merge-ort: avoid accidental API mis-use

Previously, callers of the merge-ort API could have passed an
uninitialized value for struct merge_result *result.  However, we want
to check result to see if it has cached renames from a previous merge
that we can reuse; such values would be found behind result->priv.
However, if result->priv is uninitialized, attempting to access behind
it will give a segfault.  So, we need result->priv to be NULL (which
will be the case if the caller does a memset(&result, 0)), or be written
by a previous call to the merge-ort machinery.  Documenting this
requirement may help, but despite being the person who introduced this
requirement, I still missed it once and it did not fail in a very clear
way and led to a long debugging session.

Add a _properly_initialized field to merge_result; that value will be
0 if the caller zero'ed the merge_result, it will be set to a very
specific value by a previous run by the merge-ort machinery, and if it's
uninitialized it will most likely either be 0 or some value that does
not match the specific one we'd expect allowing us to throw a much more
meaningful error.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

merge-ort.c

@@ -52,6 +52,8 @@ enum merge_side {
 	MERGE_SIDE2 = 2
 };
 
+static unsigned RESULT_INITIALIZED = 0x1abe11ed; /* unlikely accidental value */
+
 struct traversal_callback_data {
 	unsigned long mask;
 	unsigned long dirmask;
@@ -3768,6 +3770,10 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
 	assert(opt->obuf.len == 0);
 
 	assert(opt->priv == NULL);
+	if (result->_properly_initialized != 0 &&
+	    result->_properly_initialized != RESULT_INITIALIZED)
+		BUG("struct merge_result passed to merge_incore_*recursive() must be zeroed or filled with values from a previous run");
+	assert(!!result->priv == !!result->_properly_initialized);
 	if (result->priv) {
 		opt->priv = result->priv;
 		result->priv = NULL;
@@ -3927,6 +3933,7 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 	result->clean &= strmap_empty(&opt->priv->conflicted);
 	if (!opt->priv->call_depth) {
 		result->priv = opt->priv;
+		result->_properly_initialized = RESULT_INITIALIZED;
 		opt->priv = NULL;
 	}
 }

merge-ort.h

@@ -29,6 +29,8 @@ struct merge_result {
 	 * !clean) and to print "CONFLICT" messages.  Not for external use.
 	 */
 	void *priv;
+	/* Also private */
+	unsigned _properly_initialized;
 };
 
 /*