Provide a dummy compression-layer skip function which just reads data and

discards it, for use when the compression layer code doesn't know how to
skip data (e.g., everything other than the "none" compressor).  This makes
format level code simpler because that code can now assume that the
compression layer always knows how to skip and will always skip exactly
the requested number of bytes.

Discussed with:	kientzle (3 months ago)

1 files changed, 40 insertions, 0 deletions

diff --git a/lib/libarchive/archive_read.c b/lib/libarchive/archive_read.c
index c92b20c..06a4e8c 100644
--- a/lib/libarchive/archive_read.c
+++ b/lib/libarchive/archive_read.c
@@ -55,6 +55,7 @@ __FBSDID("$FreeBSD$");
 
 static int	choose_decompressor(struct archive_read *, const void*, size_t);
 static int	choose_format(struct archive_read *);
+static off_t	dummy_skip(struct archive_read *, off_t);
 
 /*
  * Allocate, initialize and return a struct archive object.
@@ -191,6 +192,13 @@ archive_read_open2(struct archive *_a, void *client_data,
 	if (e == ARCHIVE_OK)
 		a->archive.state = ARCHIVE_STATE_HEADER;
 
+	/*
+	 * If the decompressor didn't register a skip function, provide a
+	 * dummy compression-layer skip function.
+	 */
+	if (a->compression_skip == NULL)
+		a->compression_skip = dummy_skip;
+
 	return (e);
 }
 
@@ -244,6 +252,38 @@ choose_decompressor(struct archive_read *a,
 }
 
 /*
+ * Dummy skip function, for use if the compression layer doesn't provide
+ * one: This code just reads data and discards it.
+ */
+static off_t
+dummy_skip(struct archive_read * a, off_t request)
+{
+	const void * dummy_buffer;
+	ssize_t bytes_read;
+	off_t bytes_skipped;
+
+	for (bytes_skipped = 0; request > 0;) {
+		bytes_read = (a->compression_read_ahead)(a, &dummy_buffer, 1);
+		if (bytes_read < 0)
+			return (bytes_read);
+		if (bytes_read == 0) {
+			/* Premature EOF. */
+			archive_set_error(&a->archive, ARCHIVE_ERRNO_MISC,
+			    "Truncated input file (need to skip %jd bytes)",
+			    (intmax_t)request);
+			return (ARCHIVE_FATAL);
+		}
+		if (bytes_read > request)
+			bytes_read = request;
+		(a->compression_read_consume)(a, bytes_read);
+		request -= bytes_read;
+		bytes_skipped += bytes_read;
+	}
+
+	return (bytes_skipped);
+}
+
+/*
  * Read header of next entry.
  */
 int