Make it possible to peek at BIO data through BIO_f_buffer()

This is needed for the upcoming "file" scheme STORE loader.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3542)

crypto/bio/bf_buff.c

@@ -255,6 +255,11 @@ static long buffer_ctrl(BIO *b, int cmd, long num, void *ptr)
             return (0);
         ret = BIO_ctrl(b->next_bio, cmd, num, ptr);
         break;
+    case BIO_CTRL_EOF:
+        if (ctx->ibuf_len > 0)
+            return 0;
+        ret = BIO_ctrl(b->next_bio, cmd, num, ptr);
+        break;
     case BIO_CTRL_INFO:
         ret = (long)ctx->obuf_len;
         break;
@@ -380,6 +385,17 @@ static long buffer_ctrl(BIO *b, int cmd, long num, void *ptr)
             !BIO_set_write_buffer_size(dbio, ctx->obuf_size))
             ret = 0;
         break;
+    case BIO_CTRL_PEEK:
+        /* Ensure there's stuff in the input buffer */
+        {
+            char fake_buf[1];
+            (void)buffer_read(b, fake_buf, 0);
+        }
+        if (num > ctx->ibuf_len)
+            num = ctx->ibuf_len;
+        memcpy(ptr, &(ctx->ibuf[ctx->ibuf_off]), num);
+        ret = num;
+        break;
     default:
         if (b->next_bio == NULL)
             return (0);

include/openssl/bio.h

@@ -88,6 +88,7 @@ extern "C" {
 # define BIO_CTRL_SET_CALLBACK   14/* opt - set callback function */
 # define BIO_CTRL_GET_CALLBACK   15/* opt - set callback function */
 
+# define BIO_CTRL_PEEK           29/* BIO_f_buffer special */
 # define BIO_CTRL_SET_FILENAME   30/* BIO_s_file special */
 
 /* dgram BIO stuff */
@@ -494,6 +495,7 @@ size_t BIO_ctrl_wpending(BIO *b);
 
 /* For the BIO_f_buffer() type */
 # define BIO_buffer_get_num_lines(b) BIO_ctrl(b,BIO_CTRL_GET,0,NULL)
+# define BIO_buffer_peek(b,s,l) BIO_ctrl(b,BIO_CTRL_PEEK,(l),(s))
 
 /* For BIO_s_bio() */
 # define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL)