[PATCH v2 11/11] fast-export: add --always-show-modify-after-rename

[Elijah Newren <newren@gmail.com>]

I wanted a way to gather all the following information efficiently
(with as few history traversals as possible):
  * Get all blob sizes
  * Map blob shas to filename(s) they appeared under in the history
  * Find when files and directories were deleted (and whether they
    were later reinstated, since that means they aren't actually gone)
  * Find sets of filenames referring to the same logical 'file'. (e.g.
    foo->bar in commit A and bar->baz in commit B mean that
    {foo,bar,baz} refer to the same 'file', so someone wanting to just
    "keep baz and its history" need all versions of those three
    filenames).  I need to know about things like another foo or bar
    being introduced after the rename though, since that breaks the
    connection between filenames)
and then I would generate various aggregations on the data and display
some type of report for the user.

The only way I know of to get blob sizes is via
  cat-file --batch-all-objects --batch-check

The rest of the data would traditionally be gathered from a log command,
e.g.

  git log --format='%H%n%P%n%cd' --date=short --topo-order --reverse \
      -M --diff-filter=RAMD --no-abbrev --raw -c

however, parsing log output seems slightly dangerous given that it is a
porcelain command.  While we have specified --format and --raw to try
to avoid the most obvious problems, I'm still slightly concerned about
--date=short, the combinations of --raw and -c, options that might
colorize the output, and also the --diff-filter (there is no current
option named --no-find-copies or --no-break-rewrites, but what if those
turn on by default in the future much as we changed the default with
detecting renames?).  Each of those is a small worry, but they add up.

A command meant for data serialization, such as fast-export, seems like
a better candidate for this job.  There's just one missing item: in
order to connect blob sizes to filenames, I need fast-export to tell me
the blob sha1sum of any file changes.  It does this for modifies, but
not always for renames.  In particular, if a file is a 100% rename, it
only prints
    R oldname newname
instead of
    R oldname newname
    M 100644 $SHA1 newname
as occurs when there is a rename+modify.  Add an option which allows us
to force the latter output even when commits have exact renames of
files.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 Documentation/git-fast-export.txt | 11 ++++++++++
 builtin/fast-export.c             |  7 +++++-
 t/t9350-fast-export.sh            | 36 +++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-fast-export.txt b/Documentation/git-fast-export.txt
index 64c01ba918..b663b6f8af 100644
--- a/Documentation/git-fast-export.txt
+++ b/Documentation/git-fast-export.txt
@@ -129,6 +129,17 @@ marks the same across runs.
 	for intermediary filters (e.g. for rewriting commit messages
 	which refer to older commits, or for stripping blobs by id).
 
+--always-show-modify-after-rename::
+	When a rename is detected, fast-export normally issues both a
+	'R' (rename) and a 'M' (modify) directive.  However, if the
+	contents of the old and new filename match exactly, it will
+	only issue the rename directive.  Use this flag to have it
+	always issue the modify directive after the rename, which may
+	be useful for tools which are using the fast-export stream as
+	a mechanism for gathering statistics about a repository.  Note
+	that this option only has effect when rename detection is
+	active (see the -M option).
+
 --refspec::
 	Apply the specified refspec to each ref exported. Multiple of them can
 	be specified.
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index e0f794811e..31ad43077a 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -38,6 +38,7 @@ static int use_done_feature;
 static int no_data;
 static int full_tree;
 static int reference_excluded_commits;
+static int always_show_modify_after_rename;
 static int show_original_ids;
 static struct string_list extra_refs = STRING_LIST_INIT_NODUP;
 static struct string_list tag_refs = STRING_LIST_INIT_NODUP;
@@ -407,7 +408,8 @@ static void show_filemodify(struct diff_queue_struct *q,
 				putchar('\n');
 
 				if (oideq(&ospec->oid, &spec->oid) &&
-				    ospec->mode == spec->mode)
+				    ospec->mode == spec->mode &&
+				    !always_show_modify_after_rename)
 					break;
 			}
 			/* fallthrough */
@@ -1105,6 +1107,9 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 			 &reference_excluded_commits, N_("Reference parents which are not in fast-export stream by sha1sum")),
 		OPT_BOOL(0, "show-original-ids", &show_original_ids,
 			    N_("Show original sha1sums of blobs/commits")),
+		OPT_BOOL(0, "always-show-modify-after-rename",
+			    &always_show_modify_after_rename,
+			 N_("Always provide 'M' directive after 'R'")),
 
 		OPT_END()
 	};
diff --git a/t/t9350-fast-export.sh b/t/t9350-fast-export.sh
index 5690fe2810..5c20065e39 100755
--- a/t/t9350-fast-export.sh
+++ b/t/t9350-fast-export.sh
@@ -630,4 +630,40 @@ test_expect_success 'merge commit gets exported with --import-marks' '
 	)
 '
 
+test_expect_success 'rename detection and --always-show-modify-after-rename' '
+	test_create_repo renames &&
+	(
+		cd renames &&
+		test_seq 0  9  >single_digit &&
+		test_seq 10 98 >double_digit &&
+		git add . &&
+		git commit -m initial &&
+
+		echo 99 >>double_digit &&
+		git mv single_digit single-digit &&
+		git mv double_digit double-digit &&
+		git add double-digit &&
+		git commit -m renames &&
+
+		# First, check normal fast-export -M output
+		git fast-export -M --no-data master >out &&
+
+		grep double-digit out >out2 &&
+		test_line_count = 2 out2 &&
+
+		grep single-digit out >out2 &&
+		test_line_count = 1 out2 &&
+
+		# Now, test with --always-show-modify-after-rename; should
+		# have an extra "M" directive for "single-digit".
+		git fast-export -M --no-data --always-show-modify-after-rename master >out &&
+
+		grep double-digit out >out2 &&
+		test_line_count = 2 out2 &&
+
+		grep single-digit out >out2 &&
+		test_line_count = 2 out2
+	)
+'
+
 test_done
-- 
2.19.1.1063.g2b8e4a4f82.dirty