add resizeable scratch space API

Alignment support by Pieter Wuille.

src/scratch.h

@@ -0,0 +1,35 @@
+/**********************************************************************
+ * Copyright (c) 2017 Andrew Poelstra	                              *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_SCRATCH_
+#define _SECP256K1_SCRATCH_
+
+/* The typedef is used internally; the struct name is used in the public API
+ * (where it is exposed as a different typedef) */
+typedef struct secp256k1_scratch_space_struct {
+    void *data;
+    size_t offset;
+    size_t init_size;
+    size_t max_size;
+    const secp256k1_callback* error_callback;
+} secp256k1_scratch;
+
+static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t init_size, size_t max_size);
+static void secp256k1_scratch_destroy(secp256k1_scratch* scratch);
+
+/** Returns the maximum allocation the scratch space will allow */
+static size_t secp256k1_scratch_max_allocation(const secp256k1_scratch* scratch, size_t n_objects);
+
+/** Attempts to allocate so that there are `n` available bytes. Returns 1 on success, 0 on failure */
+static int secp256k1_scratch_resize(secp256k1_scratch* scratch, size_t n, size_t n_objects);
+
+/** Returns a pointer into the scratch space or NULL if there is insufficient available space */
+static void *secp256k1_scratch_alloc(secp256k1_scratch* scratch, size_t n);
+
+/** Resets the returned pointer to the beginning of space */
+static void secp256k1_scratch_reset(secp256k1_scratch* scratch);
+
+#endif

src/scratch_impl.h

@@ -0,0 +1,77 @@
+/**********************************************************************
+ * Copyright (c) 2017 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_SCRATCH_IMPL_H_
+#define _SECP256K1_SCRATCH_IMPL_H_
+
+#include "scratch.h"
+
+/* Using 16 bytes alignment because common architectures never have alignment
+ * requirements above 8 for any of the types we care about. In addition we
+ * leave some room because currently we don't care about a few bytes.
+ * TODO: Determine this at configure time. */
+#define ALIGNMENT 16
+
+static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t init_size, size_t max_size) {
+    secp256k1_scratch* ret = (secp256k1_scratch*)checked_malloc(error_callback, sizeof(*ret));
+    if (ret != NULL) {
+        ret->data = checked_malloc(error_callback, init_size);
+        if (ret->data == NULL) {
+            free (ret);
+            return NULL;
+        }
+        ret->offset = 0;
+        ret->init_size = init_size;
+        ret->max_size = max_size;
+        ret->error_callback = error_callback;
+    }
+    return ret;
+}
+
+static void secp256k1_scratch_destroy(secp256k1_scratch* scratch) {
+    if (scratch != NULL) {
+        free(scratch->data);
+        free(scratch);
+    }
+}
+
+static size_t secp256k1_scratch_max_allocation(const secp256k1_scratch* scratch, size_t objects) {
+    if (scratch->max_size <= objects * ALIGNMENT) {
+        return 0;
+    }
+    return scratch->max_size - objects * ALIGNMENT;
+}
+
+static int secp256k1_scratch_resize(secp256k1_scratch* scratch, size_t n, size_t objects) {
+    n += objects * ALIGNMENT;
+    if (n > scratch->init_size && n <= scratch->max_size) {
+        void *tmp = checked_realloc(scratch->error_callback, scratch->data, n);
+        if (tmp == NULL) {
+            return 0;
+        }
+        scratch->init_size = n;
+        scratch->data = tmp;
+    }
+    return n <= scratch->max_size;
+}
+
+static void *secp256k1_scratch_alloc(secp256k1_scratch* scratch, size_t size) {
+    void *ret;
+    size = ((size + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
+    if (size + scratch->offset > scratch->init_size) {
+        return NULL;
+    }
+    ret = (void *) ((unsigned char *) scratch->data + scratch->offset);
+    memset(ret, 0, size);
+    scratch->offset += size;
+    return ret;
+}
+
+static void secp256k1_scratch_reset(secp256k1_scratch* scratch) {
+    scratch->offset = 0;
+}
+
+#endif

src/secp256k1.c

@@ -17,6 +17,7 @@
 #include "ecdsa_impl.h"
 #include "eckey_impl.h"
 #include "hash_impl.h"
+#include "scratch_impl.h"
 
