Fix #525, ensure POSIX stack size meets requirements

The pthread_attr_setstacksize() function stipulates that it
may fail if the user-supplied stack size is not at least
PTHREAD_STACK_MIN and also possibly a multiple of page size.

This partially reverts previous PR #508 and adds back rounding
up to PTHREAD_STACK_MIN and also adds rounding up to a system
page size.  However the check for zero stack still remains
at the shared level so attempts to create a task with zero
stack will still fail.  This allows internal helper threads
to be created with a default minimum stack size, however.

src/os/posix/inc/os-posix.h

@@ -80,6 +80,7 @@ typedef struct
  pthread_key_t ThreadKey;
  sigset_t MaximumSigMask;
  sigset_t NormalSigMask;
+ size_t PageSize;
  POSIX_PriorityLimits_t PriLimits;
  int SelectedRtScheduler;
 } POSIX_GlobalVars_t;

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

@@ -38,6 +38,13 @@
 #include "os-shared-task.h"
 #include "os-shared-idmap.h"
 
+/*
+ * Defines
+ */
+#ifndef PTHREAD_STACK_MIN
+#define PTHREAD_STACK_MIN (8*1024)
+#endif
+
 /* Tables where the OS object information is stored */
 OS_impl_task_internal_record_t OS_impl_task_table [OS_MAX_TASKS];
 
@@ -416,6 +423,8 @@ int32 OS_Posix_TaskAPI_Impl_Init(void)
  }
 #endif
 
+ POSIX_GlobalVars.PageSize = sysconf(_SC_PAGESIZE);
+
  return OS_SUCCESS;
 } /* end OS_Posix_TaskAPI_Impl_Init */
 
@@ -446,14 +455,42 @@ int32 OS_Posix_InternalTaskCreate_Impl(pthread_t *pthr, uint32 priority, size_t
  return(OS_ERROR);
  }
 
-
- /*
- ** Test to see if the original main task scheduling priority worked.
- ** If so, then also set the attributes for this task. Otherwise attributes
- ** are left at default.
+ /*
+ * 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.
  */
- if (POSIX_GlobalVars.EnableTaskPriorities)
- {
+ if (stacksz < PTHREAD_STACK_MIN)
+ {
+ stacksz = PTHREAD_STACK_MIN;
+ }
+
+ stacksz += POSIX_GlobalVars.PageSize - 1;
+ stacksz -= stacksz % POSIX_GlobalVars.PageSize;
+
+ /*
+ ** Set the Stack Size
+ */
+ return_code = pthread_attr_setstacksize(&custom_attr, stacksz);
+ if (return_code != 0)
+ {
+ OS_DEBUG("pthread_attr_setstacksize error in OS_TaskCreate: %s\n",strerror(return_code));
+ return(OS_ERROR);
+ }
+
+ /*
+ ** Test to see if the original main task scheduling priority worked.
+ ** If so, then also set the attributes for this task. Otherwise attributes
+ ** are left at default.
+ */
+ if (POSIX_GlobalVars.EnableTaskPriorities)
+ {
  /*
  ** Set the scheduling inherit attribute to EXPLICIT
  */
@@ -464,15 +501,6 @@ int32 OS_Posix_InternalTaskCreate_Impl(pthread_t *pthr, uint32 priority, size_t
  return(OS_ERROR);
  }
 
- /*
- ** Set the Stack Size
- */
- return_code = pthread_attr_setstacksize(&custom_attr, stacksz);
- if (return_code != 0)
- {
- OS_DEBUG("pthread_attr_setstacksize error in OS_TaskCreate: %s\n",strerror(return_code));
- return(OS_ERROR);
- }
 
  /*
  ** Set the scheduling policy
@@ -503,7 +531,7 @@ int32 OS_Posix_InternalTaskCreate_Impl(pthread_t *pthr, uint32 priority, size_t
  return(OS_ERROR);
  }
 
-  } /* End if user is root */
+ } /* End if user is root */
 
  /*
  ** Create thread