[PATCH v3 3/4] connected: add transfer.connectivityCheck config and rev-list flag

["Kristofer Karlsson via GitGitGadget" <gitgitgadget@gmail.com>]

From: Kristofer Karlsson <krka@spotify.com>

Add the transfer.connectivityCheck configuration option, accepting
"full" (default) and "incremental".  When set to "incremental",
check_connected() passes --verify-trees-incremental to its rev-list
subprocess.

The rev-list flag is accepted but has no effect yet; the incremental
verification will be implemented in the next commit.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
---
 Documentation/config/transfer.adoc  | 20 ++++++++++++++++++++
 Documentation/rev-list-options.adoc |  6 ++++++
 builtin/rev-list.c                  |  4 ++++
 connected.c                         | 24 ++++++++++++++++++++++++
 4 files changed, 54 insertions(+)

diff --git a/Documentation/config/transfer.adoc b/Documentation/config/transfer.adoc
index f1ce50f4a6..b9939d3bde 100644
--- a/Documentation/config/transfer.adoc
+++ b/Documentation/config/transfer.adoc
@@ -1,3 +1,23 @@
+transfer.connectivityCheck::
+	Choose which algorithm to use for the connectivity check
+	performed during object transfer operations such as
+	linkgit:git-fetch[1] and linkgit:git-receive-pack[1].
+	The connectivity check verifies that all objects reachable
+	from the incoming tips are available locally or, in a partial
+	clone, promised by a promisor remote.
+	The variants are as follows:
++
+--
+`full` (default);;
+	Walk the full object closure of the boundary commits.
+`incremental`;;
+	Verify incoming commits by diffing their trees against parent
+	trees, recursively descending only into entries that differ.
+	The largest benefits occur when incoming commits change a
+	small fraction of a large tree closure.
+	Falls back to `full` for deepening fetches.
+--
+
 transfer.credentialsInUrl::
 	A configured URL can contain plaintext credentials in the form
 	`<protocol>://<user>:<password>@<domain>/<path>`. You may want
diff --git a/Documentation/rev-list-options.adoc b/Documentation/rev-list-options.adoc
index bd9f345690..35c4bf882f 100644
--- a/Documentation/rev-list-options.adoc
+++ b/Documentation/rev-list-options.adoc
@@ -1102,6 +1102,12 @@ It is an error to use `--missing-only` without `--missing=print` or
 	stronger than `--missing=allow-promisor` because it limits the
 	traversal, rather than just silencing errors about missing
 	objects.
+
+`--verify-trees-incremental`::
+	(For internal use only.)  Verify tree connectivity
+	incrementally by comparing each commit's tree against its
+	parent trees.  Used by `check_connected()` when
+	`transfer.connectivityCheck` is set to `incremental`.
 endif::git-rev-list[]
 
 `--no-walk[=(sorted|unsorted)]`::
diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index 0faa833fac..9a76bb9c9c 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -769,6 +769,8 @@ int cmd_rev_list(int argc,
 		if (!strcmp(arg, "--exclude-promisor-objects")) {
 			repo->fetch_if_missing = 0;
 			revs.exclude_promisor_objects = 1;
+		} else if (!strcmp(arg, "--verify-trees-incremental")) {
+			repo->fetch_if_missing = 0;
 		} else if (skip_prefix(arg, "--missing=", &arg)) {
 			parse_missing_action_value(repo, arg);
 		} else if (!strcmp(arg, "--missing-only")) {
@@ -848,6 +850,8 @@ int cmd_rev_list(int argc,
 
 		if (!strcmp(arg, "--exclude-promisor-objects"))
 			continue; /* already handled above */
+		if (!strcmp(arg, "--verify-trees-incremental"))
+			continue; /* already handled above */
 		if (skip_prefix(arg, "--missing=", &arg))
 			continue; /* already handled above */
 
diff --git a/connected.c b/connected.c
index 929b9bd28d..e3dcd8e2b0 100644
--- a/connected.c
+++ b/connected.c
@@ -1,6 +1,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "config.h"
 #include "gettext.h"
 #include "hex.h"
 #include "odb.h"
@@ -67,6 +68,26 @@ static int check_connected_promisor(oid_iterate_fn fn,
 	return 1;
 }
 
+static int incremental_check_applicable(struct check_connected_options *opt)
+{
+	const char *algorithm = NULL;
+
+	if (repo_config_get_string_tmp(the_repository,
+				       "transfer.connectivitycheck",
+				       &algorithm))
+		return 0;
+	if (!strcasecmp(algorithm, "full"))
+		return 0;
+	if (strcasecmp(algorithm, "incremental"))
+		die(_("unknown transfer.connectivityCheck algorithm '%s'"),
+		    algorithm);
+
+	if (opt->is_deepening_fetch)
+		return 0;
+
+	return 1;
+}
+
 /*
  * If we feed all the commits we want to verify to this command
  *
@@ -133,6 +154,9 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 	if (opt->progress)
 		strvec_pushf(&rev_list.args, "--progress=%s",
 			     _("Checking connectivity"));
+	if (incremental_check_applicable(opt))
+		strvec_push(&rev_list.args,
+			    "--verify-trees-incremental");
 
 	rev_list.git_cmd = 1;
 	if (opt->env)
-- 
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