[clang-format] Fix a serious bug in git clang-format -f (#102629)

With the --force (or -f) option, git-clang-format wipes out input files
excluded by a .clang-format-ignore file if they have unstaged changes.

This patch adds a hidden clang-format option --list-ignored that lists
such excluded files for git-clang-format to filter out.

Fixes #102459.

clang/test/Format/list-ignored.cpp

@@ -0,0 +1,60 @@
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir/level1/level2
+
+// RUN: cd %t.dir
+// RUN: echo "*" > .clang-format-ignore
+// RUN: echo "level*/*.c*" >> .clang-format-ignore
+// RUN: echo "*/*2/foo.*" >> .clang-format-ignore
+
+// RUN: touch foo.cc
+// RUN: clang-format -list-ignored .clang-format-ignore foo.cc \
+// RUN:   | FileCheck %s
+// CHECK: .clang-format-ignore
+// CHECK-NEXT: foo.cc
+
+// RUN: cd level1
+// RUN: touch bar.cc baz.c
+// RUN: clang-format -list-ignored bar.cc baz.c \
+// RUN:   | FileCheck %s -check-prefix=CHECK2
+// CHECK2: bar.cc
+// CHECK2-NEXT: baz.c
+
+// RUN: cd level2
+// RUN: touch foo.c foo.js
+// RUN: clang-format -list-ignored foo.c foo.js \
+// RUN:   | FileCheck %s -check-prefix=CHECK3
+// CHECK3: foo.c
+// CHECK3-NEXT: foo.js
+
+// RUN: touch .clang-format-ignore
+// RUN: clang-format -list-ignored foo.c foo.js \
+// RUN:   | FileCheck %s -allow-empty -check-prefix=CHECK4
+// CHECK4-NOT: foo.c
+// CHECK4-NOT: foo.js
+
+// RUN: echo "*.js" > .clang-format-ignore
+// RUN: clang-format -list-ignored foo.c foo.js \
+// RUN:   | FileCheck %s -check-prefix=CHECK5
+// CHECK5-NOT: foo.c
+// CHECK5: foo.js
+
+// RUN: cd ../..
+// RUN: clang-format -list-ignored *.cc level1/*.c* level1/level2/foo.* \
+// RUN:   | FileCheck %s -check-prefix=CHECK6
+// CHECK6: foo.cc
+// CHECK6-NEXT: bar.cc
+// CHECK6-NEXT: baz.c
+// CHECK6-NOT: foo.c
+// CHECK6-NEXT: foo.js
+
+// RUN: rm .clang-format-ignore
+// RUN: clang-format -list-ignored *.cc level1/*.c* level1/level2/foo.* \
+// RUN:   | FileCheck %s -check-prefix=CHECK7
+// CHECK7-NOT: foo.cc
+// CHECK7-NOT: bar.cc
+// CHECK7-NOT: baz.c
+// CHECK7-NOT: foo.c
+// CHECK7: foo.js
+
+// RUN: cd ..
+// RUN: rm -r %t.dir

clang/tools/clang-format/ClangFormat.cpp

@@ -210,6 +210,10 @@ static cl::opt<bool> FailOnIncompleteFormat(
     cl::desc("If set, fail with exit code 1 on incomplete format."),
     cl::init(false), cl::cat(ClangFormatCategory));
 
+static cl::opt<bool> ListIgnored("list-ignored",
+                                 cl::desc("List ignored files."),
+                                 cl::cat(ClangFormatCategory), cl::Hidden);
+
 namespace clang {
 namespace format {
 
@@ -715,7 +719,13 @@ int main(int argc, const char **argv) {
   unsigned FileNo = 1;
   bool Error = false;
   for (const auto &FileName : FileNames) {
-    if (isIgnored(FileName))
+    const bool Ignored = isIgnored(FileName);
+    if (ListIgnored) {
+      if (Ignored)
+        outs() << FileName << '\n';
+      continue;
+    }
+    if (Ignored)
       continue;
     if (Verbose) {
       errs() << "Formatting [" << FileNo++ << "/" << FileNames.size() << "] "

clang/tools/clang-format/git-clang-format

@@ -173,11 +173,12 @@ def main():
   # those files.
   cd_to_toplevel()
   filter_symlinks(changed_lines)
+  filter_ignored_files(changed_lines, binary=opts.binary)
   if opts.verbose >= 1:
     ignored_files.difference_update(changed_lines)
     if ignored_files:
-      print(
-        'Ignoring changes in the following files (wrong extension or symlink):')
+      print('Ignoring the following files (wrong extension, symlink, or '
+            'ignored by clang-format):')
       for filename in ignored_files:
         print('    %s' % filename)
     if changed_lines:
@@ -399,6 +400,16 @@ def filter_symlinks(dictionary):
       del dictionary[filename]
 
 
+def filter_ignored_files(dictionary, binary):
+  """Delete every key in `dictionary` that is ignored by clang-format."""
+  ignored_files = run(binary, '-list-ignored', *dictionary.keys())
+  if not ignored_files:
+    return
+  ignored_files = ignored_files.split('\n')
+  for filename in ignored_files:
+    del dictionary[filename]
+
+
 def cd_to_toplevel():
   """Change to the top level of the git repository."""
   toplevel = run('git', 'rev-parse', '--show-toplevel')