Rudimentary CPython 3.7 pool allocator canary implementation - useful for debugging python C/Cython extensions

ris-cpython-3.7-pool-canary.patch

:100644 100644 1248a3937e... 0000000000... M	Objects/obmalloc.c

diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 1248a3937e..29ad3081b6 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1402,10 +1402,12 @@ pymalloc_alloc(void *ctx, size_t nbytes)
     if (nbytes == 0) {
         return NULL;
     }
+    nbytes++;
     if (nbytes > SMALL_REQUEST_THRESHOLD) {
         return NULL;
     }
@@ -1564,6 +1566,7 @@ pymalloc_alloc(void *ctx, size_t nbytes)
 success:
     UNLOCK();
     assert(bp != NULL);
+    bp[INDEX2SIZE((uint)(nbytes - 1) >> ALIGNMENT_SHIFT)-1] = (uint8_t)0xca;
     return (void *)bp;
 
 failed:
@@ -1635,6 +1638,19 @@ pymalloc_free(void *ctx, void *p)
     }
     /* We allocated this address. */
 
+    int canary_offset = INDEX2SIZE(pool->szidx)-1;
+    uint8_t canary_value = ((uint8_t *)p)[canary_offset];
+    if(__builtin_expect(canary_value != 0xca, 0)) {
+        fprintf(stderr, "Canary fail for block @ p=%p, value @ %p was 0x%02x\n", p, ((uint8_t *)p) + canary_offset, canary_value);
+        if(canary_value == 0xd0) {
+            fprintf(stderr, "Possible double-free?\n");
+        }
+        // deliberate segfault
+        *((uint8_t *)0) = 123;
+    } else {
+        ((uint8_t *)p)[INDEX2SIZE(pool->szidx)-1] = 0xd0;
+    }
+
     LOCK();
 
     /* Link p to the start of the pool's freeblock list.  Since
@@ -1886,7 +1902,7 @@ pymalloc_realloc(void *ctx, void **newptr_p, void *p, size_t nbytes)
 
     /* pymalloc is in charge of this block */
     size = INDEX2SIZE(pool->szidx);
-    if (nbytes <= size) {
+    if (nbytes + 1 <= size) {
         /* The block is staying the same or shrinking.
 
            If it's shrinking, there's a tradeoff: it costs cycles to copy the