2 files changed, 30 insertions, 4 deletions

diff --git a/cgitrc.5.txt b/cgitrc.5.txt
index 4040617..c795425 100644
--- a/cgitrc.5.txt
+++ b/cgitrc.5.txt
@@ -95,7 +95,9 @@ cache-size::
 
 cache-snapshot-ttl::
 	Number which specifies the time-to-live, in minutes, for the cached
-	version of snapshots. See also: "CACHE". Default value: "5".
+	version of snapshots. A snapshot requested with a fixed object id
+	uses "cache-static-ttl" instead, since its content can never change.
+	See also: "CACHE". Default value: "5".
 
 cache-static-ttl::
 	Number which specifies the time-to-live, in minutes, for the cached
diff --git a/source/cgit.c b/source/cgit.c
index 12981c2..f284ad9 100644
--- a/source/cgit.c
+++ b/source/cgit.c
@@ -792,6 +792,25 @@ static void authenticate_cookie(void)
 	ctx.env.authenticated = cgit_close_filter(ctx.cfg.auth_filter);
 }
 
+/*
+ * Only a full object id names content that can never change. The id parameter
+ * accepts anything git can resolve, so a ref name or an abbreviation arrives
+ * here just as marked as a real id, and a page pinned to one of those must
+ * not be cached under the never-expiring static ttl.
+ */
+static int is_full_oid(const char *rev)
+{
+	size_t len = strlen(rev);
+
+	if (len != GIT_SHA1_HEXSZ && len != GIT_SHA256_HEXSZ)
+		return 0;
+	for (; *rev; rev++) {
+		if (!isxdigit(*rev))
+			return 0;
+	}
+	return 1;
+}
+
 // Every cache-*-ttl setting is written in minutes, and so is this.
 static int calc_ttl(void)
 {
@@ -804,12 +823,17 @@ static int calc_ttl(void)
 	if (!strcmp(ctx.qry.page, "about"))
 		return ctx.cfg.cache_about_ttl;
 
+	// Checked ahead of the snapshot ttl, because a tarball pinned to an
+	// object id can never come out differently and is the most expensive
+	// page to rebuild.
+	if (ctx.qry.has_oid &&
+	    (!ctx.qry.oid || is_full_oid(ctx.qry.oid)) &&
+	    (!ctx.qry.oid2 || is_full_oid(ctx.qry.oid2)))
+		return ctx.cfg.cache_static_ttl;
+
 	if (!strcmp(ctx.qry.page, "snapshot"))
 		return ctx.cfg.cache_snapshot_ttl;
 
-	if (ctx.qry.has_oid)
-		return ctx.cfg.cache_static_ttl;
-
 	if (ctx.qry.has_symref)
 		return ctx.cfg.cache_dynamic_ttl;