[PATCH v4 08/10] reftable/stack: add mechanism to notify callers on reload

[Patrick Steinhardt <ps@pks.im>]

Reftable stacks are reloaded in two cases:

  - When calling `reftable_stack_reload()`, if the stat-cache tells us
    that the stack has been modified.

  - When committing a reftable addition.

While callers can figure out the second case, they do not have a
mechanism to figure out whether `reftable_stack_reload()` led to an
actual reload of the on-disk data. All they can do is thus to assume
that data is always being reloaded in that case.

Improve the situation by introducing a new `on_reload()` callback to the
reftable options. If provided, the function will be invoked every time
the stack has indeed been reloaded. This allows callers to invalidate
data that depends on the current stack data.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/reftable-writer.h | 9 +++++++++
 reftable/stack.c           | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/reftable/reftable-writer.h b/reftable/reftable-writer.h
index c85ef5a5bd14595d75f99457fef4407040e197c5..5f9afa620bb00de66c311765fb0ae8c6f56401ae 100644
--- a/reftable/reftable-writer.h
+++ b/reftable/reftable-writer.h
@@ -68,6 +68,15 @@ struct reftable_write_options {
 	 * fsync(3P) when unset.
 	 */
 	int (*fsync)(int fd);
+
+	/*
+	 * Callback function to execute whenever the stack is being reloaded.
+	 * This can be used e.g. to discard cached information that relies on
+	 * the old stack's data. The payload data will be passed as argument to
+	 * the callback.
+	 */
+	void (*on_reload)(void *payload);
+	void *on_reload_payload;
 };
 
 /* reftable_block_stats holds statistics for a single block type */
diff --git a/reftable/stack.c b/reftable/stack.c
index d97b64a40d4ad05cfd9e6f33e8ba1e713281ef6d..5384ca9de0d1f064aebcb09308a74cc397b37463 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -548,6 +548,10 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
 		close(fd);
 	free_names(names);
 	free_names(names_after);
+
+	if (st->opts.on_reload)
+		st->opts.on_reload(st->opts.on_reload_payload);
+
 	return err;
 }
 

-- 
2.47.0.366.gd4f858ca17.dirty