 #define ARG_CHECK(cond) do { \
     if (EXPECT(!(cond), 0)) { \
@@ -114,6 +115,17 @@ void secp256k1_context_set_error_callback(secp256k1_context* ctx, void (*fun)(co
     ctx->error_callback.data = data;
 }
 
+secp256k1_scratch_space* secp256k1_scratch_space_create(const secp256k1_context* ctx, size_t init_size, size_t max_size) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(max_size >= init_size);
+
+    return secp256k1_scratch_create(&ctx->error_callback, init_size, max_size);
+}
+
+void secp256k1_scratch_space_destroy(secp256k1_scratch_space* scratch) {
+    secp256k1_scratch_destroy(scratch);
+}
+
 static int secp256k1_pubkey_load(const secp256k1_context* ctx, secp256k1_ge* ge, const secp256k1_pubkey* pubkey) {
     if (sizeof(secp256k1_ge_storage) == 64) {
         /* When the secp256k1_ge_storage type is exactly 64 byte, use its

src/tests.c

@@ -248,6 +248,41 @@ void run_context_tests(void) {
     secp256k1_context_destroy(NULL);
 }
 
+void run_scratch_tests(void) {
+    int32_t ecount = 0;
+    secp256k1_context *none = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
+    secp256k1_scratch_space *scratch;
+
+    /* Test public API */
+    secp256k1_context_set_illegal_callback(none, counting_illegal_callback_fn, &ecount);
+    scratch = secp256k1_scratch_space_create(none, 100, 10);
+    CHECK(scratch == NULL);
+    CHECK(ecount == 1);
+
+    scratch = secp256k1_scratch_space_create(none, 100, 100);
+    CHECK(scratch != NULL);
+    CHECK(ecount == 1);
+    secp256k1_scratch_space_destroy(scratch);
+
+    scratch = secp256k1_scratch_space_create(none, 100, 1000);
+    CHECK(scratch != NULL);
+    CHECK(ecount == 1);
+
+    /* Test internal API */
+    CHECK(secp256k1_scratch_max_allocation(scratch, 0) == 1000);
+    CHECK(secp256k1_scratch_max_allocation(scratch, 1) < 1000);
+    CHECK(secp256k1_scratch_resize(scratch, 50, 1) == 1);  /* no-op */
+    CHECK(secp256k1_scratch_resize(scratch, 200, 1) == 1);
+    CHECK(secp256k1_scratch_resize(scratch, 950, 1) == 1);
+    CHECK(secp256k1_scratch_resize(scratch, 1000, 1) == 0);
+    CHECK(secp256k1_scratch_resize(scratch, 2000, 1) == 0);
+    CHECK(secp256k1_scratch_max_allocation(scratch, 0) == 1000);
+
+    /* cleanup */
+    secp256k1_scratch_space_destroy(scratch);
+    secp256k1_context_destroy(none);
+}
+
 /***** HASH TESTS *****/
 
 void run_sha256_tests(void) {
@@ -4451,6 +4486,7 @@ int main(int argc, char **argv) {
 
     /* initialize */
     run_context_tests();
+    run_scratch_tests();
     ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
     if (secp256k1_rand_bits(1)) {
         secp256k1_rand256(run32);

src/util.h

@@ -76,6 +76,14 @@ static SECP256K1_INLINE void *checked_malloc(const secp256k1_callback* cb, size_
     return ret;
 }
 
+static SECP256K1_INLINE void *checked_realloc(const secp256k1_callback* cb, void *ptr, size_t size) {
+    void *ret = realloc(ptr, size);
+    if (ret == NULL) {
+        secp256k1_callback_call(cb, "Out of memory");
+    }
+    return ret;
+}
+
 /* Macro for restrict, when available and not in a VERIFY build. */
 #if defined(SECP256K1_BUILD) && defined(VERIFY)
 # define SECP256K1_RESTRICT