[PATCH v2 2/2] commit-graph: fix buggy --expire-time option

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

The commit-graph builtin has an --expire-time option that takes a
datetime using OPT_EXPIRY_DATE(). However, the implementation inside
expire_commit_graphs() was treating a non-zero value as a number of
seconds to subtract from "now".

Update t5323-split-commit-graph.sh to demonstrate the correct value
of the --expire-time option by actually creating a crud .graph file
with mtime earlier than the expire time. Instead of using a super-
early time (1980) we use an explicit, and recent, time. Using
test-tool chmtime to create two files on either end of an exact
second, we create a test that catches this failure no matter the
current time. Using a fixed date is more portable than trying to
format a relative date string into the --expiry-date input.

I noticed this when inspecting some Scalar repos that had an excess
number of commit-graph files. In Scalar, we were using this second
interpretation by using "--expire-time=3600" to mean "delete graphs
older than one hour ago" to avoid deleting a commit-graph that a
foreground process may be trying to load.

Also I noticed that the help text was copied from the --max-commits
option. Fix that help text.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 builtin/commit-graph.c        | 2 +-
 commit-graph.c                | 2 +-
 t/t5324-split-commit-graph.sh | 8 +++++++-
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 4a70b33fb5f..5bfba972498 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -140,7 +140,7 @@ static int graph_write(int argc, const char **argv)
 		OPT_INTEGER(0, "size-multiple", &split_opts.size_multiple,
 			N_("maximum ratio between two levels of a split commit-graph")),
 		OPT_EXPIRY_DATE(0, "expire-time", &split_opts.expire_time,
-			N_("maximum number of commits in a non-base split commit-graph")),
+			N_("only expire files older than a given date-time")),
 		OPT_END(),
 	};
 
diff --git a/commit-graph.c b/commit-graph.c
index f013a84e294..0d0d37787a0 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1707,7 +1707,7 @@ static void expire_commit_graphs(struct write_commit_graph_context *ctx)
 	timestamp_t expire_time = time(NULL);
 
 	if (ctx->split_opts && ctx->split_opts->expire_time)
-		expire_time -= ctx->split_opts->expire_time;
+		expire_time = ctx->split_opts->expire_time;
 	if (!ctx->split) {
 		char *chain_file_name = get_chain_filename(ctx->odb);
 		unlink(chain_file_name);
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 53b2e6b4555..b8b208fc3da 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -210,8 +210,14 @@ test_expect_success 'test merge stragety constants' '
 		git config core.commitGraph true &&
 		test_line_count = 2 $graphdir/commit-graph-chain &&
 		test_commit 15 &&
-		git commit-graph write --reachable --split --size-multiple=10 --expire-time=1980-01-01 &&
+		touch $graphdir/to-delete.graph $graphdir/to-keep.graph &&
+		test-tool chmtime =1546362000 $graphdir/to-delete.graph &&
+		test-tool chmtime =1546362001 $graphdir/to-keep.graph &&
+		git commit-graph write --reachable --split --size-multiple=10 \
+			--expire-time="2019-01-01 12:00 -05:00" &&
 		test_line_count = 1 $graphdir/commit-graph-chain &&
+		test_path_is_missing $graphdir/to-delete.graph &&
+		test_path_is_file $graphdir/to-keep.graph &&
 		ls $graphdir/graph-*.graph >graph-files &&
 		test_line_count = 3 graph-files
 	) &&
-- 
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