[PATCH 1/2] odb: add odb_source_files_try() for heterogeneous source iteration

From: Aaron Paterson via GitGitGadget <gitgitgadget@gmail.com>
Date: Mar 16, 2026, 15:29 UTC
Message-ID: <e6877b30d5dddce5720a15d724d7b381a5595915.1773674983.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.2068.git.1773674983.gitgitgadget@gmail.com>

From: Aaron Paterson <apaterson@pm.me>

The odb_source vtable introduced in this release allows multiple backend implementations via the odb_source_type enum. However, source-chain iteration sites that need files-specific internals (pack store, loose cache, MIDX) currently use odb_source_files_downcast(), which calls BUG() for non-files source types. This makes it impossible to add a non-files source to the chain without crashing.

Add odb_source_files_try() as a companion to the existing downcast function. It returns NULL for non-files sources instead of aborting. This follows the pattern used elsewhere in git where a "try" or "maybe" variant provides a fallback path while the strict version retains its safety guarantee.

The existing odb_source_files_downcast() is unchanged and continues to BUG() on type mismatch, protecting call sites that should only ever receive a files source.

A subsequent commit will convert the source-chain iteration sites to use this new helper.

Signed-off-by: Aaron Paterson <apaterson@pm.me>
---
 odb/source-files.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/odb/source-files.h b/odb/source-files.h
index 23a3b4e04b..abd093b23f 100644
--- a/odb/source-files.h
+++ b/odb/source-files.h
@@ -32,4 +32,18 @@ static inline struct odb_source_files *odb_source_files_downcast(struct odb_sour
 	return container_of(source, struct odb_source_files, base);
 }
 
+/*
+ * Try to cast the given source to the files backend. Returns NULL if
+ * the source uses a different backend. Use this in loops that iterate
+ * over heterogeneous source chains (e.g. when alternates may include
+ * non-files backends). Use odb_source_files_downcast() when the source
+ * is known to be a files backend.
+ */
+static inline struct odb_source_files *odb_source_files_try(struct odb_source *source)
+{
+	if (source->type != ODB_SOURCE_FILES)
+		return NULL;
+	return container_of(source, struct odb_source_files, base);
+}
+
 #endif

-- 
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