fcos translate.go: add warn on small or constrained root partition

[prestist]

#211

Note I somehow dropped the '1' in the version for FCOS in the commit messages.. but before I update the commit structure lets make sure we are happy with the functional changes. Look at the first 2 commits, i.e the exp changes this is basically propagated to all versions back to 1_3 as you can see later with the fixup commits. Once approved I will merge the fixup commits with a rename to the commits to be inclusive of the changes!

[aaradhak]

The first commit message says fcos v0_7_exp but there is no v0_7_exp directory. May be this should be fcos v1_7_exp?

[aaradhak]

Minor: Typo in commit message body: "contstraints" should be "constraints"

[aaradhak]

Trying to understand this wipetable change. The previous code only checked reserved partlabels when wipeTable is false. The current change now runs the root validation even when wipeTable is true. I guess it is intentional.

[prestist]

Yeah it was intentional the root partition size validation needs to happen regardless of wipeTable, because:

1. Even when wipeTable is true (creating a fresh partition table), users can still specify
   partitions that trap the root partition - this is exactly the scenario we want to warn about

2. The wipeTable check only applies to the "reserved partlabel validation" (checking that
   BIOS-BOOT is partition 1, root is partition 4, etc.). That validation should only run
   when wipeTable is false because we're matching existing partitions.

3. The root size/constraint validation is orthogonal to whether we're wiping the table -
   it's about warning users before they create a configuration that won't have enough space
   for OS updates.

[prestist]

@aaradhak

Fixed both issues! I've rebased and updated the commit messages:

1. Changed from "fcos v0_7_exp" to "fcos translate.go" / "fcos translate_test.go" since
   these changes actually apply to FCOS versions v1_3 through v1_7

2. Fixed the typo: "contstraints" → "constraints"

[aaradhak]

@prestist I happened to go over the release details of butane v0.27.0 . I bumped into this Add Fcos spec 1.8.0-experimental , Should this validation also be added to config/fcos/v1_8_exp/translate.go? Was it excluded intentionally?

[prestist]

@prestist I happened to go over the release details of butane v0.27.0 . I bumped into this Add Fcos spec 1.8.0-experimental , Should this validation also be added to config/fcos/v1_8_exp/translate.go? Was it excluded intentionally?

Nope that was an oversite, due to the latest stabilization and the age of this pr.

[prestist]

@aaradhak

If you want to test it locally you can do this:

1. Checkout and Build                                                                                                                                                  
                                                                                                                                                                         
  # Checkout the PR                                                                                                                                                      
  gh pr checkout 378                                                                                                                                                     
                                                                                                                                                                         
  # Build butane                                                                                                                                                         
  ./build                                                                                                                                                                                                               
                                                                                                                                                                         
  2. Manual Testing Scenarios
                                                                                                                                                                         
  The PR adds two warnings for root partition issues:                                                                                                                    
  
  ⚠️  Warning 1: Root Too Small (< 8 GiB)                                                                                                                                 
                                                                            
  cat > test-small.bu << 'EOF'                                                                                                                                           
  variant: fcos                                                                                                                                                          
  version: 1.7.0
  storage:                                                                                                                                                               
    disks:                                                                  
      - device: /dev/vda
        wipe_table: false                                                                                                                                                
        partitions:
          - number: 4                                                                                                                                                    
            label: root                                                     
            size_mib: 5000                                                                                                                                               
  EOF                                                                                                                                                                    
  
  ./bin/amd64/butane < test-small.bu                                                                                                                                     
  Result: warning: root should have 8GiB of space assigned ✅               
                                                                                                                                                                         
  ⚠️  Warning 2: Root Constrained by Auto-Positioned Partition
                                                                                                                                                                         
  cat > test-constrained.bu << 'EOF'                                        
  variant: fcos                                                                                                                                                          
  version: 1.7.0                                                            
  storage:
    disks:
      - device: /dev/vda
        partitions:                                                                                                                                                      
          - number: 4                                                                                                                                                    
            label: root                                                                                                                                                  
            size_mib: 0                                                                                                                                                  
          - number: 5                                                       
            label: data
            start_mib: 0
  EOF                                                                                                                                                                    
                                                                                                                                                                         
  ./bin/amd64/butane < test-constrained.bu                                                                                                                               
  Result: warning: root partition cannot expand; it is set to fill available space but is followed by an auto-positioned partition ✅                                    
                                                                                                                                                                         
  ✅ No Warning: 8 GiB or larger
                                                                                                                                                                         
  cat > test-ok.bu << 'EOF'                                                 
  variant: fcos                                                                                                                                                          
  version: 1.7.0                                                            
  storage:
    disks:                                                                                                                                                               
      - device: /dev/vda
        wipe_table: false                                                                                                                                                
        partitions:                                                         
          - number: 4                                                                                                                                                    
            label: root                                                     
            size_mib: 8192
  EOF

  ./bin/amd64/butane --strict < test-ok.bu  # No warnings  

[aaradhak]

The condition here causes an explicitly positioned partition to replace an auto-positioned one as the "nearest" candidate but the auto-positioned partition is the one that actually constrains root right?

[prestist]

oooof, okay; yees you are right; Let me update the code to account for this.

[prestist]

@aaradhak Good catch! You're right - the previous logic missed when both an auto-positioned and an explicitly positioned partition were present, the "find nearest" would prefer the explicit partitions, effectively hiding the auto-positioned partition that actually constrains root.

The startMiB of any following partition. If this is unspecified (0) and the root sizeMiB is also unspecified, then the following partition will prevent the root partition from growing at all, and we should warn.

Instead of trying to determine which partition is physically "nearest" to root (which we can't reliably do without knowing disk size and sgdisk's placement heuristics), we now simply check: if root is auto-sized AND any other partition on the disk has startMiB unspecified, warn. I also added a test case for this scenario (auto-positioned partition with an explicit one also present).

[aaradhak]

LGTM