Fixed issue in emubd where noop erases start out-of-sync with disk

Because reproducibility is extremely important, emubd always zeros
blocks on the first erase, even when erase_value=-1.

Well, at least it should be. We were correctly zeroing the blocks in
RAM, but if erase_value=-1 we were leaving the disk unzeroed, causing
the disk to fall out of sync.

Fixed by zeroing disk in lfs_emubd_createcfg, even if erase_value=-1.

Also I went ahead and dropped the bd->disk->scratch block. We're already
allocating RAM-backed blocks on erase anyways, so keeping scratch around
doesn't really gain us anything anymore. Now there is just a temporary
allocation in lfs_emubd_createcfg to zero the disk efficiently during
initialization.

bd/lfs_emubd.c

@@ -138,7 +138,6 @@ int lfs_emubd_createcfg(const struct lfs_config *cfg, const char *path,
             return LFS_ERR_NOMEM;
         }
         bd->disk->rc = 1;
-        bd->disk->scratch = NULL;
 
         #ifdef _WIN32
         bd->disk->fd = open(bd->cfg->disk_path,
@@ -153,31 +152,28 @@ int lfs_emubd_createcfg(const struct lfs_config *cfg, const char *path,
             return err;
         }
 
-        // if we're emulating erase values, we can keep a block around in
+        // go ahead and erase all of the disk, otherwise the file will not
-        // memory of just the erase state to speed up emulated erases
+        // match our internal representation
-        if (bd->cfg->erase_value != -1) {
+        uint8_t *scratch = malloc(cfg->block_size);
-            bd->disk->scratch = malloc(cfg->block_size);
+        if (!scratch) {
-            if (!bd->disk->scratch) {
             LFS_EMUBD_TRACE("lfs_emubd_createcfg -> %d", LFS_ERR_NOMEM);
             return LFS_ERR_NOMEM;
         }
-            memset(bd->disk->scratch,
+        memset(scratch,
-                    bd->cfg->erase_value,
+                (bd->cfg->erase_value != -1) ? bd->cfg->erase_value : 0,
                 cfg->block_size);
 
-            // go ahead and erase all of the disk, otherwise the file will not
-            // match our internal representation
         for (size_t i = 0; i < cfg->block_count; i++) {
-                ssize_t res = write(bd->disk->fd,
+            ssize_t res = write(bd->disk->fd, scratch, cfg->block_size);
-                        bd->disk->scratch,
-                        cfg->block_size);
             if (res < 0) {
                 int err = -errno;
+                free(scratch);
                 LFS_EMUBD_TRACE("lfs_emubd_create -> %d", err);
                 return err;
             }
         }
-        }
+
+        free(scratch);
     }
 
     LFS_EMUBD_TRACE("lfs_emubd_createcfg -> %d", 0);
@@ -216,7 +212,6 @@ int lfs_emubd_destroy(const struct lfs_config *cfg) {
         bd->disk->rc -= 1;
         if (bd->disk->rc == 0) {
             close(bd->disk->fd);
-            free(bd->disk->scratch);
             free(bd->disk);
         }
     }
@@ -418,9 +413,7 @@ int lfs_emubd_erase(const struct lfs_config *cfg, lfs_block_t block) {
                 return err;
             }
 
-            ssize_t res2 = write(bd->disk->fd,
+            ssize_t res2 = write(bd->disk->fd, b->data, cfg->block_size);
-                    bd->disk->scratch,
-                    cfg->block_size);
             if (res2 < 0) {
                 int err = -errno;
                 LFS_EMUBD_TRACE("lfs_emubd_erase -> %d", err);

bd/lfs_emubd.h

@@ -67,9 +67,8 @@ typedef int64_t lfs_emubd_ssleep_t;
 
 // emubd config, this is required for testing
 struct lfs_emubd_config {
-    // 8-bit erase value to use for simulating erases. -1 does not simulate
+    // 8-bit erase value to use for simulating erases. -1 simulates a noop
-    // erases, which can speed up testing by avoiding the extra block-device
+    // erase, which is faster than simulating a fixed erase value.
-    // operations to store the erase value.
     int32_t erase_value;
 
     // Number of erase cycles before a block becomes "bad". The exact behavior
@@ -127,7 +126,6 @@ typedef struct lfs_emubd_block {
 typedef struct lfs_emubd_disk {
     uint32_t rc;
     int fd;
-    uint8_t *scratch;
 } lfs_emubd_disk_t;
 
 // emubd state