Merge branch 'jb/required-filter'

* jb/required-filter:
  Add a setting to require a filter to be successful

Conflicts:
	convert.c

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: