[PATCH 12/17] chunk-format: create read chunk API

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

Add the capability to read the table of contents, then pair the chunks
with necessary logic using read_chunk_fn pointers. Callers will be added
in future changes, but the typical outline will be:

 1. initialize a 'struct chunkfile' with init_chunkfile(NULL).
 2. call read_table_of_contents().
 3. for each chunk to parse, call pair_chunk() with appropriate pointers.
 4. call free_chunkfile() to clear the 'struct chunkfile' data.

We are re-using the anonymous 'struct chunkfile' data, as it is internal
to the chunk-format API. This gives it essentially two modes: write and
read. If the same struct instance was used for both reads and writes,
then there would be failures.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 chunk-format.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++
 chunk-format.h | 21 +++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/chunk-format.c b/chunk-format.c
index 2ce37ecc6bb..674d31d5e58 100644
--- a/chunk-format.c
+++ b/chunk-format.c
@@ -12,6 +12,8 @@ struct chunk_info {
 	uint32_t id;
 	uint64_t size;
 	chunk_write_fn write_fn;
+
+	const void *start;
 };
 
 struct chunkfile {
@@ -89,3 +91,65 @@ int write_chunkfile(struct chunkfile *cf, void *data)
 
 	return 0;
 }
+
+int read_table_of_contents(struct chunkfile *cf,
+			   const unsigned char *mfile,
+			   size_t mfile_size,
+			   uint64_t toc_offset,
+			   int toc_length)
+{
+	uint32_t chunk_id;
+	const unsigned char *table_of_contents = mfile + toc_offset;
+
+	ALLOC_GROW(cf->chunks, toc_length, cf->chunks_alloc);
+
+	while (toc_length--) {
+		uint64_t chunk_offset, next_chunk_offset;
+
+		chunk_id = get_be32(table_of_contents);
+		chunk_offset = get_be64(table_of_contents + 4);
+
+		if (!chunk_id) {
+			error(_("terminating chunk id appears earlier than expected"));
+			return 1;
+		}
+
+		table_of_contents += CHUNK_LOOKUP_WIDTH;
+		next_chunk_offset = get_be64(table_of_contents + 4);
+
+		if (next_chunk_offset < chunk_offset ||
+		    next_chunk_offset > mfile_size - the_hash_algo->rawsz) {
+			error(_("improper chunk offset(s) %"PRIx64" and %"PRIx64""),
+			      chunk_offset, next_chunk_offset);
+			return -1;
+		}
+
+		cf->chunks[cf->chunks_nr].id = chunk_id;
+		cf->chunks[cf->chunks_nr].start = mfile + chunk_offset;
+		cf->chunks[cf->chunks_nr].size = next_chunk_offset - chunk_offset;
+		cf->chunks_nr++;
+	}
+
+	chunk_id = get_be32(table_of_contents);
+	if (chunk_id) {
+		error(_("final chunk has non-zero id %"PRIx32""), chunk_id);
+		return -1;
+	}
+
+	return 0;
+}
+
+int pair_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       chunk_read_fn fn,
+	       void *data)
+{
+	int i;
+
+	for (i = 0; i < cf->chunks_nr; i++) {
+		if (cf->chunks[i].id == chunk_id)
+			return fn(cf->chunks[i].start, cf->chunks[i].size, data);
+	}
+
+	return CHUNK_NOT_FOUND;
+}
diff --git a/chunk-format.h b/chunk-format.h
index bfaed672813..250e08b8e6a 100644
--- a/chunk-format.h
+++ b/chunk-format.h
@@ -17,4 +17,25 @@ void add_chunk(struct chunkfile *cf,
 	       size_t size);
 int write_chunkfile(struct chunkfile *cf, void *data);
 
+int read_table_of_contents(struct chunkfile *cf,
+			   const unsigned char *mfile,
+			   size_t mfile_size,
+			   uint64_t toc_offset,
+			   int toc_length);
+
+/*
+ * When reading a table of contents, we find the chunk with matching 'id'
+ * then call its read_fn to populate the necessary 'data' based on the
+ * chunk start and size.
+ */
+typedef int (*chunk_read_fn)(const unsigned char *chunk_start,
+			     size_t chunk_size, void *data);
+
+
+#define CHUNK_NOT_FOUND (-2)
+int pair_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       chunk_read_fn fn,
+	       void *data);
+
 #endif
-- 
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