply assuring that the stack is at least PTHREAD_MIN_STACK,
add this to the requested stack instead.  This in turn will ensure that
the user always has at least their requested stack size available for
actual use.

If the pthread implementation does not advertise a PTHREAD_MIN_STACK
value, then just reserve 1 extra page.

src/os/posix/src/os-impl-tasks.c

@@ -37,10 +37,26 @@
 #include "os-shared-idmap.h"
 
 /*
- * Defines
+ * Extra Stack Space for overhead -
+ *
+ * glibc pthreads implicitly puts its TCB/TLS structures at the base of the stack.
+ * The actual size of these structures is highly system and configuration dependent.
+ * Experimentation/measurement on an x64-64 system with glibc shows this to consume
+ * between 9-10kB of stack space off the top.
+ *
+ * There does not seem to be a reliable/standardized way of determining how large this
+ * is going to be, but PTHREAD_STACK_MIN (if defined) should include adequate space for it.
+ * If this is not defined, then just assume 1 page.
+ *
+ * Importantly - when the user passes a stack size to OS_TaskCreate() - the expectation is
+ * that there will be at least this amount of real _usable_ space for the task.  If 10kB
+ * is used before the entry point is even called, this needs to be accounted for, or else
+ * the stack might end up being too small.
  */
-#ifndef PTHREAD_STACK_MIN
-#define PTHREAD_STACK_MIN (8 * 1024)
+#ifdef PTHREAD_STACK_MIN
+#define OS_IMPL_STACK_EXTRA PTHREAD_STACK_MIN
+#else
+#define OS_IMPL_STACK_EXTRA POSIX_GlobalVars.PageSize
 #endif
 
 /* Tables where the OS object information is stored */
@@ -451,20 +467,9 @@ int32 OS_Posix_InternalTaskCreate_Impl(pthread_t *pthr, osal_priority_t priority
     }
 
     /*
-     * Adjust the stack size parameter.
-     *
-     * POSIX has additional restrictions/limitations on the stack size of tasks that
-     * other RTOS environments may not have.  Specifically POSIX says that the stack
-     * size must be at least PTHREAD_STACK_MIN and may also need to be a multiple of the
-     * system page size.
-     *
-     * Rounding up means the user might get a bigger stack than they requested, but
-     * that should not break anything aside from consuming extra memory.
+     * Adjust the stack size parameter, add budget for TCB/TLS overhead.
      */
-    if (stacksz < PTHREAD_STACK_MIN)
-    {
-        stacksz = PTHREAD_STACK_MIN;
-    }
+    stacksz += OS_IMPL_STACK_EXTRA;
 
     stacksz += POSIX_GlobalVars.PageSize - 1;
     stacksz -= stacksz % POSIX_GlobalVars.PageSize;