[PATCH 12/15] chunk-format: create write_chunks()

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

From: Derrick Stolee <dstolee@microsoft.com>

The commit-graph and multi-pack-index files both use a chunk-based file
format. They have already unified on using write_table_of_contents(),
but we expand upon that by unifying their chunk writing loop.

This takes the concepts already present in the commit-graph that were
dropped in the multi-pack-index code during refactoring, including:

* Check the hashfile for how much data was written by each write_fn.

* Allow write_fn() to report an error that results in a failure
  without using die() in the low-level commands.

This simplifies the code in commit-graph.c and midx.c while laying the
foundation for future formats using similar ideas.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 chunk-format.c | 23 +++++++++++++++++++++++
 chunk-format.h | 13 +++++++++++++
 commit-graph.c | 13 ++-----------
 midx.c         |  3 +--
 4 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/chunk-format.c b/chunk-format.c
index 771b6d98d0..a6643a4fc8 100644
--- a/chunk-format.c
+++ b/chunk-format.c
@@ -24,3 +24,26 @@ void write_table_of_contents(struct hashfile *f,
 	hashwrite_be32(f, 0);
 	hashwrite_be64(f, cur_offset);
 }
+
+int write_chunks(struct hashfile *f,
+		 struct chunk_info *chunks,
+		 int nr,
+		 void *data)
+{
+	int i;
+
+	for (i = 0; i < nr; i++) {
+		uint64_t start_offset = f->total + f->offset;
+		int result = chunks[i].write_fn(f, data);
+
+		if (result)
+			return result;
+
+		if (f->total + f->offset != start_offset + chunks[i].size)
+			BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
+			    chunks[i].size, chunks[i].id,
+			    f->total + f->offset - start_offset);
+	}
+
+	return 0;
+}
diff --git a/chunk-format.h b/chunk-format.h
index 4b9cbeb372..a2c7ddb23b 100644
--- a/chunk-format.h
+++ b/chunk-format.h
@@ -33,4 +33,17 @@ void write_table_of_contents(struct hashfile *f,
 			     struct chunk_info *chunks,
 			     int nr);
 
+/*
+ * Write the data for the given chunk list using the provided
+ * write_fn values. The given 'data' parameter is passed to those
+ * methods.
+ *
+ * The data that is written by each write_fn is checked to be of
+ * the expected size, and a BUG() is thrown if not specified correctly.
+ */
+int write_chunks(struct hashfile *f,
+		 struct chunk_info *chunks,
+		 int nr,
+		 void *data);
+
 #endif
diff --git a/commit-graph.c b/commit-graph.c
index 5494fda1d3..10dcef9d6b 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1809,17 +1809,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 			num_chunks * ctx->commits.nr);
 	}
 
-	for (i = 0; i < num_chunks; i++) {
-		uint64_t start_offset = f->total + f->offset;
-
-		if (chunks[i].write_fn(f, ctx))
-			return -1;
-
-		if (f->total + f->offset != start_offset + chunks[i].size)
-			BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
-			    chunks[i].size, chunks[i].id,
-			    f->total + f->offset - start_offset);
-	}
+	if (write_chunks(f, chunks, num_chunks, ctx))
+		return -1;
 
 	stop_progress(&ctx->progress);
 	strbuf_release(&progress_title);
diff --git a/midx.c b/midx.c
index 47f5f60fcd..67ac232a81 100644
--- a/midx.c
+++ b/midx.c
@@ -957,8 +957,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 	write_table_of_contents(f, header_size, chunks, num_chunks);
 
-	for (i = 0; i < num_chunks; i++)
-		chunks[i].write_fn(f, &ctx);
+	result = write_chunks(f, chunks, num_chunks, &ctx);
 
 	finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
 	commit_lock_file(&lk);
-- 
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