More custom ODB backend content

Author: ben

book/B-embedding-git/sections/libgit2.asc

@@ -128,7 +128,7 @@ git_repository *repo;
 error = git_repository_wrap_odb(&repo, odb); // <2>
 
 git_odb_backend *my_backend;
-error = git_odb_backend_mine(&my_backend, repo, …); // <3>
+error = git_odb_backend_mine(&my_backend, /*…*/); // <3>
 
 error = git_odb_add_backendodb, my_backend, 1); // <4>
 ----
@@ -140,7 +140,42 @@ _Note that error handling has been left out for this example. You should know be
 <3> Initialize a custom ODB backend.
 <4> Set the repository to use the custom backend for its ODB.
 
-// TODO
+But what is this `git_odb_backend_mine` thing?
+Well, that's your own backend, and you can do whatever you want in there, so long as you fill in the `git_odb_backend` structure properly.
+Here's what it _could_ look like:
+
+[source,c]
+----
+typedef struct {
+    git_odb_backend parent;
+
+    // Some other stuff
+    void *custom_context;
+} my_backend_struct;
+
+int git_odb_backend_mine(git_odb_backend **backend_out, /*…*/)
+{
+    my_backend_struct *backend;
+
+    backend = calloc(1, sizeof (my_backend_struct));
+
+    backend->custom_context = …;
+
+    backend->parent.read = &my_backend__read;
+    backend->parent.read_prefix = &my_backend__read_prefix;
+    backend->parent.read_header = &my_backend__read_header;
+    // …
+
+    *backend_out = (git_odb_backend *) backend;
+
+    return GIT_SUCCESS;
+}
+----
+
+The `my_backend_struct` structure contains a `git_odb_backend` member first, which ensures that the memory layout is what the Libgit2 code expects it to be.
+The rest of it is arbitrary; this structure can be as large or small as you need it to be.
+The initialization function allocates some memory for the structure, sets up the custom context, and then fills in the members of the `parent` structure that it supports.
+Take a look at the `include/git2/sys/odb_backend.h` file in the Libgit2 source for a complete set of call signatures; your particular use case will help determine which of these you'll want to support.
 
 ==== Other Bindings