diff --git a/Makefile b/Makefile
index 5776309365..3a6889d2bc 100644
--- a/Makefile
+++ b/Makefile
@@ -852,6 +852,7 @@ TEST_BUILTINS_OBJS += test-submodule-nested-repo-config.o
 TEST_BUILTINS_OBJS += test-submodule.o
 TEST_BUILTINS_OBJS += test-subprocess.o
 TEST_BUILTINS_OBJS += test-trace2.o
+TEST_BUILTINS_OBJS += test-truncate.o
 TEST_BUILTINS_OBJS += test-urlmatch-normalization.o
 TEST_BUILTINS_OBJS += test-userdiff.o
 TEST_BUILTINS_OBJS += test-wildmatch.o
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index abe8a785eb..9bd223b365 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -86,6 +86,7 @@ static struct test_cmd cmds[] = {
 	{ "submodule-nested-repo-config", cmd__submodule_nested_repo_config },
 	{ "subprocess", cmd__subprocess },
 	{ "trace2", cmd__trace2 },
+	{ "truncate", cmd__truncate },
 	{ "userdiff", cmd__userdiff },
 	{ "urlmatch-normalization", cmd__urlmatch_normalization },
 	{ "xml-encode", cmd__xml_encode },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index ea2672436c..f41a4d0aa8 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -79,6 +79,7 @@ int cmd__submodule_config(int argc, const char **argv);
 int cmd__submodule_nested_repo_config(int argc, const char **argv);
 int cmd__subprocess(int argc, const char **argv);
 int cmd__trace2(int argc, const char **argv);
+int cmd__truncate(int argc, const char **argv);
 int cmd__userdiff(int argc, const char **argv);
 int cmd__urlmatch_normalization(int argc, const char **argv);
 int cmd__xml_encode(int argc, const char **argv);
diff --git a/t/helper/test-truncate.c b/t/helper/test-truncate.c
new file mode 100644
index 0000000000..3931deaec7
--- /dev/null
+++ b/t/helper/test-truncate.c
@@ -0,0 +1,25 @@
+#include "test-tool.h"
+#include "git-compat-util.h"
+
+/*
+ * Truncate a file to the given size.
+ */
+int cmd__truncate(int argc, const char **argv)
+{
+	char *p = NULL;
+	uintmax_t sz = 0;
+	int fd = -1;
+
+	if (argc != 3)
+		die("expected filename and size");
+
+	sz = strtoumax(argv[2], &p, 0);
+	if (*p)
+		die("invalid size");
+
+	fd = xopen(argv[1], O_WRONLY | O_CREAT, 0600);
+
+	if (ftruncate(fd, (off_t) sz) < 0)
+		die_errno("failed to truncate file");
+	return 0;
+}