Merge bitcoin-core/secp256k1#1311: Revert "Remove unused scratch space from API"

3ad1027 Revert "Remove unused scratch space from API" (Jonas Nick)

Pull request description:

  This reverts commit 712e7f8.

  Removing the scratch space from the API may break bindings to the library.

ACKs for top commit:
  sipa:
    ACK 3ad1027
  real-or-random:
    ACK 3ad1027

Tree-SHA512: ad394c0a2f83fe3a5f400c0e8f2b9bf40037ce4141d4414e6345918f5e6003c61da02a538425a49bdeb5700f5ecb713bd58f5752c0715fb1fcc4950099fdc0e6

Author: sipa

include/secp256k1.h

@@ -49,6 +49,19 @@ extern "C" {
  */
 typedef struct secp256k1_context_struct secp256k1_context;
 
+/** Opaque data structure that holds rewritable "scratch space"
+ *
+ *  The purpose of this structure is to replace dynamic memory allocations,
+ *  because we target architectures where this may not be available. It is
+ *  essentially a resizable (within specified parameters) block of bytes,
+ *  which is initially created either by memory allocation or TODO as a pointer
+ *  into some fixed rewritable space.
+ *
+ *  Unlike the context object, this cannot safely be shared between threads
+ *  without additional synchronization logic.
+ */
+typedef struct secp256k1_scratch_space_struct secp256k1_scratch_space;
+
 /** Opaque data structure that holds a parsed and valid public key.
  *
  *  The exact representation of data inside is implementation defined and not
@@ -372,6 +385,29 @@ SECP256K1_API void secp256k1_context_set_error_callback(
     const void *data
 ) SECP256K1_ARG_NONNULL(1);
 
+/** Create a secp256k1 scratch space object.
+ *
+ *  Returns: a newly created scratch space.
+ *  Args: ctx:  an existing context object.
+ *  In:   size: amount of memory to be available as scratch space. Some extra
+ *              (<100 bytes) will be allocated for extra accounting.
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT secp256k1_scratch_space *secp256k1_scratch_space_create(
+    const secp256k1_context *ctx,
+    size_t size
+) SECP256K1_ARG_NONNULL(1);
+
+/** Destroy a secp256k1 scratch space.
+ *
+ *  The pointer may not be used afterwards.
+ *  Args:       ctx: a secp256k1 context object.
+ *          scratch: space to destroy
+ */
+SECP256K1_API void secp256k1_scratch_space_destroy(
+    const secp256k1_context *ctx,
+    secp256k1_scratch_space *scratch
+) SECP256K1_ARG_NONNULL(1);
+
 /** Parse a variable-length public key into the pubkey object.
  *
  *  Returns: 1 if the public key was fully valid.

src/scratch.h

@@ -21,8 +21,6 @@ typedef struct secp256k1_scratch_space_struct {
     size_t max_size;
 } secp256k1_scratch;
 
-typedef struct secp256k1_scratch_space_struct secp256k1_scratch_space;
-
 static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t max_size);
 
 static void secp256k1_scratch_destroy(const secp256k1_callback* error_callback, secp256k1_scratch* scratch);

src/secp256k1.c

@@ -219,12 +219,12 @@ void secp256k1_context_set_error_callback(secp256k1_context* ctx, void (*fun)(co
     ctx->error_callback.data = data;
 }
 
-static secp256k1_scratch_space* secp256k1_scratch_space_create(const secp256k1_context* ctx, size_t max_size) {
+secp256k1_scratch_space* secp256k1_scratch_space_create(const secp256k1_context* ctx, size_t max_size) {
     VERIFY_CHECK(ctx != NULL);
     return secp256k1_scratch_create(&ctx->error_callback, max_size);
 }
 
-static void secp256k1_scratch_space_destroy(const secp256k1_context *ctx, secp256k1_scratch_space* scratch) {
+void secp256k1_scratch_space_destroy(const secp256k1_context *ctx, secp256k1_scratch_space* scratch) {
     VERIFY_CHECK(ctx != NULL);
     secp256k1_scratch_destroy(&ctx->error_callback, scratch);
 }