[PATCH v1] read-cache: add GIT_TEST_INDEX_VERSION support

[Ben Peart <benpeart@microsoft.com>]

Teach get_index_format_default() to support running the test suite
with specific index versions.  In particular, this enables the test suite
to be run using index version 4 which is not the default so gets less testing.

Signed-off-by: Ben Peart <Ben.Peart@microsoft.com>
---

Notes:
    Base Ref: v2.19.0
    Web-Diff: https://github.com/benpeart/git/commit/52e733e2ce
    Checkout: git fetch https://github.com/benpeart/git git-test-index-version-v1 && git checkout 52e733e2ce

 read-cache.c | 47 +++++++++++++++++++++++++++++++++--------------
 t/README     |  6 +++++-
 2 files changed, 38 insertions(+), 15 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 7b1354d759..d140ce9989 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1570,26 +1570,45 @@ static unsigned int get_index_format_default(void)
 	char *envversion = getenv("GIT_INDEX_VERSION");
 	char *endp;
 	int value;
-	unsigned int version = INDEX_FORMAT_DEFAULT;
+	unsigned int version = -1;
+
+	if (envversion) {
+		version = strtoul(envversion, &endp, 10);
+		if (*endp ||
+			version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
+			warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
+				"Using version %i"), INDEX_FORMAT_DEFAULT);
+			version = INDEX_FORMAT_DEFAULT;
+		}
+	}
 
-	if (!envversion) {
-		if (!git_config_get_int("index.version", &value))
+	if (version == -1) {
+		if (!git_config_get_int("index.version", &value)) {
 			version = value;
-		if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
-			warning(_("index.version set, but the value is invalid.\n"
-				  "Using version %i"), INDEX_FORMAT_DEFAULT);
-			return INDEX_FORMAT_DEFAULT;
+			if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
+				warning(_("index.version set, but the value is invalid.\n"
+					"Using version %i"), INDEX_FORMAT_DEFAULT);
+				version = INDEX_FORMAT_DEFAULT;
+			}
 		}
-		return version;
 	}
 
-	version = strtoul(envversion, &endp, 10);
-	if (*endp ||
-	    version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
-		warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
-			  "Using version %i"), INDEX_FORMAT_DEFAULT);
-		version = INDEX_FORMAT_DEFAULT;
+	if (version == -1) {
+		envversion = getenv("GIT_TEST_INDEX_VERSION");
+		if (envversion) {
+			version = strtoul(envversion, &endp, 10);
+			if (*endp ||
+				version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
+				warning(_("GIT_TEST_INDEX_VERSION set, but the value is invalid.\n"
+					"Using version %i"), INDEX_FORMAT_DEFAULT);
+				version = INDEX_FORMAT_DEFAULT;
+			}
+		}
 	}
+
+	if (version == -1)
+		version = INDEX_FORMAT_DEFAULT;
+
 	return version;
 }
 
diff --git a/t/README b/t/README
index 9028b47d92..f872638a78 100644
--- a/t/README
+++ b/t/README
@@ -315,10 +315,14 @@ packs on demand. This normally only happens when the object size is
 over 2GB. This variable forces the code path on any object larger than
 <n> bytes.
 
-GIT_TEST_OE_DELTA_SIZE=<n> exercises the uncomon pack-objects code
+GIT_TEST_OE_DELTA_SIZE=<n> exercises the uncommon pack-objects code
 path where deltas larger than this limit require extra memory
 allocation for bookkeeping.
 
+GIT_TEST_INDEX_VERSION=<n> exercises the index read/write code path
+for the index version specified.  Can be set to any valid version
+but the non-default version 4 is probably the most beneficial.
+
 Naming Tests
 ------------
 

base-commit: 1d4361b0f344188ab5eec6dcea01f61a3a3a1670
-- 
2.18.0.windows.1