[Bug build/33616] New: [gdb/build, pre-commit] check-whitespace on files instead of diffs

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=33616

            Bug ID: 33616
           Summary: [gdb/build, pre-commit] check-whitespace on files
                    instead of diffs
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: build
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

I noticed the new pre-commit hook check-whitespace, and I realized that:
- it only works on patches, and
- it works on any file.

So I tried out the following experiment:
- git rm -rf gdb/testsuite
- git commit -a -m "Fix whitespace"
- git revert HEAD
- git rebase --whitespace=fix HEAD^
- git rebase -i HEAD^^ (to combine the two commits)
and ran the testsuite.

I came across one failing test-case: gdb.ada/unchecked_union.exp, which has
literal strings of the form:
...
set inner_string {    case ? is
        when 0 =>
            small: range 0 .. 255;
            second: range 0 .. 255;
        when ? =>
            bval: range 0 .. 255;
        when others =>
            large: range 255 .. 510;
            more: range 255 .. 510;
    end case;
}
...
which are space-indented, and "fixed" by git, breaking the test-case.

So, the git check is context-unaware, and doesn't distinguish between
code/comments/strings. [ BTW, same problem in check-gnu-style, see this (
https://sourceware.org/pipermail/gdb-patches/2025-November/222363.html ) patch
series].

One of the problems with the fact that the current check only checks a patch,
is that pre-commit run --all-files check-whitespace doesn't do anything, so
it's still possible to introduce whitespace errors without getting noticed. 

I found this article (
https://peter.eisentraut.org/blog/2014/11/04/checking-whitespace-with-git )
that shows a way to do the check for entire files instead of patches:
...
git diff-tree --check $(git hash-object -t tree /dev/null) HEAD
...

So I think that this change changes the check into a file check instead:
...
-git --no-pager diff --staged --check "$@"
+git diff-index --cached --check $(git hash-object -t tree /dev/null) -- "$@"
...
and introducing trailing whitespace here:
...
-  return gdb_main (&args);
+  return gdb_main (&args); 
...
is detected when committing, as before:
...
$ git commit -a -m try
  ...
check-whitespace.........................................................Failed
- hook id: check-whitespace
- duration: 0.01s
- exit code: 2

gdb/gdb.c:38: trailing whitespace.
+  return gdb_main (&args); 

  ...
...
but also after committing it:
...
$ git commit -a -m try --no-verify
[master 631538fbebc] try
 1 file changed, 1 insertion(+), 1 deletion(-)
$ pre-commit run --all-files 2>&1 | grep gdb/gdb.c
gdb/gdb.c:38: trailing whitespace.
$ 
...

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