config: add O_DIRECT hint to config read failures

[nvartolomei]

Config files are read with O_DIRECT (via Seastar's open_file_dma). On a
filesystem that doesn't support direct I/O the read fails with EINVAL, which
previously surfaced as an opaque Failure during startup: system_error (error system:22, Invalid argument) with no indication of the cause. This happens in
some environments that bind-mount the bootstrap config from an overlayfs whose
lower layer is squashfs.

This wraps the node- and bootstrap-config reads to report the file path and, on
EINVAL, a hint that the filesystem may not support O_DIRECT. The hint is scoped
to config files (often bind-mounted onto arbitrary filesystems); data
directories are validated separately by syschecks::disk, which does a
round-trip O_DIRECT check.

It also stops config_manager::load_bootstrap from swallowing non-ENOENT
errors: previously any filesystem_error (including the O_DIRECT EINVAL) was
treated as "bootstrap file absent", so the node started silently without
applying the bootstrap config. Now only a genuinely missing file (ENOENT) is
ignored; any other error fails startup.

See:

• fcntl, O_DIRECT, squashfs, overlayfs scylladb/seastar#3518
• Cannot start the Docker container #14473

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v26.1.x
• v25.3.x
• v25.2.x

Release Notes

Improvements

• Startup failures caused by reading config files on filesystems without
  O_DIRECT support (e.g. an overlayfs over squashfs, common when the config is
  bind-mounted) now report the file path and an O_DIRECT hint instead of an
  opaque "Invalid argument".

  Behavior change: if the bootstrap config file (.bootstrap.yaml) is
  present but unreadable, the node now fails to start instead of silently
  starting as though no bootstrap config existed. A missing bootstrap file is
  still ignored, as before.

[Copilot]

Pull request overview

This PR improves startup diagnostics when reading Redpanda config files via Seastar open_file_dma fails (notably with EINVAL on filesystems that don’t support O_DIRECT), and it hardens bootstrap config handling so unreadable-but-present bootstrap config fails startup instead of being silently ignored.

Changes:

• Add utils::format_file_io_error(...) to include file path context and an O_DIRECT hint on EINVAL.
• Wrap node config (redpanda.yaml) reads to throw clearer errors on read failures.
• Update config_manager::load_bootstrap to ignore only ENOENT (missing file) and to fail startup on other read errors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/v/utils/file_io.h	Declares new helper to format file I/O errors with an O_DIRECT hint on EINVAL.
src/v/utils/file_io.cc	Implements the formatted error helper using fmt.
src/v/utils/BUILD	Adds @fmt dependency to the file_io library.
src/v/redpanda/application.cc	Wraps node-config read failures to throw enriched error messages (including path + O_DIRECT hint on EINVAL).
src/v/cluster/config_manager.cc	Stops swallowing non-ENOENT bootstrap read errors; improves error reporting for bootstrap read/parse failures.