Add a setting to require a filter to be successful

By default, a missing filter driver or a failure from the filter driver is
not an error, but merely makes the filter operation a no-op pass through.
This is useful to massage the content into a shape that is more convenient
for the platform, filesystem, and the user to use, and the content filter
mechanism is not used to turn something unusable into usable.

However, we could also use of the content filtering mechanism and store
the content that cannot be directly used in the repository (e.g. a UUID
that refers to the true content stored outside git, or an encrypted
content) and turn it into a usable form upon checkout (e.g. download the
external content, or decrypt the encrypted content).  For such a use case,
the content cannot be used when filter driver fails, and we need a way to
tell Git to abort the whole operation for such a failing or missing filter
driver.

Add a new "filter.<driver>.required" configuration variable to mark the
second use case.  When it is set, git will abort the operation when the
filter driver does not exist or exits with a non-zero status code.

Signed-off-by: Jehan Bing <jehan@orb.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 31 insertions, 10 deletions

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index a85b187e04..80120ea14f 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -294,16 +294,27 @@ output is used to update the worktree file.  Similarly, the
 `clean` command is used to convert the contents of worktree file
 upon checkin.
 
-A missing filter driver definition in the config is not an error
-but makes the filter a no-op passthru.
-
-The content filtering is done to massage the content into a
-shape that is more convenient for the platform, filesystem, and
-the user to use.  The key phrase here is "more convenient" and not
-"turning something unusable into usable".  In other words, the
-intent is that if someone unsets the filter driver definition,
-or does not have the appropriate filter program, the project
-should still be usable.
+One use of the content filtering is to massage the content into a shape
+that is more convenient for the platform, filesystem, and the user to use.
+For this mode of operation, the key phrase here is "more convenient" and
+not "turning something unusable into usable".  In other words, the intent
+is that if someone unsets the filter driver definition, or does not have
+the appropriate filter program, the project should still be usable.
+
+Another use of the content filtering is to store the content that cannot
+be directly used in the repository (e.g. a UUID that refers to the true
+content stored outside git, or an encrypted content) and turn it into a
+usable form upon checkout (e.g. download the external content, or decrypt
+the encrypted content).
+
+These two filters behave differently, and by default, a filter is taken as
+the former, massaging the contents into more convenient shape.  A missing
+filter driver definition in the config, or a filter driver that exits with
+a non-zero status, is not an error but makes the filter a no-op passthru.
+
+You can declare that a filter turns a content that by itself is unusable
+into a usable content by setting the filter.<driver>.required configuration
+variable to `true`.
 
 For example, in .gitattributes, you would assign the `filter`
 attribute for paths.
@@ -335,6 +346,16 @@ input that is already correctly indented.  In this case, the lack of a
 smudge filter means that the clean filter _must_ accept its own output
 without modifying it.
 
+If a filter _must_ succeed in order to make the stored contents usable,
+you can declare that the filter is `required`, in the configuration:
+
+------------------------
+[filter "crypt"]
+	clean = openssl enc ...
+	smudge = openssl enc -d ...
+	required
+------------------------
+
 Sequence "%f" on the filter command line is replaced with the name of
 the file the filter is working on.  A filter might use this in keyword
 substitution.  For example: