grep --exclude option doesn't always skip named pipes

It seems grep still opens files even if the regex tells it to skip them:

$ ll
total 4.0K
p-w--w---- 1 user user 0 Feb  7 16:44 pip-fifo
--w--w---- 1 user user 4 Feb  7 16:44 pip-file
lrwxrwxrwx 1 user user 4 Feb  7 16:44 pip-link -> file

(Note: none of these have read permissions.)

$ strace -e openat grep foo --exclude='pip*' pip-file pip-link pip-fifo
openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = -1 EACCES (Permission denied)
grep: pip-file: Permission denied
openat(AT_FDCWD, "pip-link", O_RDONLY|O_NOCTTY) = -1 ENOENT (No such file or directory)
grep: pip-link: No such file or directory
openat(AT_FDCWD, "pip-fifo", O_RDONLY|O_NOCTTY) = -1 EACCES (Permission denied)
grep: pip-fifo: Permission denied
+++ exited with 2 +++

Granting read permissions, it appears that it doesn't try to read them after opening if they are excluded:

$ strace -e openat grep foo --exclude='pip*' pip-file pip-link pip-fifo
openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3
openat(AT_FDCWD, "pip-link", O_RDONLY|O_NOCTTY) = -1 ENOENT (No such file or directory)
grep: pip-link: No such file or directory
openat(AT_FDCWD, "pip-fifo", O_RDONLY|O_NOCTTY^Cstrace: Process 31058 detached
 <detached ...>

$ strace -e openat,read grep foo --exclude='pip*' pip-file
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\25\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\r\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\t\2\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260`\0\0\0\0\0\0"..., 832) = 832
openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3
+++ exited with 1 +++

$ strace -e openat,read grep foo --exclude='pipe*' pip-file
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\25\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\r\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\t\2\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260`\0\0\0\0\0\0"..., 832) = 832
openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3
read(3, "foo\n", 32768)                 = 4
foo
read(3, "", 32768)                      = 0
+++ exited with 0 +++

And since openat wasn't called with O_NONBLOCK, the opening itself hangs, and grep doesn't reach the part where it excludes it from reading.

Looking at the source code, I believe the flow is like this:

1. If not recursive, call grep_command_line_arg on each file.
2. That calls grepfile if not on stdin.
3. grepfile calls grepdesc after opening the file.
4. grepdesc checks for excluding the file.

When recursive:

1. grepdirent checks for excluding the file before calling grepfile, so the failing openat never happens.