Force DAG file creation if file already existed but with wrong size

internal.c

@@ -373,6 +373,24 @@ ethash_cache_t *ethash_light_acquire_cache(ethash_light_t light)
 	return ret;
 }
 
+static bool ethash_mmap(struct ethash_full* ret, FILE* f)
+{
+	int fd;
+	ret->file = f;
+	if ((fd = ethash_fileno(ret->file)) == -1) {
+		return false;
+	}
+	ret->data = mmap(
+		NULL,
+		(size_t)ret->file_size,
+		PROT_READ | PROT_WRITE,
+		MAP_SHARED,
+		fd,
+		0
+	);
+	return ret->data != MAP_FAILED;
+}
+
 ethash_full_t ethash_full_new(
 	char const* dirname,
 	ethash_h256_t const* seed_hash,
@@ -382,39 +400,30 @@ ethash_full_t ethash_full_new(
 )
 {
 	struct ethash_full* ret;
-	int fd;
 	FILE *f = NULL;
-	bool match = false;
 	ret = calloc(sizeof(*ret), 1);
 	if (!ret) {
 		return NULL;
 	}
 	ret->file_size = (size_t)full_size;
-	switch (ethash_io_prepare(dirname, *seed_hash, &f, (size_t)full_size)) {
+	switch (ethash_io_prepare(dirname, *seed_hash, &f, (size_t)full_size, false)) {
 	case ETHASH_IO_FAIL:
-	case ETHASH_IO_MEMO_SIZE_MISMATCH:
 		goto fail_free_full;
 	case ETHASH_IO_MEMO_MATCH:
-		match = true;
-	case ETHASH_IO_MEMO_MISMATCH:
-		ret->file = f;
-		if ((fd = ethash_fileno(ret->file)) == -1) {
+		if (!ethash_mmap(ret, f)) {
+			goto fail_close_file;
+		}
+		return ret;
+	case ETHASH_IO_MEMO_SIZE_MISMATCH:
+		// if a DAG of same filename but unexpected size is found, silently force new file creation
+		if (ethash_io_prepare(dirname, *seed_hash, &f, (size_t)full_size, true) != ETHASH_IO_MEMO_MISMATCH) {
 			goto fail_free_full;
 		}
-		ret->data = mmap(
-			NULL,
-			(size_t)full_size,
-			PROT_READ | PROT_WRITE,
-			MAP_SHARED,
-			fd,
-			0
-		);
-		if (ret->data == MAP_FAILED) {
+		// fallthrough to the mismatch case here, DO NOT go through match
+	case ETHASH_IO_MEMO_MISMATCH:
+		if (!ethash_mmap(ret, f)) {
 			goto fail_close_file;
 		}
-		if (match) {
-			return ret;
-		}
 		break;
 	}
 

io.c

@@ -26,7 +26,8 @@ enum ethash_io_rc ethash_io_prepare(
 	char const* dirname,
 	ethash_h256_t const seedhash,
 	FILE** output_file,
-	size_t file_size
+	size_t file_size,
+	bool force_create
 )
 {
 	char mutable_name[DAG_MUTABLE_NAME_MAX_SIZE];
@@ -43,35 +44,40 @@ enum ethash_io_rc ethash_io_prepare(
 		goto end;
 	}
 
-	// try to open the file
-	FILE* f = ethash_fopen(tmpfile, "rb+");
-	if (f) {
-		size_t found_size;
-		if (!ethash_file_size(f, &found_size)) {
-			fclose(f);
-			goto free_memo;
+	FILE *f;
+	if (!force_create) {
+		// try to open the file
+		f = ethash_fopen(tmpfile, "rb+");
+		if (f) {
+			size_t found_size;
+			if (!ethash_file_size(f, &found_size)) {
+				fclose(f);
+				goto free_memo;
+			}
+			if (file_size != found_size) {
+				fclose(f);
+				ret = ETHASH_IO_MEMO_SIZE_MISMATCH;
+				goto free_memo;
+			}
+			ret = ETHASH_IO_MEMO_MATCH;
+			goto set_file;
 		}
-		if (file_size != found_size) {
-			fclose(f);
-			ret = ETHASH_IO_MEMO_SIZE_MISMATCH;
-			goto free_memo;
-		}
-	} else {
-		// file does not exist, will need to be created
-		f = ethash_fopen(tmpfile, "wb+");
-		if (!f) {
-			goto free_memo;
-		}
-		// make sure it's of the proper size
-		if (fseek(f, file_size - 1, SEEK_SET) != 0) {
-			fclose(f);
-			goto free_memo;
-		}
-		fputc('\n', f);
-		fflush(f);
-		ret = ETHASH_IO_MEMO_MISMATCH;
-		goto set_file;
 	}
+	
+	// file does not exist, will need to be created
+	f = ethash_fopen(tmpfile, "wb+");
+	if (!f) {
+		goto free_memo;
+	}
+	// make sure it's of the proper size
+	if (fseek(f, file_size - 1, SEEK_SET) != 0) {
+		fclose(f);
+		goto free_memo;
+	}
+	fputc('\n', f);
+	fflush(f);
+	ret = ETHASH_IO_MEMO_MISMATCH;
+	goto set_file;
 
 	ret = ETHASH_IO_MEMO_MATCH;
 set_file:

io.h

@@ -69,13 +69,16 @@ enum ethash_io_rc {
  *                           mode, while on the case of mismatch a new file is created
  *                           on write mode
  * @param[in] file_size      The size that the DAG file should have on disk
+ * @param[out] force_create  If true then there is no check to see if the file
+ *                           already exists
  * @return                   For possible return values @see enum ethash_io_rc
  */
 enum ethash_io_rc ethash_io_prepare(
 	char const* dirname,
 	ethash_h256_t const seedhash,
 	FILE** output_file,
-	size_t file_size
+	size_t file_size,
+	bool force_create
 );
 
 /**