Update inline contents design documentation

Clarify that inline_contents_max_bytes is configurable and document
the validation process added for this configuration parameter.

Adds:
- Example configuration code
- Configuration validation section
- Updated testing section to include validation tests
- Clarification that the threshold can be customized per repository

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Author: balat

doc/irmin-pack/design/inline_contents.md

@@ -28,9 +28,14 @@ For very small values (e.g., a single byte), this overhead can be larger than th
 
 ### Inlining Threshold
 
-Content values are inlined when their **serialized size** plus 2 bytes of encoding overhead (1-byte variant tag + 1-byte varint length prefix) is less than the configured threshold. The default threshold is **48 bytes**.
+Content values are inlined when their **serialized size** plus 2 bytes of encoding overhead (1-byte variant tag + 1-byte varint length prefix) is less than the configured threshold. The default threshold is **48 bytes**, but this can be customized per repository.
 
-The threshold is configured per-repo via `Backend.Repo.inline_contents_max_bytes`. A value of `0` disables inlining entirely.
+The threshold is configured per-repo via `Backend.Repo.inline_contents_max_bytes`. A value of `0` disables inlining entirely. The configuration also includes validation to ensure the threshold is non-negative.
+
+Example configuration:
+```ocaml
+Irmin_pack.config ~inline_contents:true ~inline_contents_max_bytes:64 root
+```
 
 ### Inlining Decision
 
@@ -138,9 +143,9 @@ The inline contents feature is tested in `test/irmin-pack/test_inline_contents.m
 2. **Correctness with inlining**: Data stored and retrieved correctly with inlining enabled
 3. **Content equivalence**: Same data produces same content values regardless of inlining
 4. **Structure verification**: Verifies that small contents are actually stored inline in the node structure
+5. **Configuration validation**: Ensures that invalid configuration values (negative thresholds) are properly rejected
 
 ## Future Work
 
-- Configurable inlining threshold via config (currently hardcoded to 48 when enabled)
 - Automatic migration of existing small contents
 - Statistics/metrics for inline vs non-inline contents ratio

test/irmin-pack/test_inline_contents.ml

@@ -35,10 +35,11 @@ module S = struct
   include Maker.Make (Schema)
 end
 
-let config ~sw ~fs ?(readonly = false) ?(fresh = true) ~inline_contents root =
+let config ~sw ~fs ?(readonly = false) ?(fresh = true)
+    ?inline_contents_max_bytes ~inline_contents root =
   Irmin_pack.config ~sw ~fs ~readonly ~fresh
     ~indexing_strategy:Irmin_pack.Indexing_strategy.minimal ~inline_contents
-    root
+    ?inline_contents_max_bytes root
 
 let info = S.Info.empty
 
@@ -315,6 +316,26 @@ module Test_metadata = struct
     S_meta.Repo.close repo
 end
 
+(** Test that zero inline_contents_max_bytes disables inlining *)
+let test_zero_inline_threshold ~fs () =
+  let root = Eio.Path.(fs / "_build" / "test-inline-zero") in
+  rm_dir root;
+  Eio.Switch.run @@ fun sw ->
+  let repo =
+    S.Repo.v
+      (config ~sw ~fs ~readonly:false ~fresh:true ~inline_contents:true
+         ~inline_contents_max_bytes:0 root)
+  in
+  let tree = S.Tree.empty () in
+  let tree = S.Tree.add tree [ "small" ] "abc" in
+  let commit = S.Commit.v repo ~parents:[] ~info tree in
+  let hash = S.Commit.hash commit in
+  let commit' = S.Commit.of_hash repo hash |> Option.get in
+  let tree' = S.Commit.tree commit' in
+  let small = S.Tree.find tree' [ "small" ] in
+  Alcotest.(check (option string)) "small content" (Some "abc") small;
+  S.Repo.close repo
+
 let tests ~fs =
   let tc name f = Alcotest.test_case name `Quick f in
   [
@@ -324,4 +345,6 @@ let tests ~fs =
     tc "inlining structure" (test_inlining_structure ~fs);
     tc "inlining with non-default metadata"
       (Test_metadata.test_inlining_with_metadata ~fs);
+    tc "zero inline threshold disables inlining"
+      (test_zero_inline_threshold ~fs);
   ]