docs: create a flow diagram of the Ignition services

docs/ignition-flow.md

@@ -0,0 +1,157 @@
+```mermaid
+flowchart TB
+    %% ===== IGNITION BOOT FLOW =====
+
+    %% --- GRUB Firstboot Detection ---
+    boot["Boot"] --> grub["GRUB bootloader"]
+
+    subgraph GRUB_FIRSTBOOT ["GRUB Firstboot Detection"]
+        direction TB
+        grub_check{"/ignition.firstboot
+        stamp file on bootfs?"}
+        grub_check -->|Yes| grub_source["Source /ignition.firstboot
+        (may set ignition_network_kcmdline)"]
+        grub_source --> grub_append["Append to kernel cmdline:
+        ignition.firstboot $ignition_network_kcmdline"]
+        grub_check -->|No| grub_no_flag["No ignition.firstboot on cmdline"]
+    end
+    grub --> GRUB_FIRSTBOOT
+
+    %% --- Initramfs Generator ---
+    GRUB_FIRSTBOOT --> generator["ignition-generator
+    (reads /proc/cmdline)"]
+    generator --> firstboot_check{"ignition.firstboot
+    on kernel cmdline?"}
+
+    %% --- Subsequent Boot Path ---
+    firstboot_check -->|No| subsequent_target["ignition-subsequent.target"]
+    subsequent_target --> subsequent_diskful["ignition-diskful-subsequent.target"]
+    subsequent_diskful --> subsequent_done["Ignition services do not run.
+    Boot continues normally."]
+
+    %% --- Firstboot Path ---
+    firstboot_check -->|Yes| complete_gate["ignition-complete.target activated"]
+    complete_gate --> fetch_offline["ignition-fetch-offline.service"]
+
+    %% --- Fetch Offline Details ---
+    subgraph FETCH_OFFLINE ["Ignition Fetch Offline"]
+        direction TB
+        offline_check_cmdline{"Config provided
+        via kernel cmdline?"}
+        offline_check_user_ign{"/usr/lib/ignition/user.ign
+        exists?"}
+        offline_try_platform["Try platform provider"]
+        offline_write_cache["Write config to /run/ignition.json"]
+        offline_needs_net{"Config needs
+        network resources?"}
+        offline_signal_neednet["Signal neednet"]
+        offline_done["Done"]
+        offline_check_cmdline -->|Yes| offline_write_cache
+        offline_check_cmdline -->|No| offline_check_user_ign
+        offline_check_user_ign -->|Yes| offline_write_cache
+        offline_check_user_ign -->|No| offline_try_platform
+        offline_try_platform -->|Config found| offline_write_cache
+        offline_try_platform -->|Needs network| offline_signal_neednet
+        offline_write_cache --> offline_needs_net
+        offline_needs_net -->|Yes| offline_signal_neednet
+        offline_needs_net -->|No| offline_done
+    end
+    fetch_offline --> FETCH_OFFLINE
+
+    FETCH_OFFLINE --> fetch_check{"/run/ignition.json exists?"}
+    fetch_check -->|Yes, skip ignition-fetch.service| kargs_service
+    fetch_check -->|No| fetch_service["ignition-fetch.service"]
+
+    %% --- Fetch Service Details ---
+    subgraph FETCH_ONLINE ["Ignition Fetch"]
+        direction TB
+        online_check_cmdline{"Config provided
+        via kernel cmdline?"}
+        online_check_user_ign{"/usr/lib/ignition/user.ign
+        exists?"}
+        online_fetch_provider["Fetch from platform provider
+        (see Provider Specific Behavior - Config Fetch below)"]
+        online_write_config["Write config to /run/ignition.json"]
+        online_done["Done"]
+        online_check_cmdline -->|Yes| online_write_config
+        online_check_cmdline -->|No| online_check_user_ign
+        online_check_user_ign -->|Yes| online_write_config
+        online_check_user_ign -->|No| online_fetch_provider
+        online_fetch_provider -->|Config found| online_write_config
+        online_fetch_provider -->|No config| online_done
+        online_write_config --> online_done
+    end
+    fetch_service --> FETCH_ONLINE
+
+    %% --- Disk & Mount Services ---
+    FETCH_ONLINE --> kargs_service["ignition-kargs.service"]
+    kargs_service -->|kargs changed| reboot_kargs["Reboot & restart from top"]
+
+    kargs_service -->|no changes| disks_service["ignition-disks.service"]
+    disks_service --> diskful_target["ignition-diskful.target reached"]
+    diskful_target --> mount_service["ignition-mount.service"]
+
+    %% --- Files ---
+    mount_service --> files_service["ignition-files.service"]
+    initrd_root_fs_target["initrd-root-fs.target"] --> afterburn_hostname_service["afterburn-hostname.service"]
+    afterburn_hostname_service -.-> files_service
+
+    %% --- Files Service Details ---
+    subgraph FILES ["Ignition Files"]
+        direction TB
+        files_read_cache["Read cached config
+        from /run/ignition.json"]
+        files_apply["Merge with base configs and apply
+        (create users, write files,
+        directories, links)"]
+        files_done["Done"]
+        files_read_cache --> files_apply
+        files_apply --> files_done
+    end
+    files_service --> FILES
+
+    FILES --> complete_target["ignition-complete.target reached"]
+
+    complete_target --> delete_config["ignition-delete-config.service"]
+
+    %% ===== STYLING =====
+    classDef service fill:#42a5f5,stroke:#1565c0,stroke-width:2px,color:#000
+    classDef target fill:#ffa726,stroke:#e65100,stroke-width:2px,color:#000
+
+    class fetch_offline,fetch_service,kargs_service,disks_service,mount_service,files_service,afterburn_hostname_service,delete_config service
+    class diskful_target,complete_target,complete_gate,network_target,initrd_root_fs_target,subsequent_target,subsequent_diskful target
+
+```
+
+## Provider Specific Behavior
+### Config Fetch
+#### Azure
+```mermaid
+flowchart TB
+    %% ===== AZURE PROVIDER-SPECIFIC CONFIG FETCH =====
+
+    start["Fetch provider-specific config"] --> imds_request["HTTP GET to Azure IMDS
+    http://169.254.169.254/metadata/instance/compute/userData
+    ?api-version=2021-01-01&format=text
+    Header - Metadata: true"]
+
+    imds_request --> imds_retry{"Response code?"}
+    imds_retry -->|"404, 410, 429, or 5xx
+    Retry with exponential backoff
+    (200ms initial, 5s max)"| imds_request
+    imds_retry -->|"Network Unreachable
+    (DHCP has not completed)"| imds_request
+    imds_retry -->|200, empty body| fallback_ovf
+    imds_retry -->|200, has body| write_config["Write decoded config to /run/ignition.json"]
+    imds_retry -->|Other error| error["Error"]
+    write_config --> done["Done"]
+
+    fallback_ovf["Fallback: read OVF custom data from CD-ROM device"]
+    fallback_ovf --> scan["Scan for UDF CD-ROM (often /dev/sr0)"]
+    scan --> mount["Mount device"]
+    mount --> read["Read for ovf-env.xml and CustomData.bin"]
+    read --> available{"Config available?"}
+    available -->|Yes| write_device["Write config to /run/ignition.json"]
+    write_device --> done
+    available -->|No| wait["Wait 1s"] --> scan
+```

docs/release-notes.md

@@ -13,6 +13,7 @@ nav_order: 9
 ### Changes
 
 - Fix test script compatibility with Go 1.26 which removed the `-go` flag from `go tool fix`
+- Improved documentation for the flow of Ignition across clouds.
 
 ### Bug fixes