Merge branch 'rs/diff-exit-code-with-external-diff'

"git diff --exit-code --ext-diff" learned to take the exit status
of the external diff driver into account when deciding the exit
status of the overall "git diff" invocation when configured to do
so.

* rs/diff-exit-code-with-external-diff:
  diff: let external diffs report that changes are uninteresting
  userdiff: add and use struct external_diff
  t4020: test exit code with external diffs

4 files changed, 38 insertions, 0 deletions

diff --git a/Documentation/config/diff.txt b/Documentation/config/diff.txt
index 5ce7b91f1d..190bda17e5 100644
--- a/Documentation/config/diff.txt
+++ b/Documentation/config/diff.txt
@@ -79,6 +79,15 @@ diff.external::
 	you want to use an external diff program only on a subset of
 	your files, you might want to use linkgit:gitattributes[5] instead.
 
+diff.trustExitCode::
+	If this boolean value is set to true then the
+	`diff.external` command is expected to return exit code
+	0 if it considers the input files to be equal or 1 if it
+	considers them to be different, like `diff(1)`.
+	If it is set to false, which is the default, then the command
+	is expected to return exit code 0 regardless of equality.
+	Any other exit code causes Git to report a fatal error.
+
 diff.ignoreSubmodules::
 	Sets the default value of --ignore-submodules. Note that this
 	affects only 'git diff' Porcelain, and not lower level 'diff'
@@ -164,6 +173,15 @@ diff.<driver>.command::
 	The custom diff driver command.  See linkgit:gitattributes[5]
 	for details.
 
+diff.<driver>.trustExitCode::
+	If this boolean value is set to true then the
+	`diff.<driver>.command` command is expected to return exit code
+	0 if it considers the input files to be equal or 1 if it
+	considers them to be different, like `diff(1)`.
+	If it is set to false, which is the default, then the command
+	is expected to return exit code 0 regardless of equality.
+	Any other exit code causes Git to report a fatal error.
+
 diff.<driver>.xfuncname::
 	The regular expression that the diff driver should use to
 	recognize the hunk header.  A built-in pattern may also be used.
diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index c7df20e571..cd0b81adbb 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -820,6 +820,11 @@ ifndef::git-log[]
 
 --quiet::
 	Disable all output of the program. Implies `--exit-code`.
+	Disables execution of external diff helpers whose exit code
+	is not trusted, i.e. their respective configuration option
+	`diff.trustExitCode` or `diff.<driver>.trustExitCode` or
+	environment variable `GIT_EXTERNAL_DIFF_TRUST_EXIT_CODE` is
+	false.
 endif::git-log[]
 endif::git-format-patch[]
 
diff --git a/Documentation/git.txt b/Documentation/git.txt
index a31a70acca..4489e2297a 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -644,6 +644,16 @@ parameter, <path>.
 For each path `GIT_EXTERNAL_DIFF` is called, two environment variables,
 `GIT_DIFF_PATH_COUNTER` and `GIT_DIFF_PATH_TOTAL` are set.
 
+`GIT_EXTERNAL_DIFF_TRUST_EXIT_CODE`::
+	If this Boolean environment variable is set to true then the
+	`GIT_EXTERNAL_DIFF` command is expected to return exit code
+	0 if it considers the input files to be equal or 1 if it
+	considers them to be different, like `diff(1)`.
+	If it is set to false, which is the default, then the command
+	is expected to return exit code 0 regardless of equality.
+	Any other exit code causes Git to report a fatal error.
+
+
 `GIT_DIFF_PATH_COUNTER`::
 	A 1-based counter incremented by one for every path.
 
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 4338d023d9..80cae17f37 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -776,6 +776,11 @@ with the above configuration, i.e. `j-c-diff`, with 7
 parameters, just like `GIT_EXTERNAL_DIFF` program is called.
 See linkgit:git[1] for details.
 
+If the program is able to ignore certain changes (similar to
+`git diff --ignore-space-change`), then also set the option
+`trustExitCode` to true.  It is then expected to return exit code 1 if
+it finds significant changes and 0 if it doesn't.
+
 Setting the internal diff algorithm
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^