snappy: Refactor so ff_snappy_uncompress() uses an existing buffer

Some uses of Snappy require uncompressing to positions within
an existing buffer. Also adds a function to get the uncompressed
length of Snappy data.

libavcodec/hapdec.c

@@ -108,8 +108,13 @@ static int setup_texture(AVCodecContext *avctx, size_t length)
         compressorstr = "none";
         break;
     case HAP_COMP_SNAPPY:
+        snappy_size = ff_snappy_peek_uncompressed_length(gbc);
+        ret = av_reallocp(&ctx->snappied, snappy_size);
+        if (ret < 0) {
+            return ret;
+        }
         /* Uncompress the frame */
-        ret = ff_snappy_uncompress(gbc, &ctx->snappied, &snappy_size);
+        ret = ff_snappy_uncompress(gbc, ctx->snappied, &snappy_size);
         if (ret < 0) {
              av_log(avctx, AV_LOG_ERROR, "Snappy uncompress error\n");
              return ret;

libavcodec/snappy.c

@@ -128,7 +128,17 @@ static int64_t decode_len(GetByteContext *gb)
     return len;
 }
 
-int ff_snappy_uncompress(GetByteContext *gb, uint8_t **buf, int64_t *size)
+int64_t ff_snappy_peek_uncompressed_length(GetByteContext *gb)
+{
+    int pos = bytestream2_get_bytes_left(gb);
+    int64_t len = decode_len(gb);
+
+    bytestream2_seek(gb, -pos, SEEK_END);
+
+    return len;
+}
+
+int ff_snappy_uncompress(GetByteContext *gb, uint8_t *buf, int64_t *size)
 {
     int64_t len = decode_len(gb);
     int ret     = 0;
@@ -137,11 +147,11 @@ int ff_snappy_uncompress(GetByteContext *gb, uint8_t **buf, int64_t *size)
     if (len < 0)
         return len;
 
-    if ((ret = av_reallocp(buf, len)) < 0)
-        return AVERROR(ENOMEM);
+    if (len > *size)
+        return AVERROR_BUG;
 
     *size = len;
-    p     = *buf;
+    p     = buf;
 
     while (bytestream2_get_bytes_left(gb) > 0) {
         uint8_t s = bytestream2_get_byte(gb);
@@ -152,13 +162,13 @@ int ff_snappy_uncompress(GetByteContext *gb, uint8_t **buf, int64_t *size)
             ret = snappy_literal(gb, p, len, val);
             break;
         case SNAPPY_COPY_1:
-            ret = snappy_copy1(gb, *buf, p, len, val);
+            ret = snappy_copy1(gb, buf, p, len, val);
             break;
         case SNAPPY_COPY_2:
-            ret = snappy_copy2(gb, *buf, p, len, val);
+            ret = snappy_copy2(gb, buf, p, len, val);
             break;
         case SNAPPY_COPY_4:
-            ret = snappy_copy4(gb, *buf, p, len, val);
+            ret = snappy_copy4(gb, buf, p, len, val);
             break;
         }
 

libavcodec/snappy.h

@@ -38,14 +38,23 @@
 #include "bytestream.h"
 
 /**
- * Decompress an input buffer using Snappy algorithm. Caller is
- * responsible of freeing the memory allocated in buf.
+ * Get the uncompressed length of an input buffer compressed using the Snappy
+ * algorithm. The GetByteContext is not advanced.
  *
  * @param gb    input GetByteContext.
- * @param buf   output buffer pointer.
- * @param size  output buffer size.
+ * @return      A positive length on success, AVERROR otherwise.
+ */
+ int64_t ff_snappy_peek_uncompressed_length(GetByteContext *gb);
+
+/**
+ * Decompress an input buffer using Snappy algorithm.
+ *
+ * @param gb    input GetByteContext.
+ * @param buf   input buffer pointer.
+ * @param size  input/output on input, the size of buffer, on output, the size
+ *              of the uncompressed data.
  * @return      0 if success, AVERROR otherwise.
  */
-int ff_snappy_uncompress(GetByteContext *gb, uint8_t **buf, int64_t *size);
+int ff_snappy_uncompress(GetByteContext *gb, uint8_t *buf, int64_t *size);
 
 #endif /* AVCODEC_SNAPPY_H */