fix: Allow running on systems without /etc/fstab present

[vojtechtrefny]

When /etc/fstab is not present, we'll simply skip parsing it.

Fixes: #557
Resolves: RHEL-115033

Summary by Sourcery

Bug Fixes:

• Skip parsing /etc/fstab when the file is not present to avoid errors on systems without it

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds a guard clause in parse() to skip parsing when /etc/fstab is missing, preventing errors on systems without the file.

Class diagram for updated parse() method in blivet.py

classDiagram
class Blivet {
  +parse()
}
Blivet : +parse()  // Now skips parsing if /etc/fstab is missing

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Flow diagram for parse() method with /etc/fstab check

flowchart TD
    A["Start parse()"] --> B{"Does /etc/fstab exist?"}
    B -- No --> C["Return (skip parsing)"]
    B -- Yes --> D["Open /etc/fstab"]
    D --> E["Read lines and parse entries"]

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File-Level Changes

Change	Details	Files
Guard against missing /etc/fstab in parse method.	Check for /etc/fstab existence with os.path.exists Early return from parse() when file is absent	library/blivet.py

Possibly linked issues

• Blivet crash when /etc/fstab isn't present #557: PR adds /etc/fstab existence check, preventing FileNotFoundError from missing fstab.

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• Move the /etc/fstab existence check before resetting self._entries to avoid unnecessary state resets when the file is missing.
• Wrap the open call in try/except FileNotFoundError instead of using os.path.exists to better handle race conditions and permission errors.
• Return a consistent type (e.g. an empty list) from parse when /etc/fstab is absent instead of None to prevent downstream errors.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Move the /etc/fstab existence check before resetting self._entries to avoid unnecessary state resets when the file is missing.
- Wrap the open call in try/except FileNotFoundError instead of using os.path.exists to better handle race conditions and permission errors.
- Return a consistent type (e.g. an empty list) from parse when /etc/fstab is absent instead of None to prevent downstream errors.

---

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 10.36%. Comparing base (59fd1c6) to head (10fa59a).
⚠️ Report is 86 commits behind head on main.

Files with missing lines	Patch %	Lines
library/blivet.py	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (59fd1c6) and HEAD (10fa59a). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (59fd1c6)	HEAD (10fa59a)
sanity	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
- Coverage   16.54%   10.36%   -6.19%     
==========================================
  Files           2        8       +6     
  Lines         284     2017    +1733     
  Branches       79        0      -79     
==========================================
+ Hits           47      209     +162     
- Misses        237     1808    +1571     

Flag	Coverage Δ
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[richm]

@vojtechtrefny what platforms are affected?

[vojtechtrefny]

The original reporter mentions running a live system over PXE, not sure which exactly, but there are more systems that don't use fstab by default, like Fedora CoreOS or bootc systems (and I guess it also possible to manually configure any system replace fstab with systemd mount units if someone really wants).