commit-graph: change test to die on parse, not load

43d3561 (commit-graph write: don't die if the existing graph is corrupt,
2019-03-25) introduced the GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD environment
variable. This was created to verify that commit-graph was not loaded
when writing a new non-incremental commit-graph.

An upcoming change wants to load a commit-graph in some valuable cases,
but we want to maintain that we don't trust the commit-graph data when
writing our new file. Instead of dying on load, instead die if we ever
try to parse a commit from the commit-graph. This functionally verifies
the same intended behavior, but allows a more advanced feature in the
next change.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -564,10 +564,6 @@ static int prepare_commit_graph(struct repository *r)
 		return !!r->objects->commit_graph;
 	r->objects->commit_graph_attempted = 1;
 
-	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD, 0))
-		die("dying as requested by the '%s' variable on commit-graph load!",
-		    GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD);
-
 	prepare_repo_settings(r);
 
 	if (!git_env_bool(GIT_TEST_COMMIT_GRAPH, 0) &&
@@ -790,6 +786,14 @@ static int parse_commit_in_graph_one(struct repository *r,
 
 int parse_commit_in_graph(struct repository *r, struct commit *item)
 {
+	static int checked_env = 0;
+
+	if (!checked_env &&
+	    git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE, 0))
+		die("dying as requested by the '%s' variable on commit-graph parse!",
+		    GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE);
+	checked_env = 1;
+
 	if (!prepare_commit_graph(r))
 		return 0;
 	return parse_commit_in_graph_one(r, r->objects->commit_graph, item);

commit-graph.h

@@ -5,7 +5,7 @@
 #include "object-store.h"
 
 #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"
-#define GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD "GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD"
+#define GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE "GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE"
 #define GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS "GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS"
 
 /*

t/t5318-commit-graph.sh

@@ -436,7 +436,7 @@ corrupt_graph_verify() {
 		cp $objdir/info/commit-graph commit-graph-pre-write-test
 	fi &&
 	git status --short &&
-	GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD=true git commit-graph write &&
+	GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE=true git commit-graph write &&
 	git commit-graph verify
 }