From 0189bc3d358c5da77403c792502872629a1ebc71 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C5=BEenan=20Zuki=C4=87?= <dzenan.zukic@kitware.com>
Date: Wed, 23 Sep 2026 17:21:27 +0000
Subject: [PATCH] BUG: Fail the check when no files are marked for clang-format

clang-format.bash formats only tracked files whose hooks.style Git
attribute includes clangformat. In a repository without such a
.gitattributes entry no file is selected, and the script then runs
"clang-format -i" with no file arguments. clang-format prints
"error: cannot use -i when reading from stdin." but exits 0, so the
check reported success without checking anything.

Count the matching files first, fail with instructions for adding a
.gitattributes when there are none, and log how many files are checked.
Document the .gitattributes requirement in the README.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015k1PExcmd3DPt9VFSGGmcL
---
 README.md  | 14 ++++++++++++++
 action.yml | 21 +++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/README.md b/README.md
index db40fc0..76d55ea 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,20 @@ jobs:
 
 The linter check will fail on a pull request if style changes are required.
 
+Only files whose `hooks.style` Git attribute includes `clangformat` are checked, so the repository
+needs a *.gitattributes* file that marks its C and C++ sources, as ITK's does:
+
+```
+[attr]our-c-style  whitespace=tab-in-indent,no-lf-at-eof  hooks.style=KWStyle,clangformat
+
+*.c    our-c-style
+*.h    our-c-style
+*.cxx  our-c-style
+*.hxx  our-c-style
+```
+
+The check fails if no tracked file has this attribute, since there would be nothing to check.
+
 ## See Also
 
 When used with
diff --git a/action.yml b/action.yml
index 0be625f..2678c93 100644
--- a/action.yml
+++ b/action.yml
@@ -100,6 +100,27 @@ runs:
       shell: bash
       run: |
         set -euo pipefail
+
+        # clang-format.bash only formats tracked files whose hooks.style Git
+        # attribute includes "clangformat", as set in .gitattributes. When no
+        # file matches, it runs "clang-format -i" with no file arguments, which
+        # prints "cannot use -i when reading from stdin" but exits 0, so the
+        # check would pass without having checked anything. Fail instead.
+        file_count=$(git ls-files | git check-attr --stdin hooks.style \
+          | grep -e 'hooks.style: .*clangformat' | grep -v 'ThirdParty' | wc -l || true)
+        if [[ "${file_count}" -eq 0 ]]; then
+          echo "::error::No tracked files have the hooks.style=clangformat Git attribute, so there is nothing to check."
+          echo ""
+          echo "Mark the sources to check in a .gitattributes file at the repository root, for example:"
+          echo ""
+          echo "  [attr]our-c-style  whitespace=tab-in-indent,no-lf-at-eof  hooks.style=KWStyle,clangformat"
+          echo "  *.h    our-c-style"
+          echo "  *.hxx  our-c-style"
+          echo "  *.cxx  our-c-style"
+          exit 1
+        fi
+        echo "Checking ${file_count} files with the hooks.style=clangformat Git attribute."
+
         ./clang-format.bash --tracked
         if ! git diff-index --diff-filter=M --quiet HEAD -- ':!.clang-format'; then
           echo "::error::${{ inputs.error-message }}"