Fix bug and handle more errors

Fix bug where mprotect calls don't match the arena cap increase
(https://github.com/olemorud/arena-allocator/issues/1)

Handle syscall errors in arena_new()

Check for syscall errors in arena_delete() and switch from void to int
to return error status.

include/arena.h

@@ -15,14 +15,16 @@ typedef struct arena {
 /**
  * Allocate a new arena.
  * The underlying memory is allocated with mmap.
+ * Errors can be checked with `arena_new_failed()`
  */
 arena_t arena_new();
 
 /**
  * Delete memory mapped for arena.
  * Should only be used with arenas from arena_new().
+ * Returns 0 on success, -1 on failure
  */
-void arena_delete(arena_t *a);
+int arena_delete(arena_t *a);
 
 /**
  * Attach an arena to an existing memory region.
@@ -41,10 +43,21 @@ static inline void *arena_detatch(arena_t arena)
     return arena.data;
 }
 
+/**
+ * Returns true if creating new arena failed
+ */
+static inline bool arena_new_failed(arena_t *a)
+{
+    return a->data == NULL;
+}
+
 /**
  * Reset an arena.
  */
-void arena_reset(arena_t *a);
+static inline void arena_reset(arena_t *a)
+{
+    a->size = 0;
+}
 
 /**
  * Allocate memory from an arena.

src/arena.c

@@ -10,6 +10,7 @@
 
 #define ARENA_ALIGN (sizeof(void *))
 #define ARENA_GROW_FACTOR 2UL
+#define ARENA_OVERCOMMIT_SIZE (1UL << 40UL)
 
 #ifndef NDEBUG
 #define arena_err(msg) \
@@ -18,53 +19,74 @@
 #define arena_err(msg)
 #endif
 
-static bool arena_grow(struct arena *a)
+static bool arena_grow(struct arena *a, size_t min_size)
 {
+    size_t new_cap = a->cap * 2;
+    while (new_cap < min_size) {
+        new_cap *= 2;
+    }
+
     int ok = mprotect(
         a->data + a->cap,
-        a->cap * ARENA_GROW_FACTOR,
-        PROT_READ | PROT_WRITE);
+        new_cap - a->cap,
+        PROT_READ | PROT_WRITE
+    );
 
     if (ok == -1) {
         arena_err("mprotect");
         return false;
     }
 
-    a->cap *= ARENA_GROW_FACTOR;
+    a->cap = new_cap;
 
     return true;
 }
 
-inline void arena_reset(arena_t *a)
-{
-    a->size = 0;
-}
 
 arena_t arena_new()
 {
     size_t size = sysconf(_SC_PAGE_SIZE);
-    void  *p    = mmap(NULL, 1UL << 40UL, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
-    mprotect(p, size, PROT_READ | PROT_WRITE);
-    if (p == MAP_FAILED)
+    if (size == -1) {
+        arena_err("sysconf");
+        goto sysconf_failed;
+    }
+
+    void *p = mmap(NULL, ARENA_OVERCOMMIT_SIZE, PROT_NONE,
+            MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    if (p == MAP_FAILED) {
         arena_err("mmap");
+        goto mmap_failed;
+    }
+
+    int ok = mprotect(p, size, PROT_READ | PROT_WRITE);
+    if (ok == -1) {
+        arena_err("mprotect");
+        goto mprotect_failed;
+    }
 
     return arena_attach(p, size);
+    
+mprotect_failed:
+    munmap(p, ARENA_OVERCOMMIT_SIZE);
+mmap_failed:
+sysconf_failed:
+    return (arena_t) { 0 };
 }
 
-void *arena_alloc(arena_t *a, size_t size)
+void* arena_alloc(arena_t *a, size_t size)
 {
     size = (size + ARENA_ALIGN - 1) & ~(ARENA_ALIGN - 1); // align
 
     void *p = a->data + a->size;
     a->size += size;
     if (a->size > a->cap) {
-        if (!arena_grow(a))
+        if (!arena_grow(a, a->size))
             return NULL;
     }
     return p;
 }
 
-void *arena_calloc(arena_t *a, size_t nmemb, size_t size)
+void* arena_calloc(arena_t *a, size_t nmemb, size_t size)
 {
     void *p = arena_alloc(a, nmemb * size);
     if (p == NULL)
@@ -73,9 +95,14 @@ void *arena_calloc(arena_t *a, size_t nmemb, size_t size)
     return p;
 }
 
-void arena_delete(struct arena *a)
+int arena_delete(struct arena *a)
 {
-    munmap(a->data, a->cap);
+    int ok = munmap(a->data, a->cap);
+    if (ok == -1) {
+        arena_err("munmap");
+        return -1;
+    }
     a->cap  = -1;
     a->size = -1;
+    return 0;
 }