stack-limit: impose minimum limit.

* gc.c (MIN_STACK_LIMIT): New preprocessor symbol.
(gc_init): If the system stack limit is too low, don't
treat that the same way as a missing or unlimited limit.
Instead clamp to the minimum value and hope for the best.
So that is to say, the system limit cannot be used as a
mechanism to set a ridiculously low stack limit in TXR; the
only way to do that is to use the set-stack-limit function.

* txr.1: Documentation updated. Also fixed 326767 typo which
should be 32767.

2 files changed, 18 insertions, 8 deletions

diff --git a/gc.c b/gc.c
index 924d7d65..d29c3596 100644
--- a/gc.c
+++ b/gc.c
@@ -28,6 +28,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stddef.h>
 #include <assert.h>
 #include <wchar.h>
 #include <signal.h>
@@ -68,6 +69,8 @@
 #define DFL_STACK_LIMIT         (16384 * 1024L)
 #endif
 
+#define MIN_STACK_LIMIT         32768
+
 #if HAVE_MEMALIGN || HAVE_POSIX_MEMALIGN
 #define OBJ_ALIGN (sizeof (obj_t))
 #else
@@ -899,9 +902,12 @@ void gc_init(val *stack_bottom)
 #if HAVE_RLIMIT
   struct rlimit rl;
   if (getrlimit(RLIMIT_STACK, &rl) == 0) {
-    if (rl.rlim_cur != RLIM_INFINITY && rl.rlim_cur > 512 * 1024) {
-      rlim_t lim = (rl.rlim_cur - rl.rlim_cur / 16) / sizeof (val);
-      gc_stack_limit = gc_stack_bottom - lim;
+    rlim_t lim = rl.rlim_cur;
+    if (lim != RLIM_INFINITY) {
+      ptrdiff_t delta = (lim >= MIN_STACK_LIMIT
+                         ? (lim - lim / 16)
+                         : MIN_STACK_LIMIT) / sizeof (val);
+      gc_stack_limit = gc_stack_bottom - delta;
     }
   }
 #endif
diff --git a/txr.1 b/txr.1
index d963097d..f94972f4 100644
--- a/txr.1
+++ b/txr.1
@@ -72734,11 +72734,15 @@ derived from
 .codn error ,
 is thrown.
 
-The stack overflow guard mechanism is enabled at startup on those platforms
-where it is possible to inquire the system about the stack limit, and where the
-stack limit is at least 512 kilobytes.  \*(TX configures the limit to within a
+The stack overflow guard mechanism is configured on start-up.
+On platforms on those platforms where it is possible to
+inquire the system about the actual stack limit, and where the stack limit is
+at least 512 kilobytes, \*(TX sets the limit to within a
 certain percentage of the actual value. If it is not possible to determine the
-system's stack limit, or it is too low, then the mechanism is disabled.
+system's stack limit, or if the system indicates that the stack size is
+unlimited, then a default limit is imposed. If the system's limit is
+configured below a certain small value, then that small value is used
+as the stack limit.
 
 The
 .code get-stack-limit
@@ -72794,7 +72798,7 @@ of the stack overflow detection mechanism is compromised.
 Likewise, if
 .meta value
 is too low, the operation of \*(TX shall become unreliable. Values
-smaller than 326767 bytes are strongly discouraged.
+smaller than 32767 bytes are strongly discouraged.
 
 .SS* Modularization
 .coNP Variable @ self-path