refs/reftable: allow configuring geometric factor

Allow configuring the geometric factor used by the auto-compaction
algorithm whenever a new table is appended to the stack of tables.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

Documentation/config/reftable.txt

@@ -36,3 +36,13 @@ reftable.indexObjects::
 	are a reverse mapping of object ID to the references pointing to them.
 +
 The default value is `true`.
+
+reftable.geometricFactor::
+	Whenever the reftable backend appends a new table to the stack, it
+	performs auto compaction to ensure that there is only a handful of
+	tables. The backend does this by ensuring that tables form a geometric
+	sequence regarding the respective sizes of each table.
++
+By default, the geometric sequence uses a factor of 2, meaning that for any
+table, the next-biggest table must at least be twice as big. A maximum factor
+of 256 is supported.

refs/reftable-backend.c

@@ -247,6 +247,11 @@ static int reftable_be_config(const char *var, const char *value,
 		opts->restart_interval = restart_interval;
 	} else if (!strcmp(var, "reftable.indexobjects")) {
 		opts->skip_index_objects = !git_config_bool(var, value);
+	} else if (!strcmp(var, "reftable.geometricfactor")) {
+		unsigned long factor = git_config_ulong(var, value, ctx->kvi);
+		if (factor > UINT8_MAX)
+			die("reftable geometric factor cannot exceed %u", (unsigned)UINT8_MAX);
+		opts->auto_compaction_factor = factor;
 	}
 
 	return 0;